Seismic Clusters and their Characteristics at the Arabian Sea TripleJunction: Supportive Evidences for Plate Margin Deformations
BASAB MUKHOPADHYAY1, MANOJ MUKHOPADHYAY
2 and SUJIT DASGUPTA3
1Geological Survey of India, Central Headquarters, 27, J.L. Nehru Road, Kolkata - 700 016, India2Department of Geology and Geophysics, King Saud University, P.O. Box 2455, Riyadh 11451,
Kingdom of Saudi Arabia3Deputy Director General (Retd.), Geological Survey of India, Central Headquarters, Kolkata
Abstract: The plate margin features defining the Arabian Sea Triple Junction (ASTJ) are: the Aden Ridge (AR), ShebaRidge (SR) with their intervening Alula-Fartak Transform (AFT), Carlsberg Ridge (CR) and Owen Fracture Zone (OFZ).Exact nature of ASTJ is presently debated: whether it is RRF (ridge-ridge-fault) or RRR (ridge-ridge-ridge) type. Arevised seismicity map for ASTJ is given here using data for a period little more than a century. “Point density spatialstatistical criterion” is applied to short-listed 742 earthquakes (mb ≥ 4.3), 10 numbers of spatio-temporal seismic clustersare identified for ASTJ and its arms. Relocated hypocentres help better constraining the cluster identification whereversuch data exist. Seismic clusters actually diagnose the most intense zones of strain accumulation due to far field as wellas the local stress operating at ASTJ. An earthquake swarm emanating from a prominent seismic cluster below SRprovides an opportunity to investigate the pore pressure diffusion process (due to the active source) by means of “r-tplot”. Stress and faulting pattern in the active zones are deduced from 43 CMT solutions. While normal or lateralfaulting is characteristic for these arms, an anomalous thrust earthquake occurs in the triangular ‘Wheatley Deep’deformation zone proximal to ASTJ. The latter appears to have formed due to a shift of the deformational front fromOFZ towards a transform that offsets SR. Though ASTJ is still in the process of evolution, available data favour that thisRRF triple junction may eventually be converted to a more stable RRR type.
Keywords: Revised seismicity map, Foreshock-mainshock-aftershock sequence, Aspect ratio and b-values, Seismicclusters, CMT solutions, r-t plot, RRR plate margin kinematics.
INTRODUCTION
The ASTJ covers an area of 1,20,000 sq km in westernArabian Sea, where, its constituent ridges and transformsdefine the western margin of the Indian plate against theSomalia plate to the southwest and the Arabian plate duewest (Fig.1). The spreading ridges delineate the plate margin,namely; the CR, SR and AR or their connecting transforms- OFZ and AFT. Here our objective is to investigate thepresent kinematics of the ASTJ, so that, the nature of theplate margin below the Arabian Sea can be better understood.This is achieved by adopting the following approach:(a) By preparing a revised seismicity map using the
available earthquake catalogues maintained by ISS, ISCand NEIC (USGS) corresponding to a period little morethan a century (1904-2009); the cut-off magnitudeselected in creating the map is mb ≥ 3.3.
(b) Diagnose the seismic clusters, wherever present,surrounding the ASTJ, out to a radial distance of about
500 km. In absence of any better alternative, this cut-off distance is somewhat subjective, yet it is retainedfor the basic reason of not to lose sight of ASTJ bymoving too far away from it. For identification ofclusters, point density spatial function technique isapplied to the seismic data to constrain the extent of theclusters. Such identified clusters are shown on a mapview. The salient features of these clusters in terms ofseismicity, seismotectonics and local seabedmorphology are also summarised in text.
(c) To investigate the correlation between the major seismicclusters and their underlying crustal sections derivedfrom geophysical modelling. For this, we use both theBathymetric Shaded Relief (BSR) map and the relocatedseismic events using EHB technique (courtesy: Dr. E.R.Engdahl, pers. comm.). Available CMT solutions forearthquakes (www.seismology.harvard.edu) in such welldefined seismic zones are next utilised to draw
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inferences on the prevailing stress and faulting patternunderlying the seismic clusters found across the studyarea.
(d) The role of pore pressure caused by magmatic dykeintrusion from mantle along rift axis to generateearthquake swarms within clusters is investigated by‘r-t plot’. Seismotectonic status in correspondence tocrustal structure for tectonic features proximal to ASTJis also evaluated.
(e) Stress and faulting pattern in the active zones arededuced from 43 CMT solutions. These help studyingthe Neotectonics for ASTJ and its arms. Finally theseresults are used to infer on the evolutionary status forASTJ – between RRF and RRR types.
SEISMIC CLUSTER ANALYSIS
Spatial clusters form by close occurrence of similar pointevents in space; moderate to large magnitude earthquakesoften occur in spatial seismic clusters (McGuire, 2004). Aseismic cluster is suspected in a region if it consists ofmultiple events with magnitude greater than a threshold valueoriginating within an acceptable time period. The earthquakecatalogue used in the present study (source: ISS, ISC andNEIC- USGS) consists of a total of 862 earthquake recordsfor the period of 1904-2009 covering a rather wide range of
magnitude (mb = 3.3 to 7.0) with shallow focal depths.Earthquakes enlisted in the catalogue are plotted onbathymetric DEM with gravity data layer draped on it(Fig.2). It is known that non-uniform status of seismicmonitoring in an area introduce factors of inconsistency andincompleteness in any earthquake catalog, resulting intoinherent uncertainty in the search for ‘long-term earthquakeclustering’ (Kagan and Jackson, 1991). Further, anystatistical treatment for cluster analysis essentially dependson the completeness of the earthquake catalogue (Ansari etal. 2009). For this purpose, suitable magnitude for cataloguecompleteness has been carried out as per the Gutenberg-Richter relationship. Log N – mb curve illustrates that formb 4.3, the curve is smooth and follows a straight-line (insetin Fig. 2), thereby implying that all earthquakes of mb ≥ 4.3in the region were detected and the catalogue is by andlarge complete above this cut-off magnitude. The seismicityplot (Fig.2) exhibits zones where earthquakes are visiblyclusters.
