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ORI GIN AL PA PER
Characteristics of near-source ground motionsfrom the 2012 Varzaghan–Ahar double earthquakes,Northwest of Iran
Saman Yaghmaei-Sabegh
Received: 11 March 2013 / Accepted: 10 September 2013 / Published online: 21 September 2013� Springer Science+Business Media Dordrecht 2013
Abstract Wavelet-based analyses were used in this study to interpret the near-source
ground-motion characteristics of records obtained during the 2012 Varzaghan–Ahar double
earthquakes in the East-Azerbaijan province of Iran. For this purpose, the large pulse of
ground motions was extracted by applying continuous wavelet transforms, and then, the
residual motions were achieved by subtraction of extracted pulse from the original
motions. Analysis of ground motions illustrated that the records obtained at Varzaghan
station could be classified as pulse-like motions with pulse index around 0.9. As well, the
acceleration response spectra for horizontal directions were compared with the design
response spectrum of Iranian seismic design code (Standard No. 2800). The comparisons
showed a relatively large amplification in spectral values due to near-source pulse effects.
Keywords Pulse-like ground motions � Wavelet transform � Double
earthquakes � Iran
1 Introduction
On August 11, 2012, northwest of Iran—near the cities of Ahar and Varzaghan—was
struck by relatively moderate double earthquakes as an effect of strike-slip faulting
mechanism in the crust of the Eurasia plate. The earthquake killed 327 people, claimed
more than 3,000 injuries and left more than 30,000 homeless (Razzaghi and Ghafory-
Ashtiany 2012).
The main shock of first event with Mw 6.4 (USGS) at 16:53 local time was recorded by
57 stations. Only 11 min after first event, the second event occurred with Mw 6.3 (USGS),
suffered heavy level of damages mainly in Varzaghan and nearby villages. More number of
stations has been triggered in the study region (73 stations) and higher values of peak
S. Yaghmaei-Sabegh (&)Department of Civil Engineering, University of Tabriz, Tabriz, Irane-mail: [email protected]
123
Nat Hazards (2014) 70:1077–1097DOI 10.1007/s11069-013-0862-0
ground acceleration were observed in the second event. The maximum peak ground
acceleration was recorded by Varzaghan station of about 532 cm/s/s from the second
event.
Different estimates of the main shocks’ epicenters have been made by Institute of
Geophysics at Tehran University (IGTU), International Institute of Earthquake Engi-
neering and Seismology (IEEE), Building and Housing Research Center (BHRC) and
National Earthquake Information Center (NEIC). Map of triggered stations in first and
second earthquake along with the location of epicenter has been shown in Figs. 1 and 2.
The focal depths of the first and second events were reported as about 10 (IGTU, NEIC)
and 14–15 km (IEESS), respectively. The focal mechanisms of the both shocks are con-
sistent with right-lateral strike-slip faulting on E–W trending fault parallel to aftershock
sequence and south Ahar fault (Razzaghi and Ghafory-Ashtiany 2012). However, the
second main shock includes an extensive reverse component (CMT solution, Harvard
University; GCMT).
In this paper, 50 records from the first event and 66 records from the second event which
are available from BHRC website are used to examine the near-source characteristic of
recent double earthquakes in Northwest of Iran. It is worth noting that, the records at
Satarkhan, Nahand, Qoorichay and Aras dams were not released (by BHRC) by the time
this paper was written and therefore have not been considered in the analysis. Continuous
Fig. 1 Distribution of the triggered stations recording the first main shock of the 2012 Varzaghan–Ahardouble earthquakes (reconstructed from BHRC website)
1078 Nat Hazards (2014) 70:1077–1097
123
wavelet transform (CWT) as a quantitative method is applied to recognize and extract
strong near-fault velocity pulse from the ground-motion time histories. If the extracted
pulse is large relative to remaining features in the ground motion, the ground motion is
chosen as pulse-like motion. The preferred wavelet-based decomposition process helps us
to quantify the pulse effects on elastic acceleration response spectra when it is compared
with design response spectra of Iranian seismic design code (Standard No. 2800).
