Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Occurrence of anomalous seismic activity preceding large to greatearthquakes in northeast India region with special reference
to 6 August 1988
H.N. Singha, D. Shankerb,∗, V.P. Singhc
a Centre for Earth Science Studies, Trivandrum 695031, Indiab Department of Earthquake Engineering, Indian Institute of Technology, Roorkee 247667, Uttaranchal, India
c Department of Geophysics, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
Received 14 April 2004; received in revised form 22 July 2004; accepted 6 September 2004
Abstract
Seismicity database from 1860 to 1985 of northeast India region bounded by the area 20◦–32◦N and 82◦–100◦E have beenanalyzed for the identification of precursory swarm/anomalous seismic activity preceding large to great earthquakes withM≥ 7.5.It is observed that with the exception of three earthquakes (1908, 1912 and 1918), the large earthquakes of 1897, 1946, 1947, 1950and 1951/1952 were preceded by well-developed epoch of swarm/anomalous seismic activity in space and time well before theiroccurrence. The seismicity is observed to fluctuate in the order of low-high-low ranging from 0–0.5, 01–33 to 0–0.7 events/yearp ctively. Thed forM ock (a ther
rior to these mainshocks during the epochs of normal/background, swarm/anomalous and gap/quiescence, respeuration of precursory gap is observed to vary from 11 to 17 years for mainshocks ofM 7.5–8.0, and from 23 to 27 years
8.7 and this period is dependent on the magnitude of the mainshocks. Using the values of magnitude of mainshMm),verage magnitude of swarm (Mp) and the precursory time gap (Tp), the following predictive equations are established foregion:
m = 1.37Mp − 1.40
m = 3 logTp − 3.27
All the major earthquakes withmb ≥ 6.1 occurred during 1963–1988 have been investigated for their association with aous seismicity/precursory swarms using the events with cutoff magnitudemb ≥ 4.5. Eleven such events have occurred inegion during the period except one earthquake of 29 May 1976. All the remaining 10 earthquakes were associated in sf anomalous seismicity epochs. Well-defined patterns of anomalous seismicity are observed prior to 1964–1965,976 and 30 December 1984 (mb 5.6). All these mainshocks are preceded by seismicity patterns in the order of low-hiimilar to that observed prior to the mainshocks from 1897 to 1962. The anomalous seismicity epoch is delineated with
Studies in earthquake prediction report manyevidences that long-range correlation between theearthquakes is relected in some phenomena precursoryto strong earthquakes. It has long been suggestedthat large earthquakes are preceded by observablevariations in regional seismicity. Several model andtheory have been proposed to explain these observa-tions, including heterogeneous bond-breaking models(Sahimi and Arbabi, 1996); hierarchical fiber-bundlemodels (Newman et al., 1994); classical damagemechanics which explains that on a regional scalemay lead to a runaway process of faulting that wouldbe observed as an increased rate of seismicity priorto large earthquakes (Lyakhovsky et al., 2001). Bufe
retrospective forecast of the time-of-occurrence oftheMw = 6.9 Loma Prieta earthquake with a precisionof two months.Bowman et al., (1998)have shownthat the cumulative seismic strain release increasesas a power law time to failure before the final event.Papazachos and Papazachos (2001)used the samemethodology for studying the Benioff strain raterelease of the Aegean area. Moreover, an estimatefor the timing of an imminent large earthquake hasbeen achieved with an accuracy of±1.5 years forearthquakes of the same area (Papazachos et al.,2002).
A study of the cumulative strain release curve overa given period offers a useful means of comparing theseismic activities of different regions and also of thesame region over different periods of time. Investiga-
ary-ber
thenlyase
elyions
istics-and
and Varnes (1993)suggested a following simplepower-law time-to-failure equation derived from dam-age mechanics could be used to model the observedseismicity.
ε(t) = A + B(tc − t)m (1)
wheretc is the time of the large event,B the negativenumber andm is the usually about 0.3.A is the value ofε(t) whent= tc, i.e., the final Benioff strain up to andincluding the largest event. The cumulative Benioffstrain at timet is defined as:
tions on elastic strain rebound characteristics over ving periods of time have been carried out for a numof regions of the world (Benioff, 1951) both shallowas well deep focus earthquakes and prove that inupper elastic layer, where locking of a fault can obe relieved by subsequent fracture, the strain releoccurs in active periods which alternate with relativquiescent periods, the strain release in deeper regis almost steady. Based on strain release characterGaur and Chouhan (1968)for northeastern Indian regions, have reported that rate of strain generation
ε
wn -n entr lizeda a
maximum probable size of earthquake that may occurin near future to be 7.2× 107 (Ergs)1/2 andM 7.4 forthe Andman-Nicobar region; 20× 107 (Ergs)1/2 andM8M
reatu phe-n sses.
(t) =∑N(t)
i=1Ei(t)
1/2 (2)
hereEi is the energy of theith event andN(t) is theumber of events at timet. Employing cumulative Beioff strain release (square-root of the seismic momelease) as a measure of seismic activity, they utin empirical time-to-failure relationship to make
.2 for the Assam region and 8.5× 107 (Ergs)1/2 and7.3 for the Bihar-Nepal region.The inception of seismology, there has been a g
rgency upon the search for reliable precursoromena of earthquakes and their generative proce
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 263
Current research suggests that the seismic process ispreceded by a complex set of physical phenomena,usually known as precursors. Most of the major earth-quakes show prior seismic activity that in hindsightseems anomalous. The features include changes inregional activity rate and changes in the pattern ofsmall earthquakes, including alignments on unmappedlinear features near the (future) mainshock. Patternsrecognition in anomalous seismic activity (in spaceand time domains), which precedes major earthquakes,is a worldwide effort to understand its relation withthe impending earthquake (Mogi, 1969; Ohtake et al.,1977a, 1977b; Suzuki, 1982; Habermann, 1981; Wysset al., 1983; Liu et al., 1984; Ohnaka, 1984;Papadopoulos, 1994; Shanker et al., 1995). In recentyears, several reviews on the topic have appeared(Rikitake, 1982; Mogi, 1984; Xu, 1984). Khazanchiand Dutta (1978), andKhattri and Wyss (1978)haveinvestigated long-term premonitory phenomena for theeastern Himalayan region. Their findings show that theseismicity rate deviates from normal before and aftermajor earthquakes. Several authors have discussed theseismicity of the region and a few have dealt with possi-bility of occurrence of large earthquakes in the region(Chouhan, 1966; Gaur and Chouhan, 1968). It is re-ported that the frequency of occurrence of large earth-quakes ofM 8 and 8.5 in northeast India region is 25and 50 years, respectively based on statistical distribu-tion of maximum magnitude earthquakes using Gum-bel’s extreme value theory (Khazanchi and Dutta, 1978;G sg int hado
et al. (1980)based on� (is a peak in the sum ofearthquake energies) andS (is the spatial clusteringof earthquakes during a time interval when the seis-micity is above average before large earthquakes) pat-terns in a large area including Himalayas and TibetPlateau had indicated that the region is due for a largeearthquake.Gupta and Singh (1980)have investigatedP-wave travel time delays using data from ShillongWWSSN station.Srivastava and Chaudhary (1979)re-ported that the earthquake of 1 June 1969 (M= 5.0),which had occurred in the vicinity of Shillong seis-mological station, was associated with P-wave delays.Singh et al. (1982)had reported that medium size earth-quakes ofmb ≥ 5.8 in Burma-Szechwan regions arepreceded by well-defined precursory swarm activity.This information indicates that the northeast India re-gion might experience another series of major seismicdisturbance.
The study of precursor is an interesting subject andworth of investigating in northeast India region. Thus,in view of significantly high seismicity and occurrenceof frequent disastrous earthquakes in this region, weconsidered it as the most promising place to search forseismic precursors. The northeast India region, whichis seismically one of most active regions in the world,has been the site of 10 earthquakes withM≥ 7.5 in-cluding two earthquakes ofM 8.7 among a dozen greatknown earthquakes of the world. These two great earth-quakes have occurred in 1897 in Shillong Plateau andin 1950 at India-China border region. The 12 June1 hil-l s andc 00h lte1 icin-i rth-q gni-t d byt Eng-l rth-q sev-e manl truc-t
louss sso-c
oswami and Sarmah, 1982). If this estimation holdood, one such earthquake ofM≥ 8 has been due
he region since last such magnitude earthquakeccurred on 18 November 1951.
Singh et al., (1994)have studied the behavioureismic activity for the considered region by statiry model of seismicity rates and seismic energy
eased during 3-year window and concluded that loerm seismicity changes before large earthquakeseismic energy release has a periodicity of 20 yeaila and Narain (1971)had studied the seismicity
he area and indicated that the “A” parameter ofeismicity varies from 6.8 to 2.4.Khattri and Wyss1978)have identified an area as “Assam Gap” bouny the meizoseismal areas of 1897 and 1950 garthquakes, and reported that all earthquakesM.6 in northeast India region were preceded by aiod of significant seismic quiescence.Keilis-borok
897 earthquake, which occurred in the vicinity of Song Plateau, leveled all the stone houses, bridgehurches in Shillong and its vicinity claiming over 16uman lives (Oldham, 1889) and is the most widely fearthquake (updated fromRichter, 1958) till date. The5 August 1950 earthquake had occurred in the v
ty of India-China border and is among a few eauakes to which an instrumentally determined ma
ude ofM 8.7 has been assigned. Seiches causehis earthquake were observed as far away asand and Norway. In addition to these two great eauakes, the northeast India region has experiencedral dozen earthquakes, which have claimed hu
ives and widespread destruction to man-made sures and property.