‘Point density’ estimate is applied to constrain the extentof such seismic clusters as it successfully identifies the areasof concentration of data points or vice versa. To calculatethe point density, ‘distance’ between the adjacent earthquakesis measured on the earthquake distribution map (Fig. 2) bya spatial technique called the ‘near analysis’. From the‘distance data’, a mean distance between the earthquake
Fig.1. Generalized plate configuration map in the Arabian Sea and the Triple Junction formed by the Indian, Arabian and Somalia platesat the intersection of the Sheba Ridge and Owen Fracture Zone (after Fournier et al. 2001). Magnetic anomalies and fracturezones are adopted from Royer et al. (2002).
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points is calculated, which is nearly 6 km in the presentcase. The ‘mean distance’ is then taken as the radius of thecircular neighbourhood for point density calculation. Pointdensity is next calculated as the total number of earthquakeepicentral points that fall within the chosen circularneighbourhood with radius of 6 km divided by the area (=π*62) of the neighbourhood as a process of normalisation.The measurement is then carried out on an overlapping gridpattern where the centre of the circle has moved across themap, both along latitude and longitude, by a sliding distanceequivalent to search radius (6 km). The calculated pointdensity value is stored at a grid point placed at the centre ofthe circle. Resulting point values obtained by this slidinggrid process are interpolated to generate a continuous grid.This grid has a mean (m) 0.035 and standard deviation (sd)0.088. The areas with anomalous point density [value > (m+ 2 sd) = 0.211] are marked as zones of spatial clusters.The process identifies 10 such “spatio-temporal seismicclusters” of variable sizes representing both ridge andtransform segments for ASTJ with numbers “A” to “J”(shown by closed dark polygons on Fig.3). The clusters
contain foreshock-mainshock-aftershock (FMA) sequencesor their combination, standalone independent earthquakesas well as the swarm sequences of short temporal duration(Table 1 for the temporal sequences and their salientcharacteristics).
SEISMIC CLUSTERS ASSOCIATED WITH THERIDGES AND TRANSFORMS AT ASTJ
Aden Ridge
The AR separating the Somalia and Arabian plates isassociated with seismic cluster, ‘A’. It trends E-W, ellipticalin shape, of length 113 km and breadth 29 km (aspect ratio,length/breadth = 3.89) (Fig.3). This cluster contains 73earthquakes, with mb ranging from 3.8 to 5.6 and focaldepth varying from 2 to 33.2 km. The b-value calculated bymaximum likelihood method (Aki, 1965) is 1.08. Earthquakesequences within this cluster contain three swarm sequencesof short temporal durations, whose characteristics aresummarised in Table 1. North of the cluster ‘A’, Cenozoicsediments in the Masila basin in mainland Yemen, exhibit
Fig.2. Revised seismicity map of the study area based on events during 1906-2009 (source: ISS, ISC & NEIC/USGS global earthquakecatalogue), superposed shaded bathymetric relief with gravity data. Topographic data are taken from 1-minute global topography(Smith and Sandwell, 2007) and gravity data (Sandwell and Smith, 2009). The distribution suggests highly active neotectonismin plate margins below the Arabian Sea. Frequency magnitude relationships for 862 events of mb ≥ 3.3 suggest that the catalogueis largely complete above magnitude 4.3.