2 Analysis of ground-motion records
Uncorrected strong motion data were processed to make baseline and instrumental cor-
rections; also, high-pass Butterworth filter has been applied. Among observed seismic
wave data which are available, the records of first earthquake at four selected sites; Var-
zaghan, Khajeh, Ahar and Nahand are analyzed with more details. These stations have
experienced peak ground acceleration (PGA) larger than 0.2 g, and maximum values of
peak ground velocity (PGV) are 49.2, 20.08, 14.21 and 14.94 cm/s, respectively. Maxi-
mum displacement of about 11 cm was recorded on horizontal component of Varzaghan
station. The acceleration and velocity time series of the main shock along with the Fourier
spectrum of ground-motion records have been presented in Figs. 3, 4 and 5. As a first
feature, the concentration of energy in Varzaghan station is observed in wave trains.
Difference between Fourier spectrum amplitude of two horizontal components at Varza-
ghan station was recognized in Fig. 5. It can be seen that the lower frequencies between 0.3
Fig. 2 Distribution of the triggered stations recording the second main shock of the 2012 Varzaghan–Ahardouble earthquakes (reconstructed from BHRC website)
Nat Hazards (2014) 70:1077–1097 1079
123
and 2 Hz are dominant in the longitudinal component of Varzaghan station, which may be
due to the near-source effects. Also a spike around the frequency of 3 Hz is evident in the
transverse component of Khajeh station. Note that the longitudinal and transverse com-
ponents have been named as L and T component hereafter.
Seismic waveforms at Varzaghan station recorded in second event along with its Fourier
spectrum have been presented in Figs. 6 and 7. It is worthy to note that this site has
experienced higher value of peak ground acceleration, velocity and displacement in the
study region as 0.54 g, 39.77 cm/s and 6 cm, respectively.
3 Acceleration response spectra of observed ground motions and comparisonwith the Iranian seismic design code
The third edition of Iranian seismic design code (Standard No. 2800) published by the
BHRC in 2004 follows the conventional-force-based design method (seismic coefficient
method) for design of structures. This code is used vastly in Iran as one of the world’s most
earthquake-prone area which has been exposed to numerous destructive earthquakes in the
past. There is no doubt that improvement and putting into operation of Standard No. 2800
in seismic design of structures is one of the major important steps for reducing the
earthquake hazards in Iran.
L component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70Time(sec)
(a) T component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70Time(sec)
(a)
L component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70Time(sec)
(b) T component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
(b)
L component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70Time(sec)
(c) T component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70Time(sec)
(c)
L component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
(d) T component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
(d)
Fig. 3 Acceleration time histories of ground-motion records at a Varzaghan, b Khajeh, c Ahar andd Nahand stations from the first event
1080 Nat Hazards (2014) 70:1077–1097
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Based on the Iranian seismic design code and considering earthquake occurrence risk in
different regions, four seismic zones are defined as: very high, high, medium and low
seismic potential zones. Accordingly, the value of the design basis acceleration is assumed
as 0.35, 0.3, 0.25 and 0.2 g for these regions, respectively.
In this section of paper, the pseudo-acceleration response spectra at 5 % damping ratio
for the horizontal components at Varzaghan, Khajeh, Ahar and Nahand stations have been
analyzed and compared with the code design response spectra from Standard No. 2800.
These stations have been located at epicentral distance of 19.2, 47.1, 18.1 and 45 km,
respectively. It should be noted that based on Standard No. 2800 provisions, the selected
stations have been grouped in the high seismic potential zone. However, in order to better
understand the level of earthquake ground-motion records, design response spectra for high
and very high seismic potential zone have been presented in Figs. 8, 9, 10 and 11.