In the present paper, precursory swarm/anomaeismic activity and annual earthquake frequency aiated with major earthquakes ofM≥ 7.5 andmb ≥ 6.1
264 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
for the periods 1897–1962 and 1962–1988, respec-tively, in northeast India region with special emphasisto 6 August 1988 earthquake (M 7.5) of Arakan Yomafold belt have been discussed. It is to mention here thaton investigation of seismic precursor for the Cacharearthquake (mb = 5.6) of 30 December 1984 (Gupta,1985), Gupta and Singh (1986)have found that thisearthquake was preceded by a well-defined precursoryseismic swarm and seismic quiescence, and has encour-aged to investigate whether other major earthquakes inthis region also showed similar behaviour. The presentresults are comparable with the results obtained else-where in the other tectonic belts such as New Zealand,California and Japan (Rikitake, 1975; Evison, 1977,1982; Bowman and King, 2001).
2. Tectonics and seismicity of the region
The entire northeast India region could be consid-ered as comprising of the Eastern Syntaxis (Zone I),the Arakan Yoma and the Naga Thrust fold belts (ZoneII), the Shillong Plateau (Zone III) and the Main Cen-tral Thrust and the Main Boundary Fault of the Hi-malayan Frontal Arc (Zone IV) (Dutta, 1964). Thesefour zones and the major tectonic elements of the regionand distribution of all the earthquakes withmb ≥ 5.1occurred from 1963 to 1984 are depicted inFig. 1.Several authors have described the geology of the re-gion (Evans, 1964; Desikachar, 1974; Nandy, 1980;Singh and Shanker, 1993; Shanker and Sharma, 1998;Shanker et al., 2000) and have indicated that the north-
Ftpa
ig. 1. Seismicity of northeast India region from 1963 to 1985 for eaectonic zones (I–IV). Medium to large size earthquakes of 1964/19mreceded by swarm/anomalous seismic activity. Preparatory zones foctivities in Eastern Syntaxis during 1968, 1977 and 1980, without ma
rthquakes withmb ≥ 5.1 over the structural features and the four major65 (b 6.3 and 6.1); 1976 (mb 6.2); 1984 (mb 5.6); 1988 (M 7.5) werer these mainshocks and occurrence of three localized consecutive swarminshocks, are also indicated.
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 265
east India region is extremely complex with regard togeology and tectonics, and characterized by severalstrong earthquakes with a multiple fault system. Manyworkers have discussed about the focal mechanism ofearthquakes and the tectonics of northeast India and itsadjacent regions (Chandra, 1975; Verma et al., 1976;Le Dain et al., 1984). The area considered includesportion of northeastern region of Indian plate, westernBurma plate and southern Tibetan Plateau (Shankeret al., 2002, 2004). The General nature of seismicityand stresses in northeast India does not show a sys-tematic pattern because of the converging nature of thesurroundings (Singh et al., 1994). The seismicty distri-bution in meeting zones of Indian and Burmese platescan be explained by southeast flow of Tibetan Plateau(Singh and Shanker, 1993).
3. Seismicity database
Frequency of occurrence, distribution of epicentersand magnitude of earthquakes are important parame-ters to study earthquake related phenomena. It has beennow well understood that a reliable seismicity databasecovering wide range of magnitudes is indispensableto have proper understanding of earthquake processin a region (Rikitake, 1982). In addition,Habermannand Wyss (1984)had pointed out two very impor-tant pre-requisite requirements for estimating abnor-mal fluctuation in seismicity prior to occurrence ofs nt ofb iono thesef piledf onc thede ip-t fore1 to1 in1 rth-q st In-d SS)a rth-q ionhr
et al. (1986)have studied earthquake detection and lo-cation capability,b-values and cumulative earthquakefrequency for the four zones in northeast India region(Fig. 2(a) and (b)). This database has been used for thepresent investigations on seismic precursor studies.
4. Seismicity pattern recognition in northeastIndia region
A swarm is a series of events occurring in the areaover an interval of time without any outstanding prin-cipal event (Bullen, 1965). Sekiya (1977)observed 10cases of swarm-like seismic activity preceding small tolarge earthquakes in the magnitude range 4.1–7.9, andindicated that such anomalous activity is first to takeplace, as compared to other precursory phenomena,due to formation of various ruptures where consider-able amount of strain energy are accumulated. He alsopointed out that such activity is of special interest andwould be employed for earthquake prediction. Further,Evison (1982)observed that a swarm sequence maybe followed by other swarm sequence indicating thatpossibly a wider area will be placed at risk. In this sit-uation, revised estimates of magnitude and occurrencetime of the impending earthquake may be necessary.Twenty earthquakes of moderate to great earthquakesin Himalayas have been found to be preceded by welldefined anomalous seismicity/precursory swarm from1860 to 1988 (Singh et al., 1982; Singh and Singh,1 n-d itudeo sorsc welli op-p re-c
pre-c 988h alp asedo ili-t ailedd akes ajore patial,t ela-t
ignificant earthquakes are: (1) a clear assessmeackground/normal seismicity; and (2) identificatf anomalous changes in real sense. Keeping
acts in mind, one such database had been comor the northeast India region from 1897 to 1962ritical examination of the existing catalogues andetection capabilities of seismological network (Guptat al., 1986). This compilation consists of a descr
ion of the historic earthquakes for the period be897 and a catalogue for the period from 1897962. Subsequent to the installation of WWSSN963, the detection and location capability of eauakes have improved considerably in the northeaia region. International Seismological Summary (Ind United States Geological Survey (USGS) eauake catalogues from 1963 to 1985 for the regave been accepted for earthquakes withmb ≥ 4.5 onemoving duplicate events. Using this database,Gupta
986; Gupta and Singh, 1986). These studies have iicated dependency of precursor time on the magnf the mainshocks. The importance of such precuran be understood with the fact that these occurn advance the mainshock occur which provides anortunity to identify other medium and short-term pursors.
In order to understand seismicity patternseding major earthquakes, the period 1860–1ave been tentatively divided in to two critichases, i.e. 1860–1962; and 1963–1998 mainly bn difference in detection and location capab
ies of earthquakes and magnitude scales prevuring these episodes. Identification of earthquwarm/anomalous seismicity phases preceding marthquakes, in the present case, is based on s
emporal and magnitude distributions and their rionships with the mainshocks as detailed below:
266 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 2. (a) Estimates ofa andb values (using logN=a−bM) infour tectonic zones in northeast India region, i.e. Eastern Syntaxis(Zone I), Arakan Yoma fold belt (Zone II), Shillong Plateau (ZoneIII) and the Frontal Arc (Zone IV) for the period 1963–1984. Acutoff magnitudemb ≥ 4.5 is estimated for the region during theperiod. (b) Cumulative number of events with time in four tectoniczones in northeast India region from 1963 to 1984. Major earthquakesduring the period have been indicated with an arrow along with theirrespective magnitude.
• Depending upon the size of the mainshocks, anappropriate grid enclosing the epicenters of eachmainshock and period of seismicity data is se-lected. While doing so major tectonic features,with which the mainshocks are associated, are alsoconsidered.
• The main database of the region is targeted to searchfor seismicity data in each grid for a specified periodirrespective of magnitude and focal depths of theevents.
• Preliminary studies on spatial and temporal distri-bution of entire seismicity data within each grid as-sociated with each mainshock have been studiedand preliminary preparation zones have been de-lineated which are found to be elliptical in mostof the cases elongated in the direction of tectonicfeatures.
• If clustering of events in space and time prior tomainshocks are observed, then the second searchwas made in each grid for all events, events withM≥ 6 and 6.5 for the period 1860–1962; and forevents withmb ≥ 4.5, 5.0, 5.5 and 6 for the period1962–1988.
• For a shallow focus mainshock, precursory eventswith shallow focus (h≤ 70 km) are only considered;and for intermediate focus mainshock, only inter-mediate precursory events (h> 70 km) are used forpattern recognition.
• Using the above procedure, all the mainshocks oc-curring during 1860–1988 in the northeast India
ma-d as:
ic-ore-time
ajora n oft ationo otec-t are:s ns,d ande Ther hipso ocksp
region have been investigated and four anolous seismicity sequences have been identifienormal/background seismicity; anomalous seismity/precursory swarm; gap/quiescence; and fshocks/mainshocks/aftershocks in space anddomains.
In the present study, most of the cases the mxis of the elliptical shape zone are in the directio
he major tectonic features. In general, the separf preparation zones is based on the certain seism
onic and geomorphological criteria. Such criteriapatial clustering of seismicity, topographic variatioimensions of rapture zones of large earthquakesvidence for interactions between seismic events.esults of spatial, temporal and magnitude relationsf characteristic precursory events with the mainsharameters are presented inTables 1–4.
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 267
4.1. Seismicity patterns from 1897 to 1962
A total of 10 earthquakes ofM≥ 7.5 have occurredin northeast India region during the period as given inTable 1. Due to lack of sufficient seismicity data, whichhave restricted the present investigations on anomalousseismicity/precursory earthquake swarms and seismicquiescence to mainshocks ofM≥ 7.5 only (Fig. 3). Ma-jor aftershocks withM≥ 7.5 of 1950 mainshock areexcluded fromTable 1. Anomalous seismicity prior tooccurrence of mainshock number 2 (1908), 3 (1912)and 4 (1918) could not be undertaken due to incom-pleteness of available seismicity data and also lack offelt data. Event 5 (1931) was found to belong to theprecursory earthquake swarm sequence of mainshockof 15 August 1950 ofM 8.7.
