Conference PaperPDF Available
THE CO-SEISMIC COULOMB STRESS CHANGES IN THE
SOUTHEAST AND NORTHWEST OF IRANIAN PLATEAU
Behnam MALEKI ASAYESH
1
, Hamid ZAFARANI
2
, Majid MAHOOD
3
, Saeed ZAREI
4
ABSTRACT
Iranian Plateau has been frequently struck by catastrophic earthquakes resulting in the massive loss of life, large
masses homeless and disrupting their agricultural and industrial lifelines. Southeast of Iran experienced 8
earthquakes during 30 years from 1981 to 2011. Northwest of Iran experienced historical and instrumental large
earthquakes too. On 11 August 2012, a strong earthquake with magnitude Mw 6.4 occurred in Ahar- Varzaghan
region, NW of Iran. It followed by another strong earthquake with magnitude Mw 6.2 after 11 minutes.
The influence of static stress transfer due to moderate-to-large earthquakes on the occurrence of future
earthquakes had been proved by numerous studies. This effect in triggering future events and spatial distribution
of aftershocks can be explained by using the Coulomb stress changes theory.
We calculated the static Coulomb stress changes for the southeast of Iran due to earthquake sequence from 1981
and for the northwest of Iran due to Ahar-Varzaghan earthquakes. In the southeast of Iran, our calculations
showed a positive stress changes due to previous events on the rupture plane of next earthquake and only plane
of the Rigan earthquake of 2010 December 20 received negative stress changes. Also, we calculated imparted
stress changes on the surrounding active faults and showed that hypocenter of recent Hojedk earthquakes (2017
December 1 and 12) with a moment magnitude of 6.1, 6.2, and 6.1 respectively, received positive stress
changes. The imparted stress by the twin Ahar- Varzaghan earthquakes on North Tabriz Fault (NTF) system
showed an increase on the eastern part of the NTF near the Bostanabad about 30 km southeast of Tabriz city and
northwest part of this fault system. Furthermore our calculation showed positive Coulomb stress changes on the
North and South Bozgush faults.
Keywords: Earthquake; Coulomb stress change; Receiver fault; Iranian Plateau.
1. INTRODUCTION
The active tectonics of Iran is dominated by the convergence of Arabian and Eurasian plates (Vernant
et al. 2004). The crustal strain caused by the plate convergence is accommodated by inland active
faults so we have a verity of the earthquake with different mechanism and ranges of a magnitude in
Iranian Plateau. Southeast of Iran experienced 8 earthquakes during 30 years from 1981 to 2011. Six
of these events by magnitude more than Mw 6.5 caused great human and financial losses in the
region. A twin strong earthquake occurred in East Azerbaijan province in northwestern Iran on 11
August 2012. The first event with a moment magnitude of 6.4 in the Ahar- Varzaghan region
followed 11 min later by another event with a moment magnitude of 6.2 in the same region. The
seismicity seriously damaged about 20 villages, killing 327 people and injuring more than 3000
(Razzaghi and Ghafory-Ashtiany, 2012).
1
Ph.D Student, Department of Geophysics, International Institute of Earthquake Engineering and Seismology
(IIEES), Tehran, Iran, b.m.asayesh@gmail.com
2
Associate Professor, Earthquake Prediction Center, International Institute of Earthquake Engineering and
Seismology (IIEES), Tehran, Iran, h.zafarani@iiees.ac.ir
3
Assistant Professor, Earthquake Prediction Center, International Institute of Earthquake Engineering and
Seismology (IIEES), Tehran, Iran, m.mahood@iiees.ac.ir
4
Ph.D student, Department of Geology, Faculty of Science, University of Birjand, Birjand, Iran,
zareisaeed@birjand.ac.ir
In recent years, many seismology scientists worldwide have focused on studying Coulomb stress
changes and the influence of static stress transfer due to moderate-to-large earthquakes on the
occurrence of future events (e.g. Harris, 1998; Stein, 1999; King and Cocco, 2001). Also, the rate-
state studies suggest that co-seismic stress changes have a time-dependent effect on neighboring faults
with an immediate jump in earthquake probability that decays with time (Parsons et al. 1999; Toda et
al. 2005).