Varzaghan and Khajeh stations provide very high values at short–moderate periods,
reaching about 1.3 and 1.6 g at 0.18 and 0.36 s, respectively. These values are above the
predicted spectral values of code for high and very high seismic potential zones. The
response spectra in Ahar station indicate that the shaking level is around the provided
spectra by the code for very high seismic region. It means that the shaking levels in the
cities are strong in the period range of 0.1–0.6 which is critical for one to four story
L component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70Time(sec)
(a) T component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70Time(sec)
L component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
(b) T component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
L component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70Time(sec)
(c) T component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70Time(sec)
loc
L component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70Time(sec)
(d) T component: Velocity(cm/sec)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
(a)
(b)
(c)
(d)
Fig. 4 Velocity time histories of ground-motion records at a Varzaghan, b Khajeh, c Ahar and d Nahandstations from the first event
Nat Hazards (2014) 70:1077–1097 1081
123
buildings. Comparison of results at Nahand station shows the lower spectral values than the
code design spectral values in all range of periods.
A comparison of Figs. 8 through 11 indicates a different behavior for L component of
record at Varzaghan station. There are two main peaks in this component that exceed the
design spectra. Meanwhile, the large difference between spectral values of two components
of records at Varzaghan station is observed in the wide range of periods. Such difference
could be recognized also in displacement response spectra for L and T components (see
Fig. 12). Higher spectral values for the L component in the periods larger than 0.6 s could
be related to near-source effects. Wavelet transform provides a more detailed analysis of
records in this seismic station through the next section.
In the second event, four stations named Varzaghan, Ahar, Heris and Khajeh have
experienced motions with PGA [ 0.2 g. The acceleration response spectra of observed
ground motions in these sites and comparison with the Iranian seismic design code
(Standard No. 2800) are illustrated in Figs. 13, 14, 15 and 16. Large differences between
the response spectra at Varzaghan station and code design spectra are observed which show
that the shaking is mainly strong at periods less than 0.6 s. The lower shaking level at wide
range of periods is established at Khajeh and Heris stations. Furthermore, Fig. 15 displays
that the Standard No. 2800 design spectra are able to cover the response spectrum induced
by the second event at Heris station.
L component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
(a) T component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
L component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
(b) T component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
L component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
(c) T component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
L component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
(d) T component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20
Frequency(Hz)
(a)
(b)
(c)
(d)
Fig. 5 Fourier spectra of ground-motion records at a Varzaghan, b Khajeh, c Ahar and d Nahand stationsfrom the first event
1082 Nat Hazards (2014) 70:1077–1097
123
4 Wavelet analysis on identification of pulse-like motions
In this part, a quantitative wavelet-based procedure is illustrated and implemented for
describing principle features of Ahar–Varzaghan 2012 earthquake as a special class of
earthquake records called pulse-like motions (Baker 2007; Yaghmaei-Sabegh 2010).
Wavelet transform is able to detect and extract large pulses in time histories by separating
the contributions of different levels of frequency. It is also possible to evidence simply the
frequency contents of extracted pulse on response spectra of motion.