The results of anomalous seismicity/swarms andseismic quiescence preceding the mainshocks num-bered 1, 6, 7, 8, 9 and 10 (Table 1) are discussedhere. These earthquakes along with their preparatoryareas delineated based on spatial distribution of back-ground seismicity, anomalous seismicity/swarm activ-ity, gap events and mainshocks and associated after-shocks considering all the events in a grid enclosingmainshocks and associated precursory sequences areshown inFig. 3on a simplified four seismic zones mapof northeast India. It is observed that active seismic-ity phases with regard to large earthquakes in northeastIndia region are confined to relatively short duration,i.e. 1897–1918 and 1946–1952 as compared to entirep sents
4p
and1 pre-c mic-i e,c ob-s tiven inF ndt entsw ec ngh was Ta
ble
1Li
stof
larg
eto
grea
tear
thqu
akes
withM
≥7
1 2th
atha
veoc
curr
edin
nort
heas
tInd
iare
gion
from
1897
to19
62an
dre
late
dse
ism
icpr
ecur
sory
para
met
ers
Ser
ial
num
ber
Mai
nsho
cks
and
asso
ciat
edpa
ram
eter
sS
war
m/a
nom
alou
sse
ism
icity
and
asso
ciat
edpa
ram
eter
s
Dat
eE
pice
ntre
Foc
alde
pth
(km
)
Mag
nitu
de(M
)S
tudy
grid
Per
iod
ofda
taex
amin
edO
nset
ofsw
arm
activ
ity
Mag
nitu
de(M
m)
Pre
curs
ory
time
gapT
p
(day
s)
Ave
rage
mag
nitu
deof
swar
m(M
p)
Pre
para
tion
zone
Are
a(a
ppro
x.)
(Km
2)
mm
/dd/
yyyy
◦ N◦ E
◦ N◦ E
106
/12/
1897
26.0
91.0
608.
724
–28
89–9
518
60–1
906
2M
ay18
748.
784
367.
3a
1.18
×10
5
212
/12/
1908
26.5
97.5
–71 2
––
––
––
––
305
/23/
1912
21.0
97.0
608.
0–
––
––
––
–4
07/0
8/19
1824
.591
.0–
7.6
––
––
––
––
501
/27/
1931
25.6
96.8
607.
6–
––
––
––
–6
09/1
2/19
4623
.596
.060
73 421
–26
92–9
819
20–1
947
5D
ecem
ber
1934
73 4
4296
7.0
7.88
×10
4
707
/29/
1947
28.5
94.0
6073 4
24–2
9.588
.5–9
4.5
1920
–194
811
Feb
ruar
y19
367
3 441
836.
61.
33×
105
808
/15/
1950
28.5
96.5
258.
722
–33
93–1
0019
00–1
953
30Ja
nuar
y19
248.
796
877.
33.
79×
105
911
/18/
1951
31.1
91.4
–8.
028
–33
85–9
319
15–1
955
14D
ecem
ber
1934
8.0
6210
6.8
1.67
×10
5
1008
/17/
1952
30.5
91.5
607.
5–
––
––
––
–
Ear
thqu
akes
seria
lnum
bers
(1,
6,7,
8,9
and
10)
are
the
mai
nsho
cks
prec
eded
bysw
arm
/ano
mal
ous
seis
mic
activ
ity.
Mm
:m
agni
tude
ofm
ains
hock
;Mp:
aver
age
mag
nitu
deof
larg
esta
ndse
cond
larg
este
vent
sin
the
swar
mse
quen
ce);
Tp:p
recu
rsor
ytim
ega
pin
days
estim
ated
from
the
onse
toft
hesw
arm
tillt
heoc
curr
ence
ofth
em
ains
hock
.a
Valu
eofM
p7.
3re
late
dto
1897
mai
nsho
ckis
arbi
trar
y(f
orde
tails
plea
sere
fer
Gup
taan
dS
ingh
,198
6).
eriod from 1860 to 1962 considered for the pretudy (Gupta and Singh, 1986).
.2. Identification of swarm/anomalous seismicityreceding medium to great earthquakes
The mainshocks of 1897, 1946, 1947, 1950951–1952 in northeast India are found to beeded by well defined four sets of anomalous seisty phases as shown inFig. 4(S) and (T). In each caslustering of events in space and time has beenerved which are well evident in the plot of cumulaumber of earthquakes with time (CNET) shownig. 4(T). Such conditions of clustering in space a
ime are found to exist even when precursory evith higher levels of magnitudes (M≥ 6 and 6.5) aronsidered (Fig. 4(S)b and c and (T)). On consideriigher magnitude levels of precursory events, it
268H.N.S
inghetal./P
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andPlanetaryInteriors148(2005)261–284
Table 2Swarm/anomalous seismicity phases and related seismicity rates preceding large to great earthquakes occurred from 1897 to 1962 in northeast India region
Mainshocks precededby swarm/anomalousseismicity
Cutoffmagnitude
Swarm/anomalous seismicity phases and related seismicity parameters
Datemm/dd/yyyy
M Background seismicity Swarm/anomalous seismicity Gap/quiescence
Period D N AF Period D N AF Period D N AF
06/12/1897 8.7 All events 1860 to April 1874 14.3 3 0.2 May 1874–1877 3.7 95 25.7 1878 to 11 Jun1897
19.5 1 0.0
09/12/1946 734 All events 1920 to May 1934 14.4 7 0.5 June 1934 to 11May 1940
6.0 10 1.7 12 May 1940 to 11September 1946
6.4 0 0.0
M≥ 6 1920 to May 1934 14.4 1 0.1 June 1934 to 11May 1940
6.0 6 1.0 12 May 1940 to 11September 1946
6.4 0 0.0
M≥ 6.5 1920 to 13 April 1938 18.3 2 0.1 14 April 1938 to 11May 1940
2.1 4 1.9 12 May 1940 to 11September 1946
6.4 0 0.0
07/29/1947 734 All events 1920–1935 16.0 3 0.2 1936 to September1941
5.8 13 2.2 October 1941 to 28July 1947
5.8 0 0.0
M≥ 6 1920–1940 21.0 0 0.0 January 1941 toSeptember 1941
0.8 2 2.5 October 1941 to 28July 1947
5.8 0 0.0
M≥ 6.5 1920–1940 21.0 0 0.0 January 1941 toSeptember 1941
0.8 2 2.5 October 1941 to 28July 1947
5.8 0 0.0
08/15/1950 8.7 All events 1900–1923 24.0 2 0.1 1924–1938 15.0 60 4.0 1939 to 14 August1950
11/18/1951 and 08/17/1952 8.0 and 7.5 All events 1915–13 Dec 1934 20.0 9 0.4 14 December 1934to 15 August 1937
2.7 10 3.7 16 August 1937 to16 November 1951
14.3 2 0.1
M≥ 6 1915–14 Dec 1934 20.0 4 0.2 15 December 1934to 3 January 1935
0.1 2 33 4 January 1935 to16 November 1951
16.9 2 0.1
M≥ 6.5 1915–14 Dec 1934 20.0 2 0.1 15 December 1934to 3 January 1935
0.1 2 33 4 January 1935 to16 November 1951
16.9 0 0.0
D: duration in years;N: number of events; and AF: annual fre
quency.
H.N.S
inghetal./P
hysics
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andPlanetaryInteriors148(2005)261–284
Table 3List of significant earthquakes ofmb ≥ 6.1 occurred in northeast India region during 1963–1988 along with 30 December 1984 earthquake ofmb 5.6 and three localized swarmactivities in Eastern Syntaxis for studying swarm/anomalous seismicity changes associated with some of the mainshocks during the period
Serialnumber
Major earthquakes andassociated parameters (A)
Swarm/anomalous seismicity and associated parameters (B)
Portion (A) of the table provides basic data on the earthquakes withmb ≥ 6.1, and (B) shows information on precursory data estimates prior to mainshocks. Earthquakes serialnumbers 1–4 are identified as major earthquake swarm sequence to the Mainshock of 6 August 1988 (serial number 12); earthquakes numbers 5, 6, 10, 11 and 12 are found to be themainshocks preceded by swarm/anomalous seismic activity; earthquake serial number 8 is treated as gap event during quiescence period prior to 6 August 1988 main shock (serialnumber 12).Mm: magnitude of mainshock;Mp: average magnitude of largest and second largest events in the swarm sequence);Tp: precursory time gap in days estimated from theonset of the swarm till the occurrence of the mainshock. Except event number 12, magnitudes of all other earthquakes are in body wave scale.
a Approximated body wave magnitude prior to 1963.
269
270 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
possible to delineate reduced size of preparatory zonesand/or reorient preparation zones (Fig. 4(S)) especiallyfor 1946, 1947 and 1950 mainshocks. Such deductionsare generally helpful to reduce spatial uncertainties forthe location of epicenters of impending earthquakes. Itis also evident that size of preparation zones is directlyproportional to the size of the mainshock magnitude(Table 1). When considered all events, the preparationzones of 1897 and 1947 mainshocks are found to orientin northeast-southwest direction; of 1946 and 1950 innorthwest-southeast direction; and 1951–1952 main-shocks in east-west directions. No change in the ori-entation and/or size of preparatory zone of 1951–1952was observed on considering even higher magnitudelevel precursory events whereas it was quite promi-nent in the case of 1946, 1947 and 1950 mainshocks(Fig. 4(S)b-B and C, c-B and C and d-B and C)). Suchconditions could not be studied for 1897 earthquakedue to paucity of database during the period. Fore-shocks activity is also noticed for 1950 and 1951 main-shocks.