The objective of this study is calculating the Coulomb stress changes in two different regions of the
Iranian plateau. For this purpose, at first, we calculate the Coulomb stress change due to previous
events on the fault plane of the next events in the southeast of Iran. Then we calculate the transferred
stress due to Ahar- Varzaghan twin earthquakes on North Tabriz Fault system (northwest of Iran).
2. SEISMOTECTONIC SETTING
Active faulting, active folding, recent volcanic activities, mountainous terrain, and variable crustal
thickness, are characteristics of the Iranian Plateau. This Plateau has been frequently struck by
catastrophic earthquakes resulting in the massive loss of life, large masses homeless and disrupting
their agricultural and industrial lifelines (Berberian, 1996). As mentioned, Iran is subjected to a
convergent stress produced by a motion of the Arabian plate in an NNE-SSW direction at a few
cm/year relative to the Eurasian plate and crustal strain due to this plate convergence is
accommodated by inland active faults and folds. Shortening and earthquake deformation within Iran
is mainly accommodated by distributed faulting in the Zagros, Alborz, Kopeh-Dagh and west of the
Dasht-e-Lut (Walker et al. 2003).
Figure 1. Main tectonic features of southeast of Iran. Location and focal mechanism of the main earthquakes
that are occurred in southeast of Iran are shown (Berberian et al. 2001; Jackson et al. 2006; Rouhollahi et al.
2012). Location of the major cities also are shown. Faults are from Hesami et al. (2003).
Central Iran has lateral escape respect to the Lut Block that is the result of indentation of the Arabian
plate into a composite system of collision-oblique transpressive fold-thrust mountain belts (Berberian,
2005). The northward motion of central Iran relative to western Afghanistan results two major fault
zones that have been developed with a nearly north-south-oriented strike along the western and
eastern borders of the Lut Block in eastern border of Iran (e.g. Mohajer-Ashjai et al. 1975; Walker
and Jackson, 2002) (Figure 1). These faults with right-lateral motions reflect the subjected stress
(Meyer and Le Dortz, 2007). The earthquakes in central Iran are generally shallow (less than 25 km)
and are usually associated with surface faulting (Berberian, 1976).
The tectonics of the northwest of Iran are influenced by the northward motion of the Arabian indenter,
the westwards extrusion of the Anatolian plate along the North- and East-Anatolian faults, and the
reverse tectonics and subduction under the Greater Caucasus and the ApsheronBalkhan sill,
respectively, to the north (McKenzie, 1972; Copley and Jackson, 2006). The most dominant tectonic
feature is the right-lateral, west-northwesteast-southeast striking, subvertical North Tabriz fault
(NTF) accommodating 7 mm/yr of right-lateral motion (Djamour et al. 2011; Moradi et al. 2011).
The main tectonics of studied area are shown in Figure 2.
Figure 2. Main tectonic features of northwest of Iran. Location and focal mechanism of the main earthquakes
that are occurred in northweat of Iran (Walker et al. 2013; Donner et al. 2015). Location of the major cities also
are shown. Faults are from Hesami et al. (2003).
3. STUDIED EVENTS
As already mentioned the southeast of Iran has experienced a number of destructive earthquakes
during the last 36 years. Eight earthquakes and an aseismic slip that occurred in the mentioned time
frame are briefly elaborated in Table 1. These events are used to calculate the Coulomb stress change
in the southeast of Iran from 1981 till 2017 (Table 1). Beside that twin earthquake of Ahar-Varzaghan
are considered in our calculations of imparted stress on North Tabriz Fault system.
In this study, we used Coulomb 3.4 software which implements the elastic half-space of Okada (1992)
to calculate the co-seismic static stress changes. We assumed Young modulus, shear modulus,
Poisson ratio, and apparent coefficient of friction were considered equal to 8 × 105 bar, 3.2 × 105 bar,
0.25, and 0.4, respectively.