L component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
L component: Velocity(cm/s)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
T component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
T component: Velocity(cm/s)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
V component: Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 10 20 30 40 50 60 70
Time(sec)
V component: Velocity(cm/s)
-50
-30
-10
10
30
50
0 10 20 30 40 50 60 70
Time(sec)
Fig. 6 Acceleration and velocitytime histories of ground-motionrecords at Varzaghan stationfrom the second event
Nat Hazards (2014) 70:1077–1097 1083
123
L component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16 18 20
Frequency(Hz)
T component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16 18 20
Frequency(Hz)
V component: FAS
0
0.05
0.1
0.15
0.2
0.25
0 2 4 6 8 10 12 14 16 18 20
Frequency(Hz)
Fig. 7 Fourier spectra ofground-motion records atVarzaghan station from thesecond event
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
Fig. 8 Pseudo-accelerationresponse spectra of Varzaghanstation (recorded from the firstevent) compared with the spectraof Standard No. 2800 for highand very high seismic potentialzones at different soil conditions
1084 Nat Hazards (2014) 70:1077–1097
123
The wavelet transform of a ground motion, g(t), with respect to mother wavelet function
wð�Þ is defined by:
WT g; a; b½ � ¼ 1ffiffiffiffiffiffi
aj jp
Z
1
�1
gðtÞw� t � b
a
� �
dt ð1Þ
where a = 0 and b are real values called the scale and translation or location parameters,
respectively, and symbol * denotes complex conjugation. Dilation by the scale a which is
inversely proportional to frequency represents the periodic nature of the signal. WT g; a; b½ �as a result of the wavelet transform is representing a time-scale map (or Scalogram). Fourier
transform of wðtÞ is used to reconstruct g(t) from its wavelet transform, WT g; a; b½ �, as:
gðtÞ ¼ 1
2pCw
Z
1
�1
Z
1
�1
1
a2WT½g; a; b�w t � b
a
� �
dadb ð2Þ
Cw ¼Z
1
�1
wðxÞ�
�
�
�
�
�
2
xj j dx ð3Þ
where wðxÞ is the Fourier transform of wðtÞ, and the coefficient of Cw is a constant
depends on the selected mother wavelet.
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Khajeh_L
Khajeh_T
Very high seismic potential zone
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Khajeh_L
Khajeh_T
Fig. 9 Pseudo-accelerationresponse spectra of Khajehstation (recorded from the firstevent) compared with the spectraof Standard No. 2800 for highand very high seismic potentialzones at different soil conditions
Nat Hazards (2014) 70:1077–1097 1085
123
In this section of article, the large pulse of ground motions is extracted by applying
CWT, and the residual motions are achieved by subtraction of extracted pulse from the
original motions. Then, the original motion is compared with the size of residual motion by
means of quantitative criterion named pulse indicator (PI). PI is defined as the following
form
PI ¼ 1
1þ expð�23:3þ 14:6ðPGV ratioÞ þ 20:5ðenergy ratioÞÞ ð4Þ
The above equation which proposed by Baker (2007) consists of two variables: PGV
ratio, that is the peak ground velocity of the residual record divided by the original record’s
PGV, and energy ratio, which achieves by dividing the energy of the residual record to the
original record’s energy. Records with pulse indicator above 0.85 are classified as pulse-
like motions if satisfying two other conditions as:
• The PGV of original ground motion should be greater than 30 cm/s.
• The pulse arrives early in their velocity time history (as indicated by t20 %;orig) should be
greater than t10 %;pulse.
t20 %;orig and t10 %;pulseare the times at which original ground motion and extracted pulse
reach 20 and 10 % of its total cumulative squared velocity, respectively. More detailed
description about adopted method can be found in Baker (2007).
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Ahar_L
Ahar_T
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Ahar_L
Ahar_T
Fig. 10 Pseudo-accelerationresponse spectra of Ahar station(recorded from the first event)compared with the spectra ofStandard No. 2800 for high andvery high seismic potential zonesat different soil conditions
1086 Nat Hazards (2014) 70:1077–1097
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Among the available records from the first event, L component of record at Varzaghan
station satisfies all three criteria and is classified as pulse-like motion. It should be noted
that the pulse indicator for this motion has been estimated based on two different mother
wavelets: Daubechies wavelet of order 4 (db4) and BiorSpline wavelet of order 1 and
associated filter length 3 (bior1.3). Corresponding pulse indicator based on these mother
wavelets are 0.896 and 0.924, respectively. Figure 17 shows the velocity time history of
High seismic potentail zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Nahand_L
Nahand_L
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Nahand_L
Nahand_T
Fig. 11 Pseudo-accelerationresponse spectra of Nahandstation (recorded from the firstevent) compared with the spectraof Standard No. 2800 for highand very high seismic potentialzones at different soil conditions
0
10
20
30
40
0 1 2 3 4 5
Period(sec)
Sd
(cm
)
Sd_L
Sd_T
Fig. 12 Comparison of L and Tcomponent displacementresponse spectra at Varzaghanstation
Nat Hazards (2014) 70:1077–1097 1087
123
original ground motion and the associated extracted pulse for this motion (the results based
on db4 has been shown).