Four sets of anomalous phases, i.e. background seis-micity, precursory swarm/anomalous seismicity, pre-cursory gap/quiescence and foreshocks/mainshocks/aftershocks with seismicity in the order of low-high-low-high, respectively, associated with 1897, 1946,1947, 1950 and 1951–1952 can be delineated from theCNET curve shown inFig. 4(T). Among these, the pre-cursory swarm/anomalous seismicity phase is consid-ered to be diagnostic parameter as seismic precursor.T ma-l s areg h-q ob-s thep np uen-c es ofb city,afc tionz ob-s ears( tem-b (14D 46,1 ively. Ta
ble
4S
war
m/a
nom
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icity
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esan
dre
late
dse
ism
icity
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spr
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rthq
uake
soc
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om19
63to
1988
inno
rthe
ast
Indi
are
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Mai
nsho
cks
prec
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/an
omal
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seis
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ity
Cut
offm
agni
tude
Sw
arm
/ano
mal
ous
seis
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ityph
ases
and
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ted
seis
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itypa
ram
eter
s
Dat
em
m/d
d/yy
yyM
Bac
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und
seis
mic
ityS
war
m/a
nom
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sse
ism
icity
Gap
/qui
esce
nce
Per
iod
DN
AF
Per
iod
DN
AF
Per
iod
DN
AF
03/2
7/19
6401
/12/
1965
6.3
All
even
ts19
50to
23M
ay19
599.
41
0.11
24M
ay19
59to
25D
ecem
ber
1961
2.6
93.
4626
Dec
embe
r19
61to
26M
ar19
642.
30
0.0
08/1
2/19
766.
2A
llev
ents
1974
to29
May
1975
1.4
32.
1430
May
1975
to23
July
1975
0.15
533
.33
24Ju
ly19
75to
11A
ugus
t197
61.
043
2.88
12/3
0/19
845.
6A
llev
ents
1980
to10
Jan-
uary
1982
2.03
00.
011
Janu
ary
1982
to2
Feb
ruar
y19
831.
0712
11.2
13
Feb
ruar
y19
83to
29D
ecem
ber
1984
1.9
21.
05
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6/19
887.
5m
b≥
5.0
1950
toM
ay19
6313
.42
372.7
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63to
1967
4.5
823
5.02
1968
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8517
.75
392.2
0
mb≥
5.5
1950
to27
Sep
tem
ber
1963
13.7
45
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mbe
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63to
June
1965
1.76
84.
55Ju
ly19
65to
Sep
tem
ber
1985
20.2
56
0.30
mb≥
6.0
1950
–196
314.
02
0.14
Janu
ary
1964
toJu
ly19
640.
594
6.78
Aug
ust
1964
toS
epte
mbe
r19
8521
.17
20.
09
D:d
urat
ion
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ars;N
:num
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ents
;and
AF
:ann
ualf
requ
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.
he onset and termination of all the four sets of anoous seismicity phase and associated parameteriven in Tables 1 and 2. During the process of eartuake generation, spurt in seismicity have beenerved well in advance prior to all the mainshocks inreparation zones.Table 2depicts onset-terminatioeriod, duration, number of events, and annual freqies of earthquakes during the anomalous phasackground seismicity, swarm/anomalous seismind gap/quiescence before five mainshocks (Table 1)
or all the events and events withM≥ 6 and 6.5. Ononsidering all the events occurred in the preparaones, duration of anomalous seismicity/swarm areerved to be 3.7 years (May 1874–1877), 6.0 yJune 1934 to 11 May 1940); 5.8 years (1936 to Seper 1941); 15 years (1924–1938) and 2.7 yearsecember 1934 to 15 August 1937) for 1897, 19947, 1950 and 1951–1952 mainshocks, respect
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 271
Fig. 3. Large to great earthquakes ofM≥ 7.5 from 1897 to 1962, andmb ≥ 6.1 from 1963 to 1988 in northeast India region are shown on themajor tectonic zones (I–IV) of the region. Orientations of preparatory zones associated with the mainshocks of 1897, 1946, 1947, 1950 and1951/1952, which were preceded by precursory swarm/anomalous seismic activity, are also shown.
A drastic reduction in the duration of this phase isobserved when events withM≥ 6 and 6.5 are onlyconsidered.
Magnitudes of events in the four identified phasesand their relationships with the mainshock magni-tudes are shown inFig. 5 except 1897 mainshock forthe reasons mentioned elsewhere in the text. Magni-tudes of events in the preparation zones during nor-mal/background phase prior to 1946, 1947 and 1950mainshocks did not exceedM 6 maintaining low-level seismic activity and hence are not shown inFig. 5. On the other hand, during the same phase priorto 1951 mainshock, earthquakes up toM∼ 7 haveoccurred before 1924 followed by lower magnitudeevents (M∼ 5.5) till the onset of swarm/anomalousseismicity phase. Interestingly, this phase is followedby a sequence of relatively higher frequency of eventshaving slightly higher level of magnitude range clus-
tered in time which constitute swarm/anomalous seis-micity phase. Most of the events during this phasehave magnitude belowM 6 while a few exceeded thislimit (Fig. 5). The annual frequency of events duringgap/quiescence phase has been considerably low withmagnitudes not exceeding 6.5 as compared to preced-ing phase in all the cases. In essence, the magnitude oflargest events during swarm/anomalous phase has beenobserved to be lower by at least 1 unit in magnitude ascompared to the magnitude of the largest event in themainshock sequence indicating direct relationships be-tween these two.
A very interesting feature is derived, as shown inFig. 6(a) (Table 2), through the plot of logarithm ofannual frequency of earthquakes (seismicity rates) inthe preparation zones of each of the three anomalousphases preceding mainshocks occurred during 1897 to1951–1952 in the region. The seismicity rates are ob-
272 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 4. Patterns of spatial (S) and temporal (T) distribution of events in four anomalous phases, i.e. background seismicity, precursoryswarm/anomalous seismic activity, gap/quiescence seismicity and foreshocks/mainshocks/aftershocks, preceding five large earthquakes (M≥ 71
2)of 1897, 1946, 1947, 1950 and 1951–1952 that have occurred in northeast India region from 1897 to 1962.
served to fluctuate significantly in all the phases butmaintaining always above normal value (>1 event/year)during swarm/anomalous seismicity phases whereas itis always below normal value in other remaining twoanomalous phases preceding all the mainshocks and forall the sets of magnitude range. This indicates that ifa phase with higher annual frequency rate is precededand followed by a lower annual frequency rates, theformer phase and its characteristics have direct cor-relation with location, time of occurrence and mag-nitude of the impending earthquake. However, occur-
rence of any significant earthquake during the precur-sory gap period (zone with low level annual earthquakefrequency/rates) will affect the converging trend of thestress field and hence it will enlarge the gap period,which eventually delay the occurrence of the impend-ing earthquake.
The overall information derived through spatial,temporal and magnitude relationships indicate thatswarm/anomalous seismicity phase can be consideredas precursor to all the five major earthquakes withM≥ 7.5 of 1897, 1946, 1947, 1950 and 1951–1952 in
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 273
Fig. 5. Patterns of magnitude distribution of events during four anomalous phases of background seismicity, precursory swarm/anomalousseismic activity, gap/quiescence seismicity and foreshocks/mainshocks/aftershocks, preceding four large earthquakes (M≥ 7.5) of 1946, 1947,1950 and 1951–1952 that have occurred in northeast India region from 1897 to 1962 (please referFig. 4 for details on spatial and temporalpatterns).
the region which began some 23, 12, 11, 27 and 17years, respectively, prior to the mainshocks occurrence(Table 1).
5. Predictive regression
Precursory parameters furnished inTable 1indicateinterdependence betweenMm, Mp andTp. Due to lackof instrumental data and also limited available felt re-ports; no attempt has been made to estimate magnitudesof precursory swarm for the 1897 mainshock. A mag-nitude ofM 7.3 has been assigned tentatively to thelargest swarm event for the 1897 mainshock based ontwo factors: (1) the 1950 mainshock of similar mag-nitude ofM 8.7 is preceded by a swarm of magnitudeMp 7.3; and (2)Tp for both the earthquakes are compa-rable. It is assumed that, for statistical analysis, these
three parameters are related and their distribution isnormal. The best-fit equations relatingMm,Mp andTpare derived as given below:
Mm = 1.37Mp − 1.40
(R2 = 0.75 for estimation ofMm) (3)
logTp = 0.44Mp + 0.73
(R2 = 0.66 for estimation ofTp) (4)
logTp = 0.33Mm + 1.08
(R2 = 0.94 for estimation ofTp) (5)
or
Mm = 3 logTp − 3.27 (6)
274 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 6. Annual number of earthquakes during three anomalous phases of background seismicity, precursory swarm/anomalous seismic activity,gap/quiescence seismicity preceding (a) large earthquakes ofM≥ 71
2 of 1897, 1946, 1947, 1950 and 1951–1952 from 1897 to 1962 for allevents, events withM ≥6 and 6.5; (b) earthquakes ofmb ≥ 6.1 of 1964–1965, 1976, 1984 and 1988 from 1963 to 1988 considering the eventswith cutoff magnitudesmb ≥ 5, 5.5 and 6 occurred in northeast India region.
whereMm is the magnitude of the principal mainshock,Mp the characteristic magnitude estimated by averag-ing magnitudes of the largest and second largest swarmevents for each mainshock,Tp the precursory gap pe-riod (days) estimated from the onset of the precursoryswarm sequence to the onset of the principal mainshockandR2 is the coefficient of regression.
Three sets of regression lines representing the Eqs.(3)–(5)are shown inFig. 7(a–c), respectively. All thethree sets of lines are within 95% confidence limits andfor three degree of freedom.