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Table 1. Major characters of seismic clusters for the Arabian Sea Triple Junction and their correspondence to CMT Solutions
Cluster No of Magnitude Depth CMT Length (L) b-value by Type of Characteristics of temporal sequencesEarth- range range Solutions Breadth (B) (km) maximum Segment of earthquakes in Clustersquakes (km) Aspect Ratio (A R) likelihood
= (length/ breath) method(Aki, 1965)
A 73 3.8 - 5.6 2 - 33.2 6 nos (Normal L = 113 km 1.08 Aden Ridge 3 nos of swarm sequencesSolutions on low to B = 29 km (a) 1.11.2003 (11:38:4.5) to 2.11.2003 (15:11:30.10) –moderate dipping A R = 3.89 4 Earthquakes (4.1-4.7mb)fault planes) (b) 3.2. 2008 (8:19:53.98 to 25.02.2008(23:22:21.49 –
B 44 3.9 - 6.0 9.9 – 32.8 6 nos (Strike-slip L = 89 km 0.62 Alula – Fartak Isolated 2 nos of floating events without foreand aftershocksSolutions on steep B = 35 km Transform (a) 21.07.1975 (13:27:44.9) mb 5.9dipping fault planes) A R = 2.54 (b) 15.06.2001 (16:19:7.61) mb 6.0
C 47 3.7 - 5.7 4.1 – 32.2 1 no (Normal L = 77 km 0.72 Sheba Ridge 2 nos of swarm sequencesSolution on low B = 42 km (a) 25.12.1966 (5:42:1) to 25.12.1966 (5:49:58) –dipping fault plane) A R = 1.83 3 Earthquakes (4.5-4.9 mb)
(b) 25.03.2001 (7:14:9.58) to 25.03.2001 (17:6:22.23) –4 Earthquakes (4.8-5.1mb)
2 nos of FMA (Foreshock-Mainshock-Aftershock sequence)(a) 14.05.1982 (14:42:57.23) mb 5.2 as mainshock
with 2 foreshocks in the same day(b) 1.02.2008 (1:32:27.82) mb 5.0 as mainshock with
6 aftershocks
D 46 3.9 –5.3 4.5 – 35 2 nos (Normal L = 122 km 1.08 Sheba Ridge One swarm sequenceSolutions on low to B = 38 km (a) 13.05.2002 (8:24:44.4) to 21.05.2002 (9:41:55.7) –moderate dipping A R = 3.21 9 Earthquakes (3.9 – 5.2 mb)fault planes) One FMA sequence
(a) 27.01.1987 (0:36:27.35) mb 5.0 as main-shockwith 2 foreshocks and 1 aftershock
E 157 3.4 – 5.8 0.5 – 31.6 9 nos (Normal L = 138 km 0.72 Sheba Ridge 4 nos of swarm sequencesSolutions on low to B = 57 km (a) 6.08.1972 (15.37:0.2) to 20.09.1972(20.52:37) –moderate dipping A R = 2.42 5 Earthquakes (4.6-5.0 mb)fault planes) (b) 19.04.1975 (13:45:50.1) to 25.05.1975 (20:30:11.1) –
Cluster No of Magnitude Depth CMT Length (L) b-value by Type of Characteristics of temporal sequencesEarth- range range Solutions Breadth (B) (km) maximum Segment of earthquakes in Clustersquakes (km) Aspect Ratio (A R) likelihood
= (length/ breath) method(Aki, 1965)
3 nos of FMA sequences(a) 13.08.2002 (8:37:22.77) mb 5.8 with 1 foreshocks
and 10 aftershocks(b) 03.03.2008 (18:1:40.75) mb 5.4 with 1 foreshocks
and 3 aftershocks(c) 21.04.2009 (19:45:3.79) mb 5.4 with 1 foreshocks and
3 aftershocks
F 48 3.7 – 5.5 6.2 – 29.1 3 nos (Normal L = 141 km 1.08 Sheba Ridge One swarm sequenceSolutions on low to B = 50 km (a) 02.03.2000 (4:24:18.86) to 24.05.2000 (13.03.5.15) –moderate dipping A R = 2.82 4 Earthquakes (4.3 – 4.7 mb)fault planes) One FMA sequence
(a) 03.03.2007 (18:08:53.29) mb 5.3 with 1 foreshocksand 1 aftershock
G 30 3.9 – 6.5 9 – 35 4 nos (Strike-slip L = 113 km 0.54 Owen Fracture 2 nos of FMA sequencesSolutions on steep B = 26 km Zone (a) 20.04.1980 (2:37:49.30) mb 6.2 with 2 aftershocksdipping fault planes) A R = 4.34 (b) 26.02.1992 (3:45:19.78) mb 6.0 with 2 aftershocks
Isolated 2 nos of floating events without fore and aftershocks
H 31 3.7 – 6.4 7.9 – 34.4 3 nos (Strike-slip L = 99 km 0.62 Owen Fracture One FMA sequenceSolutions on steep B = 36 km Zone (a) 07.07.1986 (16:26:56.61) mb 6.4 with 5 aftershocksdipping fault planes) A R = 2.75
I 32 4.1 – 5.5 3.5 – 32.1 4 nos (Normal L = 105 km 0.72 Carlsberg Ridge One FMA sequenceSolutions on low to B = 51 km (a) 18.02.1989 (13:13:34.08) mb 5.2 with 1 aftershockmoderate dipping A R = 2.05fault planes) Isolated 2 nos of floating events without fore and aftershocks
J 36 4.1 – 5.9 9.1 – 33.1 5 nos (Normal L = 134 km 0.73 Carlsberg Ridge 2 nos of FMA sequencesSolutions on low B = 45 km (a) 14.12.1969 (18:37:9) mb 5.9 with 1 foreshockdipping fault planes) A R = 2.97 (b) 10.08.1983 (02:02:36.18) mb 5.1 with 1 foreshock
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two extensional phases: NNE (N20°E) around 18 Ma andNNW (N160°E) around 16 Ma (Al Kotbah, 1996, 2000;Huchon and Khanbari, 2003) (Fig.3). Similar extensionaldomain has also been identified from the tectonic analysisin the Qamar basin (Huchon and Khanbari, 2003). The Oligo– Miocene stretching of Arabia – Somalia plates caused areactivation of previous structures of these Mesozoic basinswithout influencing the direction of propagation of rifttowards west – southwest from the CR in the Indian Oceaninto the Afar region in east Africa (Huchon and Khanbari,2003). Similar tectonics is operating even today as indicatedby the stress axes obtained from six CMT solutions. CMTsolutions indicate gravity movement along E-W striking lowto moderate dipping fault planes (Table 2). Notice that thesolutions are dominated by high plunging compression axisand NNE-SSW trending sub-horizontal tensional axis. Thus,the movement direction along NNE is unaltered from theMiocene till date.