Table 1. Parameters of earthquakes that are studied in this paper.
Number
Earthquake
Date
Longitude
Magnitude
(Mw)
Depth
(km)
Length
(km)
Width
(km)
Mean
Slip(m)
Moment
(*1018)N.m
Main Plane
Strike( °)
Dip( ° )
Southeast of Iran events
1
Golbaf
1981/06/11
57.680a
6.6e
20e
14h,e
15h,e
1.4j
09.48e
169e
52e
57.360
6.6
20
------
-----
-----
09.82
172
37
2
Sirch
1981/07/28
57.790a
7.1e
18e
60h,e
16h,e
2.7j
36.69e
177e
69e
57.580
7.2
15.2
------
------
-----
90.10
150
13
3
S. Golbaf
1989/11/20
57.720a
5.8e
10e
10.2h
6.1h
0.22j
00.70e
145e
69e
------
------
-----
------
-----
------
------
-----
------
4
Fandoqa
1998/03/14
57.580a
6.6e
5e
23h,e
12.4h,e
1.7j
09.09e
156e
54e
57.600
6.6
15
------
-------
------
09.43
154
57
5
Shahdad
1998/03/14
-----------
-------
---
30i
20i
0.08e
02.00e
149e
06e
-----------
--------
-----
-----
-----
-----
-----
-----
-----
6
Bam
2003/12/26
58.268b
6.6f
5.5f
20h,f
12h,f
2.14f
07.60f
354f
86f
58.240
6.6
15
------
------
-------
09.31
172
59
7
Zarand
2005/02/22
56.736c
6.5g
9g
18g
14g
1.4j
07.00g
260g
60g
56.810
6.0
25.4
------
-----
------
05.20
266
47
8
Rigan1
2010/12/20
59.188d
6.5d
5d
15d
13d
1.3d
07.10d
213d
85d
59.110
6.5
14.8
------
------
------
8.26
.36
87
9
Rigan2
2011/01/27
59.044d
6.2d
9d
7d
17d
0.63d
02.60d
311
86
-------
------
------
-----
------
-------
------
-------
------
Northwest twin earthquake
10
Ahar-
Varzaghan1
2012/08/11
46.842k
6.4k
6k
24.50h
10h
0.59h
05.04k
268l
86l
46.800
6.5
15
------
-----
-----
06.04
084
84
11
Aahar-
Varzaghan2
2012/08/11
46.777k
6.2k
12k
17.14h
9.40h
0.54h
02.58k
264l
80l
46.780
6.4
19.2
-----
------
-----
04.24
255
63
a) Engdahl et al. (1998). b) Engdahl et al. (2006). c) Talebian et al. (2004). d) Walker et al. (2013). e) Berberian et al. (2001). f) Jackson et al. (2006). g) Rouhollahi
et al. (2012). h) Calculated based on the slip-seismic moment relation of Wells and Coppersmith (1994). i) Fielding et al. (2004). j) Based on empirical relation of
Kanamori and Anderson (1975). k) Donner et al. (2015). l) Momeni et al. (2016). Second row of each event shows the parameters of that event from CMT catalog.
4. THE COULOMB STRESS TRIGGERING HYPOTHESIS
The permanent deformation of the surrounding crust is the consequence of an earthquake fault rupture.
Such an earthquake changes the stress on nearby faults as a function of their locations; geometry and
sense of slip (Toda et al. 2011). The Coulomb Failure Function, ΔCFF, which is the Coulomb stress
changes, depend on both changes in shear (Δτ) and normal stress (Δσ), and is calculated as follow.
(1)
Where μ' is the apparent coefficient of friction which includes the unknown effect of pore pressure
change as well (King et al. 1994). Depending on pore fluid content of the fault zone, µ' changes
between 0.2 and 0.8. Lower than 0.2 suggested for well-developed and repeatedly ruptured fault zones
because on these zones sliding friction drops cause of trapped pore fluids. On the other hand higher
than 0.8 amount can be used for young minor faults, since they did not have enough displacement for
trapping pore fluids (King et al. 1994; Stein, 1999).