The wavelet-based process of paper aids quantification of the effect of extracted pulses
on elastic acceleration response spectrum that is used as a straightforward tool in earth-
quake engineering. For this purpose, an acceleration spectrum of identified pulse-like
motion is compared with the spectrum of the residual motion in Fig. 18. The design
response spectra from Standard No. 2800 have been superimposed in this figure. Also,
residual ground-motion time history after pulse removing has been shown in this figure. In
conclusion, it has to be remarked that the pulse causes amplification of record acceleration
spectrum in the period range of 0.45–0.8 s. Removing of pulse from the original motion
removes the second peak from the response spectrum and decreases the level of motion
under the design code spectra in the period range that is more significant for seismic design
of typical buildings in the study area.
Similar procedure has been carried out for pulse detection of records at second event.
Analysis of second event records showed that the T horizontal component of record at
Varzaghan station could be classified as pulse-like motion with pulse indicator of 0.96 and
0.85 based on bior1.3 and db4 mother wavelet, respectively. Peak ground velocity for this
record is 39.8 cm/s. Figure 19 displays the velocity time history of original motion and
extracted pulse at Varzaghan station obtained during the second event. Residual ground-
motion time history and its response spectrum along with the design response spectra for
second pulse-like motion has illustrated in Fig. 20. As shown in this figure, the maximum
Very high seismic potential zonePGA=0.35g
0
0.5
1
1.5
2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
High seismic potential zonePGA=0.3g
0
0.5
1
1.5
2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
Fig. 13 Pseudo-accelerationresponse spectra of Varzaghanstation (recorded from the secondevent) compared with the spectraof Standard No. 2800 for highand very high seismic potentialzones at different soil conditions
1088 Nat Hazards (2014) 70:1077–1097
123
spectral value of record is reduced from 1.95 to 1.3 when the pulse is removed. Change of
spectral values in the range of 0.1–0.4 s is recognized, as well.
5 Other evidences of near-source ground-motion characteristics
Evidences of near-source ground-motion characteristics have been observed in both field
observations and recorded strong ground-motion data in Varzaghan station. The large-
amplitude displacement pulses have led to the increase in damage at the vicinity of Var-
zaghan city which has been located in forward directivity region. Such motions have higher
amplitude and shorter duration as shown in Varzaghan station. These evidences have been
validated in this section of paper by various indicators as PGV, PGA, Arias Intensity and
significant duration.
The value of peak ground velocity in near-source area can be estimated by the empirical
relations proposed by Bray and Rodriguez-Marek (2004). These relations were developed
using a database consisting exclusively of forward directivity records measured based on
Eqs. 5 and 6 for rock and site conditions, respectively.
lnðPGVÞ ¼ 4:46þ 0:34 Mw � 0:58lnðR2 þ 72Þ � 0:39 ð5Þ
lnðPGVÞ ¼ 4:58þ 0:34 Mw � 0:58lnðR2 þ 72Þ � 0:49 ð6Þ
where R is the closest distance from the site to the rupture surface in km, PGV is peak
ground velocity in cm/s and the last terms (±0.39, ±0.49) are the standard deviation (in
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Ahar_L
Ahar_T
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Ahar_L
Ahar_T
Fig. 14 Pseudo-accelerationresponse spectra of Ahar station(recorded from the second event)compared with the spectra ofStandard No. 2800 for high andvery high seismic potential zonesat different soil conditions
Nat Hazards (2014) 70:1077–1097 1089
123
logarithmic scale) of the predictions. The peak ground velocity value recorded at Varza-
ghan station is 49.2 cm/s, which is larger than the value of 33 cm/s predicted by Eq. (6)
and is within one standard deviation (53 cm/s) from the proposal of Bray and Rodriguez-
Marek (2004).