Sheba Ridge
The SR defines the rift-ridge segment between Somalia,Arabian and Indian plates located to the immediate west of
ASTJ. It has spawned four seismic clusters, “C through F”(Fig. 3). All four clusters are elliptical in shape, with aspectratio varying from 1.83 to 3.21 and b-value between 0.72and 1.08. Seismicity is most intense here compared to theother ridges in the study area. This is reflected by earthquakeoccurrences, varying from numbers 46 to 157, highestoccurrence registered for cluster “E” (Table 1). Theearthquakes are of low to moderate magnitudes with shallowcrustal focal depths. The earthquakes within these clustersshow short duration temporal sequences in the form ofearthquake swarm as well as FMA sequences, characteristicsof which are given in Table 1. The largest swarm sequencewith 56 numbers of earthquakes between April 19 – May25, 1975 that has been registered in cluster “E” —seismically the most active segment of the SR (refer belowfor the “r-t plot” and its analysis for this earthquake swarm).Fault plane solutions of earthquakes within these clustersshow typical normal motion on ESE-WNW oriented low tomoderate dipping fault planes (Table 2). Compression axisplunges high with variable orientations, whereas, tensionalaxis is stable plunging sub-horizontally along NNE-SSWdirection. On either side of SR, the tectonics in onshore and
Fig.3. Seismic clusters, “A through J”, for the study area illustrated on Fig. 2. Crustal sections (black dash lines) taken across the seismicclusters are illustrated together with the relocated hypocenters and Free-air anomalies in their respective areas in Fig. 4. Relocatedhypocenters help better defining the seismic zones. Interpreted CMT solutions belonging to the seismic zones are depicted withusual symbols.
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offshore Oman along the north and passive margins in thesouth along Socotra is worked out (Fournier et al. 2007).They have pointed out that the asymmetry of the conjugatemargins on either side of the gulf is due to presence of ocean-ward dipping normal faults in the Socotra whereas the Omanmargin is dominated by horst and graben structure. Thestructures at the margin of the Gulf and the seismic clusters“C through F” reported here are the product of the rifting ofcontinental lithosphere and top crust; followed by seafloorspreading across SR. The spreading was initiated at 18 Maago in the early Miocene at the eastern part of the Gulf(Leroy et al. 2004) and continued with current full spreadingrate 22 mm/yr in N25°E direction (Fournier et al. 2001).The direction of opening of the Gulf of Aden is thusnearly ~ 40° from orthogonal to its overall N75°E trend(Cochran, 1981; Fournier and Petit, 2006).
Carlsberg Ridge
Two elliptical seismic clusters “J and I” are noticed forthis NW-SE trending ridge-rift segment present betweenSomalia and Indian plates south of ASTJ. The aspect ratiovaries from 2.05 to 2.97 and its calculated b-value is 0.72.The ridge associated seismicity is somewhat lower ascompared to those with the other Clusters; number ofearthquakes constituting the clusters varies from 32 to 36 inthe magnitude range 4.1 - 5.9. The earthquakes are of shallowcrustal depths. Both clusters are characterized by occurrenceof FMA sequences and isolated floating events (Table 1),but earthquake swarms are typically absent here.Earthquakes are produced by gravity motion along NW-SEtrending low to moderate dipping fault planes parallel tothe trend of the ridge segment (Table 2). The extensionalaxis is sub-horizontal, dips in NNE-SSW direction, but thecompression axis plunges rather steeply with variableorientations.
Alula–Far tak Transform
This NNE-SSW trending transform transects the AR andSR, has only one elliptical cluster “B”, with aspect ratio2.54 (Fig. 3). The cluster contains 44 earthquakes in themagnitude range of 3.9 to 6.0 with shallow crustal focaldepths. Its corresponding b-value is 0.62. Isolated floatingevents of moderate magnitude characterise the temporalsequence in the cluster but with typical absence of the swarmsequences. Six fault plane solutions (Table 2) within thiscluster are suggestive of sinistral strike-slip motion alongNNE-SSW oriented steep dipping fault planes. The tensionalaxis is sub-horizontal, oriented along NW-SE direction,whereas, compressional axis is again sub-horizontal alongENE-WSW direction. The cluster B is apparently formed
due to strain accumulation caused by segmentation of theridge axis by AFT, the latter is formed by oblique openningof the Gulf (Manighetti et al. 1997).
Owen Fracture Zone
The OFZ morphology is best described by a ridgecomprising of a steep easterly facing scarp and a gentlerwestern scarp (Edwards et al. 2000) as it forms part of thewestern margin of the Indian plate. This seismically activefracture zone is described as submarine fault scarp systemrunning for over 800 km from Sheba ridge to Makran coastin Pakistan (Fournier et al. 2011). The southern part of thisfracture zone is shown in the Figs.2 and 3. The fracture zoneis defined as series of clearly delineated strike slip faultsegments separated by several releasing and restrainingbends with a strike slip displacement of 10 - 12 km for last3 – 6 Ma (Fournier et al. 2011). This NNE-SSW orientedtransform present between the Arabian and Indian plateshas two elliptical spatial seismic clusters, “G and H”(Fig. 3), with aspect ratio 4.34 and 2.75 respectively. Theb-value in these clusters ranges from 0.54 to 0.62. Thetemporal sequences within these clusters either haveFMA sequences or isolated moderate size earthquake events(Table 1). The CMT solutions show dextral strike slip motionalong steeply dipping fault planes. Here the tensile axis islow plunging ENE-WSW and the compression axis is alsolow plunging NW-SE (Table 2). Focal mechanisms givenearlier suggest right-lateral slip along the active segment ofthe OFZ (also refer Quittmeyer and Kafka, 1984; Gordonand DeMets, 1989). The OFZ represents a plate boundarythat moves 3 – 4 mm/year (Reilinger et al. 2006) implyingthat Arabia is currently moving northward more rapidly thanIndia with respect to Eurasia (Fournier et al. 2008). Thisdifferential motion on either side of OFZ causes the strainaccumulation in its active segments and promotes theclustering of earthquakes.