If Mp andTp are known, Eqs.(3) and(6) are predic-tive for estimation of magnitude of principal mainshockMm and it requires recognition of swarm/anomalousseismic activity well before the mainshock. Theseequations hold good provided no major outstandingearthquake with size equal or greater than the size ofthe largest earthquakeMp occurs within the delineatedpreparation zone during the quiescence period. Mech-
anism of large earthquake generation requires conver-gence of direction of tectonic forces leading to slowbuild up of elastic strain in the pending focal region.The occurrence of a swarm sequence is indicative ofconverging trend of stress accumulation towards pend-ing focal region (Singh and Singh, 1984). The occur-rence of any outstanding events in the preparation zoneduring the gap period may divert the converging trendof stress accumulation, which may enhance the gapperiod, and also the magnitude of the principal main-shcok. If so, certain corrections are needed in Eqs.(3)and(4)before estimatingMm andTp for pending earth-quake. AsMm andTp are dependent onMp, the requiredcorrection needs to be applied for recalculation ofMp. Itis evident fromFig. 7(c) that the magnitudes of princi-pal mainshock (Mm) and logarithmic of precursory gapperiods (Tp) are linearly correlated (Eq.(6)). Rikitake(1979)had defined the first kind of precursor on thebasis of dependence of logarithmic precursory gap pe-
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 275
Fig. 7. Relations amongMm, Mp and Tp. The best fit equations to the data (a)Mm = 1.37Mp−1.40; (b) logTp = 0.44Mp + 0.73; and (c)logTp = 0.33Mm+ 1.08 represent the central best-fit lines and the outer curves are the 95% tolerance limits.
riod on the magnitude of mainshock. It shows, in thepresent case, that the swarm/anomalous seismic activ-ity is a precursor of first kind and may be used forlong-range earthquake forecasting.
6. Anomalous seismicity phases from 1963 to1988
It may also be noted that magnitude of earthquakesduring 1897–1962 are listed asM (local magnitude).On the other hand, it ismb (body wave magnitude) forthe period 1963–1985 being reported regularly in theNOAA hypocentral data file (HDF). This data, beinghomogeneous, have been considered for the presentstudy and have not been converted intoM using alreadyavailable relations betweenmb andM (Richter, 1958;Gupta and Rastogi, 1972).
Table 3(A) shows that a total of 11 earthquakeswithmb ≥ 6.1 have occurred in the region from 1963 to1988 excluding the mainshock of 30 December 1984Cachar earthquake (mb 5.6) which was found to bepreceded by well defined anomalous seismicity phases(Gupta and Singh, 1986). This results have been quiteencouraging and likewise other 10 earthquakes withmb ≥ 6.1 have been investigated using the data from1963 to 1985. The mainshock of 1988 (serial number12) did not occur by that time which will be discussedseparately. Four earthquakes withmb ≥ 6.1 (Table 3:serial numbers 1–4) have occurred in quick successionwi eltw bec hich
will also be discussed at later stage. The 1975 earth-quake (serial number 8) is an isolated event, which hasalso occurred within the Arakan Yoma fold belt closeto the event number 2 and was not preceded by anyseismicity pattern of my interest. The event number9 of 1976 is an isolated event occurred in the remoteeast and did not precede by any anomalous seismic-ity pattern. Event number 10 of 1976, which occurredin Eastern Syntaxis, is reported to be associated withwell-defined anomalous seismicity patterns (Singhet al., 1982). Earthquake swarms/anomalous seismicactivity associated with these earthquakes and the 30December 1984 (mb 5.6) Cachar earthquake using thedata from 1963 to 1985 are discussed here.
6.1. Mainshocks of Frontal Arc (27 March 1964,mb 6.3 and 12 January 1965, mb 6.1)
These two earthquakes, separated by a segment ofabout 150 km in northwest-southeast, had occurredwithin 10 months during 1964–1965 in the HimalayanFrontal Arc zone (Figs. 1 and 3; Tables 3 and 4). Guptaand Singh (1986)have discussed seismicity patterns as-sociated with this sequence of mainshock. Nine eventshave occurred in quick succession during 24 May 1959to 25 December 1961 in preparation zone of about275× 500 km2 size oriented in northwest-southeast di-rection with an annual frequency of over 3 events/yearas compared to almost zero annual frequency during thepreceding background and following gap/quiescencep n-s ain-s yh ake
ithin a short span of 6 months duration (Figs. 1 and 3)n a north-south segment in Arakan Yoma fold bithout any outstanding principal events which canonsidered as anomalous seismicity-details on w
eriod (Fig. 8). The temporal and magnitude relatiohips among four epochs associated with the mhock are shown inFig. 8(b) and (c). This significantligh seismicity constitute a well defined earthqu
276 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 8. Patterns of spatial (a), temporal (b) and magnitude distribution (c) of premonitory seismicity during background, swarm/anomalousseismicity and gap/quiescence epochs in the northwest-southeast trending preparatory zone of mainshocks of 1964 (mb 6.3) and 1965 (mb 6.1)in Frontal Arc, northeast India region.
swarm/anomalous seismicity for the 1964–1965 main-shock sequence which occurred some 5 years after theburst of swarm activity.
6.2. Mainshock of Eastern Syntaxis (12 August1976, mb 6.2)
This earthquake had occurred in Eastern Syntaxis(Figs. 1 and 3) and precursory swarm/anomalous seis-micity preceding the mainshock was reported bySinghet al. (1982). They reported a relatively short durationswarm activity from 30 May to 23 July 1975, with theoccurrence of five events ranging in magnitude from4.8 to 5.2 followed by a quiescence period from 24
Fig. 9. Patterns of spatial (a), temporal (b) and magnitude distribution (c) of premonitory seismicity during background, swarm/anomalousseismicity and gap quiescence epochs in the northeast-southwest trending preparatory zone of mainshock of 1976 (mb 6.2) in Eastern Syntaxisz
July 1975 to 11 August 1976 preceding the mainshock(Tables 3 and 4). The swarm events and the gap eventspreceding mainshock are located in a very small areaaround the epicenter of the mainshock (Fig. 9 (a) and(b)) and the preparation zone is elongated in northeast-southwest direction. It is observed that some back-ground seismicity in the southern part of the prepara-tion zone and also outside of it was present till Septem-ber 1974 whereas burst of swarm/anomalous seismicityhas taken place at a distance of about 100 km almost inthe northern part of the preparation zone (Fig. 9). The12 August 1976 mainshock had occurred after 440 daysof burst of swarm activity between the spatial concen-trations of swarm and gap events (Fig. 9(a)).
one.
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 277
Fig. 10. Patterns of spatial (a), temporal (b) and magnitude distribution (c) of premonitory seismicity during background, swarm/anomalousseismicity and gap/quiescence epochs in the northwest-southeast trending preparatory zone of 30 December 1984 mainshock (mb 5.6) of Cacharregion, Shillong Plateau.
6.3. Mainshock of Cachar area, Shillong Plateau(30 December 1984, mb 5.6)
The 1984 Cachar earthquake is the latest amongthe killer earthquakes that have occurred in the north-east India region which claimed over 20 human livesand rendered thousands homeless. The epicenter is lo-cated very close to the boundary of the Shillong Plateauand Arakan Yoma tectonic zones (Figs. 1 and 3).Gupta and Singh (1986)have studied the precursoryswarm preceding this earthquake. The area boundedby 23◦–28◦N and 90◦–94◦E has been investigatedon considering shallow earthquakes (h≤ 70 km) ofmb ≥ 4.5 (Fig. 10) for the period from 1980 to Septem-ber 1985. The swarm/anomalous seismicity, whichstarted from the beginning of 1982 and continued tillFebruary 1983, is well distributed almost through-out the NW-SE trending elliptical preparation zone(Fig. 10(a)). The period from March 1983 to 29 De-cember 1984 is marked by a very low seismicity andis considered as quiescence period for the earthquake(Tables 3 and 4). The mainshock had occurred af-ter 3 years from the onset of the anomalous seismic-ity in the southeastern part of the preparation zone(Fig. 10(a)) where none of the precursory events hadoccurred earlier (Fig. 10(a)). Magnitude distributionamong the events of four precursory epochs is shownin Fig. 10(c).
6.4. Swarms of 1968, 1977 and 1980 in EasternSyntaxis
A very peculiar case of three swarm activities,well placed in space and time domains under theswarm hypothesis, have been observed in EasternSyntaxis during 1968, 1977 and 1980 (Gupta andSingh, 1986) without mainshocks (Table 3). Thesethree swarm activities are like burst of seismicity andlocated in a very small preparation zone almost orientedin northwest-southeast direction (Figs. 1, 3 and 11).For the present investigations, only shallow focusearthquakes (h≤ 70 km) have been considered. Theelliptical areas of these swarms is bound by lati-tudes 30.1◦–30.4◦N, and longitudes 94.5◦–95.1◦E. TheCNET curve shown inFig. 11(b) indicates that the ter-mination of one set of swarm activity and the onset ofthe following set are separated by quiescence of verylow activity. During the first swarm sequence, whichbegan on 28 June 1968 lasted for about 2 months dura-tion till 3 September 1968, 14 events ofmb ≥ 4.5 haveoccurred in the preparation zone. During the gap periodfrom 4 September 1968 to 20 July 1977, i.e. about 9years duration, only three such events occurred withinthis elliptical area (Fig. 11(a)A and (b)A). The sec-ond swarm sequence, which started on 21 July 1977and lasted for 2 months till 28 August 1977, is markedby very high seismicity when sixteen events occurred
278 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 11. Patterns of spatial (a), temporal (b) and magnitude distribution (c) of premonitory seismicity during background, swarm/anomalousseismicity and gap/quiescence epochs during 1968 (28 June to 3 September 1968), 1977 (21 July to 28 August 1977) and 1980 (7 August to 24September 1980) observed in Eastern Syntaxis. No mainshock had occurred in association with these individual swarms sequence.
confining to almost one-third of the preparation zonedefined earlier (Fig. 11(a)B and (b)B). Only two eventshave occurred during the gap period from 28 August1977 to 6 August 1980. Nature of third swarm se-quence (Fig. 11(a)C and (b)C) is similar to second one,which also occurred in the same region. During thethird swarm sequence period from 7 August 1980 to 24September 1980, 14 earthquakes ofmb ≥ 4.5 had oc-curred in just 1.5 months period. Magnitude of events inthree swarm activities is shown inFig. 11c. This regionhas been quiet since 25 September 1980 till Septem-ber 1985 with the occurrence of only two events ofmb ≥ 4.5 during the gap period of 5 years.