Large difference between maximum peak ground acceleration at Varzaghan and Ahar
station with approximately same epicentral distance were observed (as 0.45 and 0.272 g,
respectively). Phung et al. (2004) attempted to quantify the directivity effects on peak
ground acceleration based on near-field strong ground motions recorded during the 1999
Chi–Chi, Taiwan (03), earthquake. They quantified directivity effect by relating the
computed residuals to directivity parameters h;X that are the angle between fault plane and
ray path to site and length ration as defined by Somerville et al. (1997).
Based on work of Phung et al. (2004), the residuals for strike-slip faulting were fit using
the following equation:
R ¼ B1 þ B2X cos h ð7Þ
where R is the natural log of residual and B1;B2 are the regression coefficients calculated as
-0.082 and 0.464, respectively. The residual parameter (R) takes the value 0.38 by con-
sidering X cos h ¼ 1, which shows the maximum directivity effect. It means that directivity
effect could increase PGA by a factor of 1.462. For our case study and by applying this
amplification factor, the recorded PGA at Ahar station would be amplified as
(0.272 g 9 1.462 = 0.4 g), which is close to recorded PGA at Varzaghan station. However,
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Heris_L
Heris_T
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Heris_L
Heris_T
Fig. 15 Pseudo-accelerationresponse spectra of Heris station(recorded from the second event)compared with the spectra ofStandard No. 2800 for high andvery high seismic potential zonesat different soil conditions
1090 Nat Hazards (2014) 70:1077–1097
123
High seismic potential zonePGA=0.3g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Khajeh_L
Khajeh_T
Very high seismic potential zone
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)
Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Khajeh_L
Khajeh_T
Fig. 16 Pseudo-accelerationresponse spectra of Khajehstation (recorded from the secondevent) compared with the spectraof Standard No. 2800 for highand very high seismic potentialzones at different soil conditions
-50
-30
-10
10
30
50
15 20 25 30 35 40
Time(sec)
Vel
oci
ty(c
m/s
ec)
-50
-30
-10
10
30
50
15 20 25 30 35 40
Time(sec)Vel
oci
ty(c
m/s
ec)
Fig. 17 Velocity time history ofL component and extracted pulseat Varzaghan station obtained inthe first event
Nat Hazards (2014) 70:1077–1097 1091
123
Residual motion_Acceleration(g)
-0.5
-0.3
-0.1
0.1
0.3
0.5
0 20 40 60
Time(sec)
Very high seismic potential zonePGA=0.35g
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
Residual motion_L
Fig. 18 Residual ground-motiontime history of Varzaghan stationrecord in the first event and itsresponse spectrum along with thedesign response spectra (standardNo. 2800)
-50
-30
-10
10
30
50
Time(sec)Vel
oci
ty(c
m/s
ec)
-50
-30
-10
10
30
50
0 5 10 15 20
0 5 10 15 20Time(sec)
Vel
oci
ty(c
m/s
ec)
Fig. 19 Velocity time history ofL component and extracted pulseat Varzaghan station obtained inthe second event
1092 Nat Hazards (2014) 70:1077–1097
123
difference between soil conditions of these two sites could help us to illustrate the difference
between the amplified (PGA = 0.4 g) and observed peak values (PGA = 0.45 g).
The distribution of corrected PGA averaged on the two horizontal components in the
first event has been shown as contour map in Fig. 21. The distribution shape of the PGA
contour is generally centered at Varzaghan station, elongated in the direction of south Ahar
fault.
Figure 22 shows the Husid diagram (as Arias Intensity vs. time) for the horizontal
components of the Varzaghan accelerogram.