REVIEW ON CRUSTAL STRUCTURE
The global marine gravity data compiled from Geosatand ERS-1 altimetry by Sandwell and Smith (2009) has beenused in this study. The data have an error limit to 2 - 4 mgalin comparison to the shipboard gravity data. With such alower error limit, this data can be used to interpret mantleanomalies in relation to structure for regional tectonicinterpretation. Free-air anomalies are draped on thebathymetric DEM as base (Figs. 2 and 3) to merely inspecttheir correlation to seabed morphology and underlyingcrustal configuration. The bathymetric DEM is constructedfrom 1-minute global topography data compiled by Smith
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Table 2. Fault plane solution data for earthquake events corresponding to seismic clusters A to J.
Fault plane Auxiliary plane
NO Date H:M:S Lat. Long. Mw T_ T_ N_ N_ P_ P_ Strike Dip Rake Strike Dip Slip Solution Cluster SourcePlunge Azimuth Plunge Azimuth Plunge Azimuth
SEISMIC CLUSTERS AND THEIR CHARACTERISTICS AT THE ARABIAN SEA TRIPLE JUNCTION 139
and Sandwell (2007) (http://topex.ucsd.edu/cgi-bin/get_data.cgi).
The gravity map (Fig. 3) of the area shows that gravityanomalies bear linear relationship with seabed topography.High elevation zone shows positive gravity values and viceversa (refer below). Gravity, bathymetric and seismicsections taken across the clusters “A through J” along thesection lines are shown on Fig.4. Of these; Cluster “A”locates on AR (Fig. 4a); Clusters “C through F” on SR (Fig.4c through d) and clusters “I and J” on CR (Fig. 4i and j).Relocated seismic events using EHB technique (courtesy:Prof. E.R. Engdahl, pers. comm.) from ISC data are plottedbelow the seismic sections. Geophysical sections, soprepared are used to study the relationships between theseismic clusters and the stress orientation, faulting patternand their characteristic crustal configuration. This in turnhelps inferring on the tectonic status for the most intenselyactive seismic zones under the ASTJ and its constituentridges and transforms.
Free-air gravity anomalies together with correspondingbathymetric values and crustal sections underlying thelocations of the seismic clusters are illustrated on Fig. 4.Thickness of oceanic crust underlying the AR varies from4.8 to 8.4 km (Cochran, 1982); thickness variation indifferent parts of the ridge system is due to asymmetricspreading. Further, two-dimensional gravity modellingsuggests thin 5-6 km thick crust in AR (Leroy et al. 2004).In all these sections, the focal depth of relocated seismicityis as deep as 30 km with majority of seismicity is originatingfrom 15 – 20 km depth. If crustal models are valid, most ofseismic activity is therefore of sub-crustal origin. Thesections also indicate a narrow zone of seismicity,approximately 60 – 65 km of horizontal width and extendingfrom 25-30 km vertically. Negative gravity anomalies outlinethe rift segments and positive gravity anomalies flanking it.The CMT solutions along these rifts-ridges are similar. Theseindicate gravity movement along NW-SE dipping conjugatefault planes, dipping, low to moderate on either side. Itindicates a mechanism of repeated collapsing of rift wall bygravity faults during spreading, to generate seismicity alongthese ridge segments. We have suggested earlier a probablemechanism for the related process (Mukhopadhyay et al.2010). Basic features involved here are: crustal thinning,deformation on the top basement and sub-crustal massanomalies, with exhumed mantle material (Leroy et al.2004).
Bathymetric and Free-air anomaly profiles together withtheir underlying crustal sections for seismic clusters relatingto the two transforms AFT and OFZ are also provided onFig. 4. Here, the cluster B is noticed on AFT and clusters
“G and H” are noticed on OFZ. Wide-angle seismic datademonstrate a typical oceanic crust of thickness ~6 km,present to the northwest of OFZ. But there is an abruptchange both in crustal thickness and velocity structure atthe northwestern edge of the Dalrymple trough, and thetrough itself is underlain by 12-km-thick crust interpretedas thinned continental crust (Edwards et al. 2008). Similarly,the observed thickness of the oceanic crust in the LaxmiRidge, NW of the study area, is 11.7 km (Naini and Talwani,1983). It is thus apparent that the crustal thickness underlyingthe transforms is slightly more than that for the rift-ridgeaxis. Distinct flat and elevated bathymetry can be seen atthe location of the clusters “G and H” on OFZ (Fig. 4 g andh). The fault planes in the AFT are NNE-SSW trending withsteep dip showing sinistral slip motion. Similarly, the faultplanes in the OFZ are NNE-SSW to NE-SW trending dippingsteeply showing dextral to pure strike slip motion.