These three swarms have occurred within the prepa-ration zone of 1950 earthquake at its northern boundary
(Figs. 1 and 3). Although upper limit of magnitudesin these three swarm cases are atmb ∼ 5.0, whereaslower magnitude limit aremb 4.7 in 1968 and 4.4 inboth 1977 and 1980 cases (Fig. 11(c)). The magnitudesof gap events are either equal or less than the swarmmagnitudes except in the case of 1968 in which oneof the gap events occurred on 15 August 1969 hasmb5.2. Occurrence of these short periods intense consec-utive swarm activities in the same region of highly lo-calized nature without any outstanding principal eventis a matter of great discussions and requires suitableexplanation. Such concentrated swarms are certainlyrelated with the earth’s internal tectonic activity andusually observed in volcanic terranes due to movementof magma in the crust (Hill et al., 1984).
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 279
Table 5Annual earthquake frequency associated with 1964 Swarm activity in Arankan Yoma fold belt
CutoffMagnitude(mb)
1964 Swarm/anomalous seismicity and associated seismicity parameters
≥5.0 1950 to May 1963 13.42 37 2.76 June 1963–1967 4.58 23 5.02 1968 to September1985
17.75 39 2.20
≥5.5 1950 to 27September 1963
13.74 5 0.36 28 September 1963to June 1965
1.76 8 4.55 July 1965 toSeptember 1985
20.25 6 0.30
≥6.0 1950–1963 14.0 2 0.14 January to July 1964 0.59 4 6.78 August 1964 toSeptember 1985
21.17 2 0.09
D: duration in years;N: number of events; and AF: annual frequency.
7. Patterns of anomalous seismicity in ArakanYoma fold belt during 1963–1965
The Arakan Yoma fold belt is seismically most ac-tive among the four tectonic zones identified in north-east India region in which intermediate earthquakesoccur frequently (Figs. 1 and 3). Table 3indicates thatfour intermediate earthquakes (88–155 km focal depth)with mb ≥ 6.1 had occurred in quick succession fromJanuary to July 1964 in a north-south segment withinthe main tectonic zone of the Arakan Yoma fold belt.This activity is similar to one observed prior to oc-currence of a dozen of medium to great earthquakesfrom 1897 to 1985 in northeast India discussed ear-lier in this paper and hence constitute a well definedswarm/anomalous seismicity pattern. In addition, afterthe occurrence of 1946 event ofM 7.8, no major earth-quake had occurred in this region. After the terminationof swarm activity since August 1964, no major seismicdisturbance with magnitude higher than the swarm hasoccurred within the region occupied by them for a quietlong period of over 20 years till 1985 except those oc-curred in 1969 (mb 5.9) and 1975 (mb 6.5). This situ-ation meets the necessary requirements of seismic ac-tivity being anomalous seismicity/swarms as seismicprecursor followed by a long quiescence period withvery low seismicity rate in both space and time do-mains, and hence the region could be the site for futuremajor earthquake (Tables 3 and 4). In order to studythis anomalous seismicity in detail from seismic pre-c kesw ridbt have
been scrutinized in space-time domains and their mag-nitude relationships considering events formb ≥ 5.0,5.5 and 6.0 (Table 5; Fig. 12).
Spatial, temporal and magnitude distribution ofevents for three cutoff magnitudesmb ≥ 5.0, 5.5 and 6.0in the selected grid are shown inFig. 12. When all theearthquakes withmb ≥ 5.0 are considered, over 90%events are centrally concentrated in a north-south seg-ment (Fig. 12(a)A) which is general trend of tectonicfeatures in the region. This spatial pattern has been usedto delineate preparation zone tentatively, which is el-liptical in shape and elongated approximately in north-south direction. When the events with cutoff magni-tudemb ≥ 5.5 and 6.0 are considered, the same spacewindow of earlier defined preparation zone encloses al-most all the events (Fig. 12(a)B and C) indicating thatthese events have spatial relation with the tectonic fea-tures within the preparation zone. Three epochs show-ing anomalous seismicity patterns can be identified us-ing the cumulative number of events with time shownin Fig. 12(b) (Table 5) as: background seismicity, theswarm/anomalous seismicity, gap/quiescence. It is ev-ident that the swarm/anomalous seismicity epoch ismarked with significantly high annual frequency ratesas compared to both preceding background and follow-ing gap/quiescence epochs as this ratio is estimated tobe 1.2:2.3:1; 1.2:15.2:1; 1.5:75:1 for the cutoff magni-tudesmb ≥ 5.0, 5.5 and 6.0 during background seismic-ity, the swarm/anomalous seismicity, gap/quiescenceepochs respectively (Table 5). The annual earthquakef ochi lue)i allt i-
ursor point of view, seismicity data for earthquaith mb ≥ 5 from 1950 to September 1985 in a gounded by 20◦–26◦N latitudes and 92◦–98◦E longi-
udes enclosing the epicenters of 1964 swarms
requency during swarm/anomalous seismicity eps observed to be always more than 1 (normal van the case of 1963–1964 swarm with respect tohe cutoff magnitudesmb ≥ 5.0, 5.5 and 6.0 as is ev
280 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Fig. 12. Spatial (a), temporal (b) and magnitude distribution (c) patterns of premonitory seismicity during background, swarm/anomalousseismicity and gap/quiescence epochs in approximately north-south trending preparatory zone. This zone is delineated considering the spatialpatterns of events formb ≥ 5 from 1950 to 1986 and it defines the 1963–1965 swarm sequence in Arakan Yoma fold belt. No major earthquakehad occurred till 1986 in the delineated zone. Using the existence of anomalous seismicity/swarm sequence observed during 1963–1965, a largeearthquake ofM 8± 0.5 was forecast to occur by the end of 1990. Subsequently, a large earthquake ofM 7.5 had occurred on 6 August 1988 indelineated preparation zone at its northern extremity.
dent fromFig. 6(b) for the mainshock of 1988. On theother hand, it is always less than normal value duringbackground and gap/quiescence epochs for cutoff mag-nitudemb ≥ 5.5 and 6.0 except for cutoff magnitudemb ≥ 5.0 which is slightly above normal but lower thanthe annual earthquake frequency for the same cutoffmagnitude during swarm/anomalous epoch (Fig. 6(b)).This deduction suggests that a proper understandingof cutoff magnitude of events is required which helpestimates better and reliable seismicity phases preced-ing major earthquakes. It is also inferred that duringthe earthquake generation process in an active regionalong certain segment of tectonic feature, there wouldbe sudden jump in the annual earthquake frequencyover the background seismicity, which can be consid-ered as anomalous seismicity. This phase may continue
for certain period as is evident fromFig. 12(b), andwould be followed by another phase of low annual fre-quency rate (gap/quiescence) similar to that of back-ground epoch without any outstanding principal event.
Magnitude distribution of events occurred in threedistinct epochs in the order of low-high-low seismic-ity rate are shown inFig. 12(c). Magnitudes of swarmevents have always been higher than the magnitudeof largest events in the preceding background and fol-lowing gap/quiescence epochs. Magnitude of only twoevents, which occurred prior to 1958 within the prepa-ration zone of 1964 swarm, exceeded 6 during the back-ground phase whereas it is only one event of 1975during the quiescence phase. It is to note that most ofthe events in the background epoch have magnitudesaround 5 and slightly above it whereas it is 5–5.5 dur-
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 281
ing quiescence epoch. On the other hand, more than50% of swarm events have magnitudes more than 5.5(Fig. 12(c)). This information suggests that the swarmevents have higher magnitude levels as compared tothe events in the preceding background and followingquiescence epochs.
7.1. Possibility of a future earthquake in thepreparatory zone of Arakan Yoma fold belt
The foregoing discussions suggest that the epoch ofswarm sequence during 1963–1964 in Arakan Yomafold belt was preceded and followed by well definedextremely low levels of anomalous seismicity as back-ground and quiescence phases, respectively, in the de-lineated preparatory zone. Further, this zone had beenquiet for a long duration of over 25 years since the onsetof swarm sequence in 1963 (Gupta and Singh, 1986).It is noteworthy that, except 1975 (mb 6.5) earthquake,no other earthquake comparable with the largest earth-quake in the swarm sequence had occurred till July1988 in the delineated zone, which would have influ-enced the existing stress regime. In view of this, the pat-tern of swarm/anomalous seismicity in Arakan Yomafold belt during 1963–1964 may be considered as aseismic forerunner for an impending large earthquakein the delineated preparation zone.
In the present case,Mp(mb) = 6.6 was estimatedfor the 1963–1964 swarm sequence. On substitutingMp = 6.6 in Eq. (3) derived earlier for the region; aM ake.C ag-nc1 ra-t )i ar int s-s ed bys pec-t iedf elt,t uakew theo gust1 do riodf e de-
lineated preparatory zone. It is to be noted that the Eqs.(3)–(6)are established using magnitudes on local scalewhich seems to hold reasonable in the present case alsosince both the magnitude scales (local as well as bodywave) at∼6.6 are found to be the same (Richter, 1958).
It is well known that none of the geological sci-ences are complete from prediction point of view intotality even if many such cases are known for a regionor from other regions with similar geological and tec-tonic set up. Any geological prediction is influencedby many factors and mostly governed by earth’s inter-nal processes about which we have very limited under-standing. Considering these facts, we have very limitedcontrol over the problem of predicting an earthquakein terms of space, time and magnitude—if it occurs;it would be 100% success, if not 100% failure. Dataon many such cases, if available, may help to the ex-tent of minimizing the uncertainty in the space, time,magnitude and focal depth only.