Arias Intensity ðIaÞ is defined by Arias (1970) as:
Ia ¼p2g
Z
td
0
½aðtÞ�2 ð8Þ
where td is total duration of earthquake ground motion and a(t) is the acceleration time
history in units of g (g is the acceleration of gravity).
The most interesting feature of the plot in Fig. 22 is that the maximum value of Arias
Intensity as a measure of the total energy content of a seismic excitation in L component is
around 2 times larger than the value for T component. It should be noted that, unlike peak
ground parameters as PGA, Arias Intensity considers the full range of frequencies and the
duration of the ground motion.
Significant duration is defined as the interval between the times at which different
specified values of Arias Intensity are reached. The advantages of the significant duration
to the other definitions are that it considers the characteristics of the entire accelerogram
and defines a continuous time window. Consequently, it is relatively stable with respect to
the definitions of beginning and end thresholds. One of the generic measures of significant
Residual motion_Acceleration(g)
-0.40
-0.20
0.00
0.20
0.40
0 10 20 30 40 50 60 70
Very high seismic potential zonePGA=0.35g
0
0.5
1
1.5
2
0 1 2 3 4
Period(sec)
Res
po
nse
Sp
ectr
al A
ccel
erat
ion
, S
a(g
)Soil Type I
Soil Type II
Soil Type III
Soil Type IV
Varzaghan_L
Varzeghan_T
Residual_T
Fig. 20 Residual ground-motiontime history of Varzaghan stationrecord in the second event and itsresponse spectrum along with thedesign response spectra (standardNo. 2800)
Nat Hazards (2014) 70:1077–1097 1093
123
duration is used as the time intervals between 5 and 75 % Arias Intensity in this paper. The
median significant duration for Varzaghan station is 2.21 s. Large difference is observed
with corresponding mean value at Ahar station (at same distance) as 6.83 s. This difference
could not be explained by differences in site classifications, and the difference between
duration at rock and soil sites are not considerable in general. The predicted values for
Varzaghan station without considering directivity effect based on Abrahamson and Silva
(1996) and Yaghmaei-Sabegh et al. (2011) models are 4.5 and 5.1, respectively. Somerville
et al. (1997) evaluated the spatial variation in strong motion duration due to rupture
directivity. They suggested a duration factor to modified ground-motion duration depended
on directivity condition. For maximum directivity condition, the duration is about 0.55
times the average duration (expected based on conventional GMPEs without directivity
effect) for both strike-slip and dip-slip faulting. Thus, the modified predicted significant
duration considering maximum directivity effect based on Abrahamson and Silva (1996)
and Yaghmaei-Sabegh et al. (2011) models takes the value of 2.47 and 2.8, respectively,
which relatively match with the observed value.
As a result, Varzaghan and Ahar have been located in approximately same distance
(*20 km) from the epicenter of first event (see Fig. 1), but as shown in this section,
Fig. 21 Contour maps of peak ground acceleration (cm/s/s) obtained based on the first event data
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20 40 60
Time(sec)
Ari
as I
nte
nsi
ty(m
/s)
Varzaghan_L Varzaghan_T
Fig. 22 Husid plot for thehorizontal component of ground-motion records at Varzaghanstation in the first event
1094 Nat Hazards (2014) 70:1077–1097
123
Varzaghan has experienced pulse-like motion and consequently much affected by this
event due to this aspect.
6 Comparison with pulse-like ground motions of past events
Figure 23 compares the computed displacement response spectrum for the pulse-like
motion at Varzaghan station with those induced by pulse-like ground motions listed in
Table 1. As seen in this Table, earthquake magnitude and closest distance of Varzaghan
station (&16 km) are more close to 1999 Chi–Chi, Taiwan (03), earthquake (Mw 6.2) and
two selected stations named CHY080 and TCU076.
Figure 23 shows that the spectrum of Varzaghan records within period range up to
0.65 s is comparable with those of the other earthquakes. For period range larger than
0.65 s, the spectral values of Bam record are considerably larger than other case studies.