b–VALUE IN THE CLUSTERS
The earthquake size distribution based on Gutenberg andRichter frequency-magnitude distribution (FMD) followsthe well-known power law, designated as b-value. Theparameter ‘b’ depends on the effective stress regime andtectonic character of the region (Hatzidimitriou et al. 1985;Tsapanos, 1990). Decreasing of ‘b’ value within seismogenicvolume correlates the increasing effective stress levels priorto a major shock (Kanamori, 1981) or increase in appliedshear stress / effective stress (Urbancic et al. 1992). Smith(1986) observed that the Cape Campbell earthquake ofJanuary 1977 was preceded by and located close to a highb-value. In one of our earlier study in search of precursor ofgreat 26th December 2004 Sumatra earthquake (Mw = 9.3)in Sumatra region, a temporal low in b-value (0.68-0.8) isreported prior to the mega-events of 2.11.2002 (M = 7.6)and 26.12.2004 (Mw 9.3) (Dasgupta et al. 2007) whichcorroborated well with the observed and experimental bvalue data of Schorlemmer et al. (2005). In the present case,the b-value calculated by maximum likelihood method (Aki,1965) shows that the clusters B, G and H located on AFTand OFZ are actually associated with very low b-value 0.54to 0.62. Whereas, in the ridge-rift segments, low b - value(~ 0.72) is characteristics of seismic clusters “C and E”belonging to SR as well as the clusters “I and J” for CR.Isolated moderate b values of 1.08 are observed in clusters“A, D and F” belonging to AR and SR segments (Table 1).The estimated b-values lead us to speculate that the clusterswith very low b-values like “B, C, E, G, H, I and J” in thetransform and ridge segments are probably vulnerable toproduce moderate size seismicity in future.
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Fig.4. Crustal sections underlying the geophysical sections crossing the seismic clusters “A through J” (Fig.3 for section line locations).
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narrow zone of volcanic activity, which delineates a volcanicridge, marks the volcanic nature of this axis of spreadingwhich is relatively straight, shallow and continuous, withdepths between 3000 and 3800 m (Fournier et al. 2008).Though similar exercise is necessary for all the swarmsequences registered in Table 1, but due to paucity ofearthquake data in those swarms it is not possible to constructthe r-t plots. However, it is conjectured that similarmechanism as described above is responsible for generatingsuch swarms.
ON THE EVOLUTIONAR Y CHARACTERISTICSOF ASTJ
The stability of the triple junction depends mostly onthe evolutionary behaviour of the plate boundaries in a localas well as in a regional scale depending on plate motion(Fournier et al. 2008). Though Ridge-Ridge-Ridge (RRR)triple junctions are most common in the earth, the Ridge-Ridge-Fault (RRF) triple junction where one transform faultmeets two spreading ridges though rare but present in threeoccasions: the Azores triple junction in Atlantic Ocean, theJuan – Fernandez triple junction in Pacific Ocean and thepresently studied area of Aden - Owen – Carlsberg triplejunction in the Indian Ocean (Searle, 1980; Larson et al.1992; Gordon and DeMets, 1989). Kinematically, the RRFtriple junctions are unstable and evolve into RFF triplejunction. The RFF type triple junction is absent here becauseone of the transform faults has evolved into a ridge system(Fournier et al. 2008). This can be viewed from a plot of theearthquake and CMT data, active structures, correspondingbathymetric and Free-air gravity anomalies (Fig.6). Inwestern part of the map, the AR is offset over 200 km by
ROLE OF PORE-PRESSURE PERTURBATION AND1975 SWARM CORRESPONDING TO CLUSTER ‘E’
There are several small swarm sequences in the ridgesegments, of which, the largest swarm sequence with 56numbers of earthquakes between 19.04.1975 and 25.05.1975was registered in cluster “E” (Table 1). The relationshipbetween the swarm sequence and its causative pore pressureperturbation is then studied by a process called the ‘r-tmethod’ what is based on the diffusion equation for a pointpore pressure source in a homogeneous and isotropic fluidsaturated poro-elastic medium having specific hydraulicproperties. Shapiro et al. (1997, 2002) predict that fluid flowmay trigger an earthquake at a location ‘r’, at any time ‘t’,after the pressure perturbation. The distance ‘r’ of thepropagating pore pressure front from the injection point (thatacts as a source with t = 0), with r = √(4πDt), is estimated asa function of time (t). The equation actually defines theenveloping parabola in the r-t plot with variable hydraulicdiffusivity (D) values, where, seismicity points should liebelow the modelled parabolic curve. The ‘D’ is scalar; whosevalue depends on permeability (k), uniaxial specific storagecoefficient (S) and viscosity of the fluid (m) by the equationD = k/(mS) (Kuempel, 1991; Wang, 2000). On the contrary,if the earthquake triggering occurs shortly after the porepressure perturbations (Noir et al. 1997), we should observea narrow cluster of seismicity along the line of the modelledparabola in the r-t plot with variable scalar D values. TheD-value in the earth’s crust usually ranges between 0.1 and10m2/s (Shapiro et al. 1999) but can reach up to 90 m2/s(Antonioli et al. 2005). In this scheme the r-t plot basicallyrepresents spatial distance ‘r’ of an individual event fromthe injection point as a function of time ‘t’. Anunambiguously defined injection point source, whichcorresponds to the origin of the graph at time 0, is thereforea pre-requisite for calculation purpose (Shapiro et al. 1997).The injection point from which the fluid diffusion starts inthe case of 1975 swarm is identified as the epicentral locationof the first earthquake (date 19.04.1975, 13 Hr, 45 Min,50.1 Sec., latitude 14.42 and longitude 56.51) from wherethe fluid actually begins to propagate (for further explanationon injection point, refer Shapiro et al. 1997, 2002, 2003).The r-t plot for D values of 8 m2/s for earthquakes of 1975swarm follows the parabolic equations (Fig.5), where, theswarm relates to the diffusion of pore pressure perturbationsin a poro-elastic fluid saturated medium. The pore-pressureperturbations that generate earthquake swarm in divergentpull-apart basins are initiated by magmatic dyke intrusionfrom mantle source (also refer Mukhopadhyay et al. 2010).Similar condition seemingly exists in this ridge segment. A
Fig.5. The “r-t plot” for the 1975 swarm sequence correspondingto cluster ‘E’ found for the Sheba Ridge, with modelledparabolic envelope of diffusivity (D = 8 m2/s). Refer textfor discussion.