8. Discussions and conclusions
It has already been established that mediumto large earthquakes precede epochs of abnormalseismicity patterns as background/normal seismicity,swarm/anomalous seismicity, quiescence and fore-shocks/mainshock/aftershocks (Kelleher and Saving,1977; Evison, 1977; Mogi, 1977; Kanamori, 1981;Singh et al., 1982; Singh and Singh, 1984; Ohnaka,1 t al.,1 uss ringo ofm astI izedf ma-j df 2,s thee n-s ts forM edf andd ce.T ob-s udesb mpa-
m ∼ 7.6 was estimated for an impending earthquonsidering the inherent level of error in the mitude estimation,Gupta and Singh (1986)had indi-ated that an earthquake withM 8± 0.5 (focal depth00± 40 km) would occur in the delineated prepa
ion zone (21◦–25◦N latitudes and 93–96◦E longitudesn which probable epicenter was shown as open sthe central part of it (Fig. 12(a)). Further, the great Aam earthquakes of 1897 and 1950 were precedimilar swarm activity some 23 and 27 years, resively, prior to their occurrence. If this fact were applor the present swarm activity in Arakan Yoma fold bhe precursory gap period for the impending earthqould lie between 23 and 27 years measured fromnset of swarm sequence in 1963. In fact, the 6 Au988 mainshock (M= 7.5; focal depth = 115 km) haccurred after about 25 years of precursory gap pe
rom the onset of 1963–1964 swarm sequence in th
984; Gupta and Singh, 1986, 1989; Shanker e995). Identification of precursory swarm/anomaloeismic activity requires spatial and temporal clustef events in a region, which is function of magnitudeainshock. In view of this, seismicity data of northe
ndia region from 1860 to 1985 have been scrutinor understanding anomalous seismicity precedingor earthquakes ofM≥ 71
2 during 1860 to 1962; anor mb ≥ 6.1 from 1963 to 1988. During 1860 to 196warm activity/anomalous seismicity preceding allarthquakes withM≥ 71
2 have been investigated coidering: the entire events in the database; even≥ 6 and≥6.5. This condition could not be appli
or 1897 great earthquake due to very poor locationetection capabilities prevailed prior to its occurrenhe trend of anomalous seismicity patterns areerved to be the same for all the three cut off magnitut the last two schemes have yielded stable and co
282 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
rable results. About 80% of major earthquakes are pre-ceded by abnormal seismicity. The swarm/anomalousseismicity were initiated about 23 and 27 years prior toboth the great earthquakes of 1897 and 1950 ofM 8.7,respectively, while it is 11–17 years forM 71
2 and 8earthquakes. In comparison, the Japanese earthquakesfor M 6.6–7.9 have longer precursory gap period rang-ing from 11 to 40 years (Evison, 1982). Ohnaka (1984)has estimated 27 yearsTp for 1923 Kanto earthquakeof M 7.9.Sekiya (1977)has estimated precursory du-ration of 10 years and 4 months forM 6.9; and 19 yearsand 3 months forM 7.3 earthquakes of Fukui area of28 June 1948 based on anomalous seismic activity.
Most of the earthquakes ofmb ≥ 6.1 since 1963 innortheast India region were observed to be preceded byanomalous seismic pattern. In these cases, the precur-sory gap period is observed to vary from as low as 440days to 1768 days formb ≥ 5.6; and 9169 days forM7.5of 6 August 1988. In this direction, a swarm/anomalousseismicity sequence was identified during 1963–1965in the Arakan Yoma fold belt in an elliptical area boundby 21◦–25◦N latitude and 93◦–96◦E longitude whereno major shock had occurred till 1986. On consideringall the events formb ≥ 5, the 1963–1965 swarm se-quence was associated with considerable annual earth-quake frequency over five events as compared to about3 and 2 events/year during the preceding backgroundand following gap/quiescence epochs, respectively. Sit-uation improves and becomes more appealing on con-sidering events formb ≥ 5.5 which helped to minimizes rgedt ncee h (28S bout1 encya 0 to2 ence( vely.F akef poch( und( tem-bd ctu-a nd,a spec-t es.I and
seismicity gap epochs are observed to be very low ofmore or less same magnitude. It is clear that the ellip-tical area, bound by 21◦–25◦N latitudes and 93◦–96◦Elongitudes which encloses all the swarm events, is ex-periencing a seismicity quiescence in both space andtime domains since July 1965, and hence may be con-sidered a probable zone for an impending large/greatearthquakes. UsingMp 6.6 for 1963–1965 swarm se-quence in the earlier established statistical relations formainshocks during 1897–1962, an earthquake of mag-nitudeM 8± 1/2 with focal depth 100± 40 km waspredicted in the delineated region in 1986 (Gupta andSingh, 1986). This earthquake may occur at any timefrom 1986 till 1990 since already 23 years are passedfrom the onset of swarm in 1963 in the region. Theupper limit of this forecast is arrived at 1990 since the1950 great Assam earthquake was preceded by similarswarm sequence some 27 years prior to its occurrence.
8.1. Would it have been possible to predict the 6August 1988 earthquake in the preparation zone?
At the present stage of research, any prediction ofan individual earthquake is a test of the validity of ahypothesized casual link between some kind of pre-cursor(s) and the focal parameters of a forthcomingearthquake. The 6 August 1988 Earthquake foci lie ina well confined seismic volume of Arakan Yoma foldbelt of the northeast India region with the estimates(date: 6 August 1988; origin time: 00 h 36 min 26.9 sG ter:2S hise thirdm ni leds de-l etersf -i ocald arma ear-i ock.T rela-t ob-s rn ofa es inn f the
pace-time windows for swarm sequence and enlahe preceding background and following quiescepochs. In this case, the swarm sequence epoceptember 1963 to June 1965) was marked by a2–15-fold increase in the annual earthquake frequs compared to the preceding background (1957 September 1963) and the following gap/quiescJuly 1965 to September 1985) epochs, respectiurther, about 75-fold increase in annual earthqu
requency is observed during swarm sequence eJanuary to July 1964) over the epochs of backgro1950–1963) and quiescence (August 1964 to Seper 1985) on considering events formb ≥ 6. It is evi-ent that seismicity rates from 1950 to 1985 had fluted in the order of low-high-low during backgrounomalous seismicity and quiescence epochs, re
ively, with respect to all the three cutoff magnitudn addition, the seismicity rates during background
CT; focal depth: 115 km; magnitude: 7.5; epicen5.116◦ N, 95.171◦ E (NEIS, USA data—Gupta andingh, 1989), has proved that prediction was true. Tarthquake had occurred near to the epicenter ofajor swarm event of 12 July 1964 (mb 6.7) as show
n Fig. 12(a) (epicenter of mainshock is shown as filtar) and located at its northern extremity in theineated preparation zone. The estimates of paramor this earthquake fall well within the predicted limts of parameters in space, time, magnitude and fepth and it indicate that anomalous seismicity/swctivity observed during 1963–1964 had direct b
ng on the occurrence parameters of the mainshhis precursory time gap seems to have definite
ion with the mainshock magnitudes as has beenerved in other precursory phenomena. The pattenomalous seismicity preceding major earthquakortheast India region can be regarded as one o
H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284 283
potential seismic precursors. Database constraints havebeen the main hindrance to search this precursor prior tosmaller earthquakes, which, otherwise, certainly wouldhave provided additional information on its existence.The entire exercise shows that anomalous seismicitypreceding major shocks is a real seismic pattern forthe northeast India region and can be employed forlong-range earthquake prediction when better qualityseismological datasets covering a wide range of mag-nitudes are available.
Acknowledgements
The authors are grateful to the anonymous review-ers for critically examining the manuscript and offer-ing very useful comments and suggestions, and to theEditor Professor Brian L.N. Kennett, for his effortsthroughout publication. The first author is thankful tothe Director, CESS for according permission to publishthis paper. Second author is indebted to Professor andHead, Department of Earthquake Engineering, IndianInstitute of Technology Roorkee, Roorkee for provid-ing excellent computational facilities.
References
Benioff, H., 1951. Global strain accumulation and release as revealed
B gy.
B ette,cept.
B icity9.
B smic. Res.
C tonics81.
C cs for
D torySoc.
D vity.
E Soc.
Evison, F.F., 1977. Fluctuations of seismicity before major earth-quakes. Nature (Lond.) 226, 710–712.
Gaur, V.K., Chouhan, R.K.S., 1968. Quantitative measures of seis-micity applied to Indian regions. Bull. Indian Soc. EarthquakeTech. 5 (1–2), 63–78.
Goswami, H.C., Sarmah, S.K., 1982. Probabilistic earthquake ex-pectancy in northeast Indian region. Bull. Seism. Soc. Am. 72,999–1009.
Gupta, H.K., Rastogi, B.K., 1972. Earthquakemb versusMs rela-tions and source multiplicity. Geophys. J. R. Astron. Soc. 28, 65–89.
Gupta, H.K., Singh, V.P., 1980. Teleseismic P-wave residual inves-tigations at Shillong, India. Tectonophysics 66, T19–T27.
Gupta, H.K., 1985. Cachar earthquake of December 31, 1984—is ita signal for the beginning of seismic activity? J. Geol. Soc. India26, 145–147.
Gupta, H.K., Rajendran, K., Singh, H.N., 1986. Seismicity of North-East India region. Part I. The database. J. Geol. Soc. India 28,345–365.
Gupta, H.K., Singh, H.N., 1986. Seismicity of northeast India region.Part II. Earthquake swarms precursory to moderate magnitude togreat earthquakes. J. Geol. Soc. India 28, 367–406.
Gupta, H.K., Singh, H.N., 1989. Earthquake swarms precursory tomoderate to great earthquakes in northeast India region. Tectono-physcis 167, 285–298.