This record with large value of PGV (=100 cm/s which is more than 2 times of PGV value
in Varzaghan station) has been obtained in closest distance of 1 km from the fault. The
other aspect is that the overall form of displacement response spectrum of Varzaghan is
generally similar to those of SFB record from the 2008 Wenchuan, China, earthquake and
CHY080 record from the 1999 Chi–Chi, Taiwan (03), earthquake. However, the peak
value for displacement in Varzaghan is around 2.5 s which is larger than the corresponding
values in CHY080 and SFB (i.e., 1.2 and 1.5 s, respectively). Meanwhile the peak values
of amplitude for these three records are comparable in the ranges of 35–42 cm.
Table 1 Information of the pulse-like motions used in comparisons
Earthquake Station Mw Distance (km) PGV (cm/s)
Chi–Chi Taiwan (03), 1999 CHY080 6.2 22.4 69.6
Chi–Chi Taiwan (03), 1999 TCU076 6.2 14.7 59.4
Bam, Iran, 2003 Bam 6.8 \1 100
Wenchuan, China, 2008 SFB 7.9 1.2 79
0
20
40
60
80
100
0 0.5 1 1.5 2 2.5 3 4
Period(sec)
Sd
(cm
)
Varzaghan(First event)Chi-Chi_Taiwan(03)_CHY080Chi-Chi_Taiwan(03)_TCU076Wenchuan 2008_SFBBam 2003_Bam
Fig. 23 comparison of the near-field record spectrum fromVarzaghan to Ahar 2008earthquake at Varzaghan stationwith those from other near-fieldevents
Nat Hazards (2014) 70:1077–1097 1095
123
7 Summary and conclusions
Ground-motion records during the 2012 Varzaghan–Ahar double earthquakes were used in
this paper to analyze the pulse-like motion characteristics. For this purpose, CWT as a
quantitative method was applied to extract strong near-fault velocity pulse from the
ground-motion time histories. Results showed that among the 66 stations, the records
obtained at Varzaghan station with epicentral distances 19.2 and 12 km (in the first and
second event, respectively) could be classified as pulse-like motions with pulse index
around 0.9. In agreement with this finding, the accelerograms analyzing showed larger
value of PGA and PGV at this site located in the direction of rupture. Other evidences of
near-source ground-motion signatures were presented for this station as well.
The pseudo-acceleration response spectra at 5 % damping for the horizontal compo-
nents at Varzaghan, Khajeh, Ahar and Nahand stations (with PGA values larger than 0.2 g)
were compared with the code design response spectra. Analysis showed that stations
Varzaghan and Khajeh provide very high values at short–moderate periods, reaching about
1.3 and 1.6 g at 0.18 and 0.36 s, respectively. The values were above the predicted values
by the code for both of high and very high seismic potential zones.
In order to clarify the pulse effects in response spectra, pulse-like motion is compared
with the spectrum of the residual motion. The comparisons helped us to recognize a
relatively large amplification in spectral values due to near-source pulse effects. As one of
the major important steps for reducing the earthquakes hazards, the results suggest further
reexamination and improving of the Standard No. 2800 in earthquake hazard zoning of the
study area, in Northwest of Iran.
The analyses results were compared with other near-source pulse-like ground motions
to explain similarities and differences. The displacement response spectrum of Varzaghan
was generally similar to those of SFB record from the 2008 Wenchuan, China, earthquake
and CHY080 record from the 1999 Chi–Chi, Taiwan (03), earthquake. The maximum
spectral values of ground-motion record at Varzaghan station was recognized in long
period values around 2.5 s, while the corresponding values for CHY080 and SFB records
were 1.2 and 1.5 s, respectively.
Acknowledgments The author gratefully acknowledges the contributions by Sasan Yaghmaei-Sabegh forpreparing maps of the 2012 Varzaghan–Ahar, Iran double earthquakes.
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