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AFT (Radhakrishna and Searle, 2006). In central part, minorSocotra Transform (ST) offsets SR. In the central portionof the map, the arcuate orientation of SR between transformsAFT, ST and OFZ is also delineated by epicentral pattern.The OFZ offsets CR by 330 km and connects to the SR(Matthews, 1963, 1966; Laughton, 1966; Matthews et al.1967; Laughton et al. 1970). Some representative CMT‘beach ball diagram’ is placed to better represent the crustalmotion (strike-slip mechanism for the transforms and normalmechanism for the ridges) in different parts of this activeplate boundary. The rectilinear E-W disposition of
earthquake epicentres just north of the eastern segment ofthe SR, with prominent moderate to small crustal earthquakebetween, forms a triangular zone represented by present dayactive deformation. The beach ball diagrams for fouravailable CMT solutions within this zone are analysed: twostrike-slip mechanisms (solution numbers 1 and 2, Fig. 6),one with thrust type of solution (solution number 3, Fig. 6)while the remaining is a normal mechanism (solution number4, Fig.6) occurred at the western end of the extensionalBeautemps – Beaupre basin where OFZ ends. The strike-slip mechanism earthquakes occurred in the year 1981 and
Fig.6. The RRF triple junction in the western end of Indian plate with active deformational structures surrounding it,represented by contrasting CMT solutions. The triple junction is formed by Sheba ridge, Carlsberg ridge on twosides with OFZ as the connector. A new active triangular deformational zone between the ridges and transform is inthe process of evolution where the deformational front is shifted from OFZ to another transform NW of the WheatlyDeep. Small white circles with black outlines are the earthquake points. Large white circles denote Strike-slip, box –Normal and star – thrust events. Other features are as in Fig.3. AFT: Alula-Fartak Transform, ST: Socotra Transform.
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1985, with NNE – SSW striking steep dipping fault planesand sub-horizontal tension axis along NNW-SSE. These lendsupport to infer sinistral motion, possibly along a transformfault, that joins the SR. Along the inferred fault line, a thrustearthquake (solution number 3 that occurred on 22.11.2003)with N - S fault plane and sub horizontal (15°) E-W orientedcompression axis has taken place. This N-S planecorresponds to one of the transform that offsets SR, andalso acts as boundary of an active deformation zone. It isalso evident that east of this active front the OFZ betweenthe Wheatly Deep and eastern end of the Beautemps –Beaupre basin (with occurrence of a normal mechanism forthe event number 4 in 1990 having E-W fault plane) isseismically inactive presently. In fact, the presence of thisactive deformational front is also elaborated (Fournier etal. 2010). They also concluded that the evolution of the triplejunction was marked by a change of geometry of the Arabia-India plate boundary around 10 Ma and the formation ofthe Beautemps-Beaupré Basin. A small part of the Arabianplate was then transferred to the Indian plate. This smallpart of the plate is the area between active deformation frontand aseismic OFZ scarp zone. This change of geometrywas coeval with a regional kinematic reorganizationcorresponding to the onset of intraplate deformation in theIndia-Australia plate and a change of kinematics along theSR, CR, and southern Central Indian ridges (Fournier et al.2010). It is interesting to note that seismic cluster ‘E’ nearthis active deformational front is the most active cluster,with occurrence of, repeated FMA sequence and earthquakeswarms. Though purely speculative at the moment, but it islikely that this active deformation front may propagatefurther to the west – if the present activity continues. Thereis also a possibility that this transform (due to oblique platemotion and rotation) may be exposed to the tensile domainoblique to it and will form a separate ridge segment. Ittherefore remains conjectural that the ASTJ, what is presentlydefined as RRF triple junction, will continue in its presenttectonic form, or it will eventually be converted to a morestable RRR triple junction.
RESULTS AND DISCUSSION
It is apparent from the results of cluster analysis thatclusters on the ridges produce more number of earthquakescompared to their counterparts on the transforms. It is ofinterest to note that clusters on the ridges are dominated bynormal events on low to moderate dipping fault planesstriking along the ridge axis, whereas, the clusters ontransforms are characterized by strike-slip events on steepdipping fault planes. It summarises that transforms are steep
dipping mantle penetrating fault planes whereas faults orfracture systems in ridges are mainly shallow angle shearfractures probably produced by penetration of mantle dykesbeneath the active rifts. The clustered earthquakes on theridge segments generate low magnitude swarm sequencesof small durations and isolated FMA sequences, whereas,earthquakes in the transforms produce moderate size isolatedfloating earthquake events or FMA sequences. The swarmsare the product of pore-pressure perturbation initiated bymagmatic fluids generated from magmatic dyke intrusionas indicated by “r-t plot”. The transforms however did notproduce any swarm sequence as such. This indicates thatstrain accumulation is more in the transforms by probablestrike-slip mechanism compared to the ridge segments where strain release is due to strong decoupling and aseismiccreep.