Habermann, R.E., 1981. Precursory seismicity patterns: stalking themature seismic gap in earthquake prediction—an internationalreview. Maurice Ewing Series 4, AGU Publication, pp. 29–42.
Habermann, R.E., Wyss, M., 1984. Background seismicity rates andprecursory seismic quiescence: imperial Valley, California. Bull.Seismol. Soc. Am. 74 (5), 1743–1755.
Hill, D.P., Bailey, R.A., Ryall, A.S. (Eds.), 1984. In: Proceedingsof the Workshop XIX: Active Tectonic and Magmatic Processes
gical
K ofnda75–
K fore.W.,U,
K n-hys.
K arges. 80,
K city
K turalf In-En-
by great earthquakes. Bull. Geol. Soc. Am. 62, 331–338.ullen, K.E., 1965. An Introduction to the Theory of Seismolo
Cambridge University Press.owman, D.D., Ouillon, G., Sammis, C.G., Sornette, A., Sorn
D., 1998. An observational test of the critical earthquake conJ. Geophys. Res. 103, 24359–24372.
owman, D.D., King, C.P., 2001. Stress transfer and seismchanges before large earthquakes. Earth Planet. Sci. 0, 1–
ufe, C.G., Varnes, D.J., 1993. Predictive modelling of the seicycle of the greater San Francisco bay region. J. Geophys98, 9871–9883.
handra, U., 1975. Seismicity, earthquake mechanisms and tecof Burma, 20◦N–28◦N. Geophys. J. R. Astron. Soc. 40, 367–3
esikachar, S.K., 1974. A review of tectonic and geological hisof Eastern India in terms of plate-tectonic theory. J. Geol.India 15, 137–149.
utta, T.K., 1964. Seismicity of Assam-zone of tectonic actiBull. Nat. Geophy. Res. Inst. 2, 152–163.
vans, P., 1964. The tectonic framework of Assam. J. Geol.India 5, 80–96.
Beneath Long Valley Caldera. Eastern California, US GeoloSurvey Open File Report, vol. 84, pp. 939–942.
aila, K.L., Narain, H., 1971. A new approach for preparationquantitative seismicity maps as applied to Alpide Belt—SuArc and adjoining areas. Bull. Seismol. Soc. Am. 61, 121291.
anamori, H., 1981. The nature of seismicity patterns belarge earthquakes, in earthquake prediction. In: Simpson, DRichards, P.G. (Eds.). Maurice Ewing Series, vol. IV. AGWashington, DC, pp. 1–19.
eilis-borok, V., Knopoff, L., Allen, C.R., 1980. Long-term premoitory seismic patterns in Tibet and the Himalayas. J. GeopRes. 85, 813–820.
elleher, J., Saving, J., 1977. Distribution of seismicity before lstrike slip and thrust-type earthquakes. J. Geophys. Re260–271.
hattri, K.N., Wyss, M., 1978. Precursory variation of seismirate in the Assam area, India. Geology 6, 685–688.
hazanchi, A.C., Dutta, T.K., 1978. Seismic loads and strucsafety with particular reference to the northeastern region odia. In: Proceedings of the Third Symposium on Earthquakegineering, University of Roorkee, vol. 1, pp. 523–528.
284 H.N. Singh et al. / Physics of the Earth and Planetary Interiors 148 (2005) 261–284
Le Dain, A.Y., Tapponnier, P., Molnar, P., 1984. Active faulting andtectonics of Burma and surrounding regions. J. Geophys. Res.89, 453–472.
Liu, P., Huang, D., Wang, L., Wang, Z., Zheng, D., Feng, H., 1984.Seismicity pattern over the preparatory process of strong earth-quakes. In: Gu, G., Ma, X.-y. (Eds.), Proceedings of the Inter-national Symposium on Continental Seismicity and EarthquakePrediction. Seismological Press, Beijing, pp. 100–110.
Lyakhovsky, V., Ben-Zion, Y., Agnon, A., 2001. Earthquake cycle,fault zones and seismicity patterns in a rheologically layeredlithosphere. J. Geophys. Res. 106, 4103–4120.
Mogi, K., 1969. Some features of recent seismic activity in andaround Japan, activity before and after great earthquakes. Bull.Earthquake Res. Inst., Univ. Tokyo 47, 395–417.
Mogi, K., 1977. An interpretation of the recent tectonic activity inthe IZU-Tokai district. Bull. Earthquake Res. 52, 315–330.
Mogi, K., 1984. Fundamental studies on earthquake prediction. In:Gu, G., Ma, X.-y. (Eds.), Proceedings of the International Sym-posium on Continental Seismicity and Earthquake Prediction.Seismological Press, Beijing, pp. 619–652.
Nandy, D.R., 1980. Tectonic patterns in North-Eastern India. IndianJ. Earth Sciences 7, 3–107.
Newman, W., Gabrielov, A., Durand, T., Phoenix, S.L., Turcotte,D., 1994. An exact renormalization model for earthquakes andmaterial failure. Statics Dynam., Physica D 77, 200–216.
Ohnaka, M., 1984–1985. A sequence of seismic activity in the Kantoarea precursory to the 1923 Kanto earthquakes. Pure Appl. Geo-phys. 122, 848–862.
Ohtake, M., Matumoto, T., Latham, G.V., 1977a. Seismicity gap nearOaxaca, Southern Mexico as a probable precursory to a largeearthquake. Pure Appl. Geophys. 115, 375–386.
Ohtake, M., Matumoto, T., Latham, G.V., 1977b. Temporal changesin seismicity preceding some shallow earthquakes in Mexico andCentral America. Bull. Int. Inst. Seismol. Earthquake Eng. 15,105–123.
O 1897.
P atingeece.om-hing
P lerated44,
P achos,the.
R and
R Am.
Rikitake, T., 1979. Classification of earthquake precursors. Tectono-physics 54, 293–309.
Rikitake, T., 1982. Earthquake Forecasting and Warning. D. ReidelPublishing Company, Japan, p. 402.
Sahimi, M., Arbabi, S., 1996. Scaling laws for fracture of heteroge-neous materials and rock. Phys. Rev. Lett. 77, 3689–3692.
Sekiya, H., 1977. Anomalous seismic activity and earthquake pre-diction. J. Phys. Earth. 25, S85–S93.
Singh, V.P., Singh, H.N., Singh, J., 1982. On the possibilities of pre-monitory swarms for three sequences of Earthquakes of Burma-Szechwan region. Tectonophysics 85, T21–T29.
Singh, V.P., Singh, H.N., 1984. Precursory swarm and mediumsize earthquake occurrences in Pamirs and its adjoining regions.Earthquake Predict. Res. 2, 245–258.
Singh, V.P., Singh, H.N., 1986. Swarm hypothesis for occurrenceof medium size earthquakes. Earthquake Predict. Res. 4, 83–94.
Singh, V.P., Shanker, D., 1993. Flow of Tibetan Plateau and tectonicalong Burmese Arc. Geophys. Trans. 38 (2–3), 135–149.
Singh, V.P., Shanker, D., Hamada, K., 1994. A study of seismicity ofnorth-east India and adjoining areas based on statistical analyses.Curr. Sci. 66 (12), 922–926.
Shanker, D., Sharma, M.L., 1998. Estimation of seismic hazard pa-rameters for the Himalayas and its vicinity from complete datafiles. Pure Appl. Geophys. 152, 267–279.
Shanker, D., Singh, H.N., Singh, V.P., 1995. Anomalous seismic ac-tivity and long-range earthquake prediction in Himachal Pradesh,India. Acta Geod. Geophy. Hung. 30 (2–4), 379–395.
Shanker, D., Singh, B., Singh, V.P., 2000. Earthquake time clusterin North-East India during February to April 1998. Acta Geod.Geophy. Hung. 35 (2), 195–204.
Shanker, D., Kapur, N., Singh, Bhawani, 2002. Thrust-wedge me-chanics and coeval development of normal and reverse faults inthe Himalayas. J. Geol. Soc., Lond. 159, 273–280.
Shanker, D., Kapur, N., Singh, V.P., 2004. Modelling quaternaryuth-
S bser-m 30,
S . Sci.
V ic-orth-
W uies-. Bull.
X X.-ental, Bei-
ldham, T., 1889. Report on the great earthquake of 12 JuneMember Geol. Surv. India 29, 1–349.
apadopoulos, G.A., 1994. Making and post-seismically evaluearthquake predictions based on seismicity patterns in GrIn: Hayakawa, M., Fujinawa (Eds.), Electromagnetic Phenena related to Earthquake prediction. The Scientific PublisCompany, Tokyo, pp. 97–102.
apazachos, C.B., Papazachos, B.C., 2001. Precursory acceBenioff strain in the Aegean area. Annali di Geofisica461–474.
thrusting in Himalayas and its geodynamic constraints in soern Tibet. Acta Geod. Geophys. Hung. 39 (1), 73–87.
rivastava, H.N., Chaudhary, H.M., 1979. Precursory seismic ovations in Himachal Pradesh and Shillong Plateau. Mausa289–296.
uzuki, Z., 1982. Earthquake prediction. Ann. Rev. Earth Plant10, 235–256.
erma, R.K., Mukhopadhyay, M., Ahluwalia, M.S., 1976. Seismity, gravity and tectonic features of northeastern India and nern Burma. Bull. Seismol. Soc. Am. 66, 1683–1694.
yss, M., Habermann, R.E., Helinioer, Ch., 1983. Seismic qcence, stress drops and asperities in the New Hebrides ArcSeismol. Soc. Am. 73, 219–236.
u, S., 1984. A Review of Seismicity Patterns. In: Gu, G., Ma,y. (Eds.), Proceedings, International Symposium on ContinSeismicity and Earthquake Prediction. Seismological Pressjing, pp. 11–42.