Positive ∆CFF promotes failure, and negative inhibits it; both increased shear and unclamping of
faults are taken to promote failure, with the role of unclamping modulated by fault friction (Toda et al.
2011).
5. DISCUTION AND RESULTS
5.1 Coulomb stress changes in the southeast sequence
This section starts by considering the first event (Golbaf earthquake) as a source to calculate stress
changes due to this event in the ruptured plane of the next event. Then the stress changes due to the
last two events will be calculated in the ruptured plane of the third event and this process is continued
until the last event. Then we calculate imparted stress changes on the surrounding faults.
By considering the Golbaf earthquake of 1981 June 11 as a source we calculated the static Coulomb
stress changes on the causative fault of Sirch earthquake of 1981 July 28. We used 1.4 m slip (Table 1)
for source fault and obtained the transferred stress on Sirch fault. Sirch fault plane received positive
Coulomb stress changes with a minimum of ~0.0038 MPa and maximum of ~0.55 MPa (Table 2 and
Figure 3a). The southern part of the Sirch rupture is located in the northern fault-end lob of Coulomb
stress changes and received the maximum amount of ∆CFF (more than 0.55 MPa) (Figure 3a). This
positive resolved stress has brought Sirch part of the fault closer to failure. Most of the gap between
these events that did not rupture during these earthquakes is placed where that transferred stress due to
Golbaf event decreases and stress changes have negative values (Figure 3a).
In the second step, we calculated the static resolved stress due to Golbaf and Sirch earthquakes on the
fractured plane in the South-Golbaf earthquake. For this part of calculation we considered 1.4 m mean
slip for Golbaf event and 2.7 m for Sirch event based on empirical relation of Kanamori and Anderson
(1975). The calculated stress showed a maximum increase about 3.84 MPa and a maximum decrease
about 3.43 MPa (Table 2).
Then we calculated static Coulomb stress changes on the Fandoqa earthquake rupture resolved from
the previous three events and our calculation showed about 5.0 MPa positive stress changes in the
location of a 6-km-long gap that did not rupture during the Golbaf and Sirch events (Figure 3c). This
high amount of positive transferred stress is sufficient for accumulating stress and causing 14th March
1998 (Mw 6.6) in the region that had experienced two moderate and large events 17 years ago.
For the Shahdad fault, there is no recorded event but based on the previous studies such as Berberian
et al. (2001) as well as InSAR modeling of Fielding et al. (2004) it can be concluded that the fault has
experienced approximately 8 cm reverse slip during the 1998 March 14 Fandoqa earthquake.
Therefore, we calculated transferred stress on this fault due to the previous events and considered it as
one of our sources for next calculations. By considering this plane as a receiver fault, resolved stress
on this reverse fault showed a maximum positive amount about 0.17 MPa and a maximum negative
amount about -0.02 MPa (Table 2). As it is shown in Figure 3d most part of this plane receive positive
Coulomb stress changes and one possible hypothesis is that this transferred stress triggered this fault
with a reverse mechanism.
In all panels of figure 3, we observed lobes of increased stress at the source fault ends. These lobes of
6
increased shear stress that concentrated at the fault ends, tend to extend the fault, as discussed by King
et al. (1994) and Das and Scholz (1983).
The calculated Coulomb stress changes on the Bam fault plane due to the five previous events showed
a little positive and negative amount (Table 2). This little effect can be attributed to large distance. In
the next step, the Bam earthquake is added as a source, in order to calculate the Coulomb stress
changes on the Zarand ruptured plane as a reverse fault. We considered right-lateral variable-slip for
the Bam rupture based on the Talebian et al. (2004) model. Transferred stress on Zarand fault has
positive amount with maximum about 0.022 MPa (Table 2).