The geometric dispositions of the rift-ridges compare tothe transforms (Fig. 2) indicate that SR segment is arcuatein shape. This arcuate nature of SR is due to the motionalong two strike slip faults with contrasting slip vectors, AFTon the western end with sinistral motion and OFZ withdextral motion at the eastern end. This configuration alsoindicates that in the region circumventing the ASTJ, theSomalia plate is moving faster northerly in comparison toArabian plate. The junction between Somalia and Arabianplate is highly active and represented by thick zone ofseismicity. The OFZ represents segments of both passiveand active seismic zone. The junction between Arabianand Indian plates is highly seismic and also representedby seismic clusters “G and H”. The junction betweenthe Arabian and Indian plates along this fracture zonebeyond 12º N latitude is passive. This is probably an outcomeof passive tectonic process where the lithospheric slab isbeing dragged northward through the surroundinglithosphere. The effect of dragging is visible at the easternend of the Error and Sharbithat ridge in contact withOFZ (Fig. 1). A total displacement of 10-12 km alongOFZ for the past 3-6 Ma is also indicated by Fournier et al.(2011).
Free-air anomalies in the study area exhibit goodcorrespondence to bathymetric relief. The rift zones show abroad U-shaped bathymetric profile indicative of highspreading rate along this constructive margin. The lowgravity anomalies in the rift zone proper and higher gravityanomalies on the ridge flanks are quite characteristic. Thehigh heat flow and extreme fractured nature of the crustalong the rift segment contributes elevated pore pressurezones due to underlying magmatic fluid activity from mantlesource. The time bound pore pressure perturbation hascaused several swarms of small temporal duration including
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a large swarm in cluster “E”. On the other hand, the ridgesegments on either side of the rift is made up of fresh un-fractured basalts, the pore-pressure effect within this solidfine grained quickly chilled rock may be considerably lessor altogether absent.
Majority of these clusters seem to be related to dykeintrusions, propagation, pore fluid fluctuations, hydrothermalactivity etc. inferred from the “r-t plot” (Fig. 5). These dykeintrusions interact with the FMA producing more prolificshorter duration seismic sequences (also refer Simão et al.2010). These sequences are already detailed in Table 1.Higher level of ridge (or near-ridge) seismicity seen heremay be the result of modification or enhancement of themore typical near-ridge thermoelastic stress state associatedwith plate cooling, or by active secondary convectionbeneath the young lithosphere of the Indian plate (Bergmanand Solomon, 1984). Generally, the low shear velocitiesassociated with the oceanic spreading centre indicate partialmelting that triggers dynamic upwelling driven by eitherthe buoyancy of retained melt or by the reduced density ofdepleted mantle (Wang et al. 2009). The anisotropy in theupwelling mantle in this zone is due to the preferentialalignment of the fast axes of olivine crystals in the directionof mantle flow and also to generate preferential alignmentof cracks that feed melt towards the spreading axis (Kendall,1994). Hydrothermal activity associated with asymmetricalaccretion in the ridge segment as in the present case showshigh levels of near-continuous hydroacoustically andrecorded teleseismic events (Escartín et al. 2008). The bandof seismicity above the magma chamber suggests thathydrothermal circulation and generation of cracks may bestrongly aligned along the ridge axis and hydrothermal cellsare oriented across-axis (Tolstoy et al. 2008). Furthermore,long-term data from several spreading ridge sites have shownthat microearthquakes at seafloor hydrothermal systems arestrongly tidally influenced, implying a seismic pulsing ofthe seafloor and the hydrothermal flow moving through it(Tolstoy, 2008). This stress change results into generationof preferential alignment of cracks. Failure of this cracksresults into teleseismically - recorded seismicity alongridge axis.
CONCLUSIONS
The details of kinematics for ASTJ formed between theIndian, Arabian and Somalia plates is already well known,though, its exact nature is debated. Here we have analyseda revised seismicity to identify ten numbers of seismicclusters distributed both in rift-ridge and transform systemssurrounding ASTJ. This forms the most active segmentsacting as prominent earthquake source zones of strainaccumulation and repeated subsequent release in the region.
Stress and faulting pattern, derived through a number ofCMT solutions, are characteristic for the spreading ridgesand transforms. The spreading ridge defines predominantlynormal earthquakes whereas the transforms produce strike-slip mechanism events. Majority of the earthquakesoriginating in ASTJ are entirely of mantle origin; theearthquakes derived from oceanic crust are rare. Theseearthquakes are thus generated by changes in thermo-elasticstress due to dyke intrusions, pore fluid fluctuations,hydrothermal activity from underlying mantle flow alongridge axis.
Individual clusters exhibit meaningful relationships withthe seabed morphology, and to a lesser extent, with the deepgravity anomalies. The elevated pore pressure zones are dueto underlying magmatic fluid activity from mantle source.The relationship between dyking event, pore-pressureperturbations and swarm generation are established.
ASTJ is still in the process of tectonic evolution. Likeother triple junctions, the ASTJ is also characterised bytensional domain with earthquakes having normal or lateralfaulting sources. Compressional earthquakes are rare. Theanomalous thrust earthquake occurs in the triangulardeformation zone proximal to ‘Wheatley Deep’ is such anexception. The active deformation in this triangular zonethat is formed due to a shift of deformational front fromOFZ towards a transform that offsets SR.
Acknowledgements: One of the authors (ManojMukhopadhyay) acknowledges that this project wassupported by King Saud University, Deanship of Scientific
Research, College of Science Research Center.
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(Received: 31 January 2011; Revised form accepted: 3 March 2011)