In the following step, we added the Dahuiyeh-Zarand earthquake as a source in our calculation. The
variable-slip on this reverse event calculated and added to input files based on Rouhollahi et al. (2012)
and the resolved stress due to this earthquake and previous events on the first Rigan earthquake has
been calculated (Table 2).
Figure 3. Calculated Coulomb stress changes due to earthquakes that occurred on the Golbaf-Sirch fault system.
a) Coulomb stress change due to Golbaf 1981 June 11 event on the Sirch event ruptured plane, which is
considered as receiver fault. b) Transferred stress on the South-Golbaf ruptured plane due to Golbaf and Sirch
events. c) The ruptured plane of the Fandoqa event is considered as a receiver and the three previous events are
considered as source. As are shown, some parts of this fault plane that ruptured 6 km gap between the Golbaf
and Sirch events surface rupture, received maximum positive stress of 5.0 Mpa. d) In this picture transferred
stress due to the Golbaf-Sirch fault system on the slipped part of the Shahdad fault on the 14th March 1998 are
shown. The depth of all calculation are 3 km.
Then we calculated the transferred stress on the second Rigan event. For this purpose, we added right-
lateral variable-slip of the first Rigan event in our calculations based on slip model presented in
Walker et al. (2013). The transferred stress on the second Rigan rupture showed a maximum stress
changes about 0.546 MPa (Table 2). By calculating the transferred stress due to only first Rigan event
on the second Rigan rupture plane we found that majority of this transferred stress is because of the
first Rigan event and previous events increased it only ~0.003 MPa. This resolved stress on the second
Rigan plane advanced the 6.2 magnitude earthquake on this fault with a left-lateral mechanism.
7
Table 2. . Minimum and maximum Coulomb stress changes for ruptured faults in south east of Iran.
Number
Earthquake
Fault Name
Date
Coulomb Stress Changes (MPa)
Max.
Min.
1
Golbaf
Golbaf-Sirch
1981/06/11
--------
-------
2
Sirch
Golbaf-Sirch
1981/07/28
0.551
0.004
3
S. Golbaf
Golbaf-Sirch
1989/11/20
3.844
-3.434
4
Fandoqa
Golbaf-Sirch
1998/03/14
4.999
-15.484
5
Shahdad
Shahdad
1998/03/14
0.170
-0.020
6
Bam
Bam
2003/12/26
0.0014
-0.003
7
Zarand
Kuhbanan
2005/02/22
0.022
0.007
8
Rigan1
Rigan1
2010/12/20
-0.006
-0.007
9
Rigan2
Rigan2
2011/01/27
0.546
-0.338
In the last part of this section, we calculated the Coulomb stress changes due to mentioned events on
the surrounding faults. For this purpose, we considered vertical right-lateral strike-slip faults with
northwest-southeast trending as receiver faults. Our calculation showed that southern end of the
Kuhbanan fault, northern and southern part of the Gowk fault, and entire of the Bam fault received
positive stress changes (Figure 4). Then we calculated the Coulomb stress changes on the reverse
faults with depth about 55 degree. We found that southern end of the Shahdad fault and entire of the
Lakarkuh fault received positive Coulomb stress changes. The 2017 December 1 and 12 Hojedk
earthquakes with moment magnitude 6.1, 6.2, and 6.1 (Iranian Seismological Center) respectively,
triggered in the region recently. The first event received positive Coulomb stress changes about 0.056
MPa in its hypocenter, the second even received about 0.026 MPa in its hypocenter, and the third
event received about 0.043 MPa in its hypocenter (Figure 5).
Figure 4. Coulomb stress change due to 9 events on the northwest-southeast orinted righ-lateral strike slip faults.
The depth of the calculations is 7.5 km.
8
5.2 COULOMB STRESS CHANGES IN THE NORTHWEST OF IRAN
The area of NW of Iran, especially the NTF, is the site of most destructive historical earthquakes.
Right-lateral strike-slip faults dominantly express in this region. The Ahar- Varzaghan earthquakes,
which occurred in this region, were the strongest instrumental earthquakes close to the Tabriz city.
The high earthquake hazard for Tabriz city is related to the activity of the NTF system. A slip rate of 7
mm/year has been estimated by Djamour et al, (2011) for NTF. This fault shows a right-lateral strike-
slip mechanism. While, the north and south Bozgosh faults, which locate at the east of NTF, show
reverse motion. The west edge of NTF, which known as Mishu fault, shows right-lateral strike slip
motion. The Sofian and Tasuj, which located in the west of NTF system and south of Mishu fault,
show reverse motion (Figure 2). Latest earthquakes of the eastern and western segments of the NTF
was in 1721 and 1780, respectively and return period of earthquake (Mw>6) in this fault is about 300
years. So we calculated the Coulomb stress changes due to 2012 twine Ahar-Varzaghan earthquake on
the NTF system to expand insights as to which fault system are now more hazardous for Tabriz city.
Figure 5. Coulomb stress change due to 9 events on the northwest-southeast orinted reverse faults. Epicenter of
the Hojedks earthquakes that occurred recently are showen with big stars. The depth of the calculations is 7.5
km.
We have considered the 2012 twine Ahar-Varzaghan earthquakes as sources that the first event has
about 54 cm mean slip and second event has about 80 cm mean slip (Wells and Coppersmith, 1994)
and calculated the Coulomb stress changes on the NTF system. For this purpose, the NTF system and
reverse faults at east and west ends have been modeled and subdivided into more than 300 parts
according to Moradi et al, (2011). The Coulomb stress changes imparted by the twine Ahar-
Varzaghan earthquake on strike-slip faults of NTF system was shown in Figure 6. The positive
Coulomb stress changes have been observed on the southeast end of NTF, where the 1721 destructive
earthquake occurred (Figures. 2 and 6), and northwest of NTF (Mishu fault). A positive stress changes
was calculated for the December 23, 2012, and the April 8, 2013, earthquakes, which occurred close to
west Mishu and east Mishu segments, 4 and 8 months after the twine 2012 Ahar- Varzaghan
9
earthquakes, respectively. While, the negative Coulomb stress changes were observed at the central
part of NTF, which the Tabriz city is located (Figure 6). Our calculation also show an increase in
on southeastern part of the NTF near the Bostanabad about 30 km southeast of Tabriz city and
indicated that positive due to the 2012 twin earthquakes make this part of the NTF as the most
likely site of the next strong to large earthquake, which can affect the Tabriz city (Figure. 6).
Figure 7 shows the Coulomb stress changes imparted by the twine Ahar- Varzaghan earthquakes on
reverse faults. The positive Coulomb stress changes have been observed in north and south Bozqosh
faults.
Figure 6. Coulomb stress changes imparted by the twine Ahar- Varzaghan earthquakes on the NTF system.
Right-lateral strike slip faults are receiver faults. Location of the December 23, 2012 and the April 8, 2013
earthquakes are shown with black stars.
Figure 7. Coulomb stress changes imparted by the twine Ahar- Varzaghan earthquakes on the NTF system.
Reverse faults are receiver faults.
10
6. CONCLUSION
We investigate the stress interaction relationship among the M 6.0 events that occurred in the
southeast of Iran since 1981. We considered 9 events as source and calculated Coulomb stress changes
due to them. The southeast of Iran showed clear stress load to failure relationships. Since we did not
consider any earthquake before the 1981 June 11, Golbaf event, we could not calculate Coulomb stress
changes imparted on the rupture plane of this event. Imparted stress on the fault plane of the next
events due to previous events was calculated and results showed that fault plane of 7 events received
positive Coulomb stress changes and only fault plane of the 2010 December 20 Rigan earthquake
received negative negligible (about thousandth) negative Coulomb stress changes. Also calculated
Coulomb stress changes due to mentioned events on the surrounding faults showed that southern end
of the Kuhbanan fault, northern and southern part of the Gowk fault, entire of the Bam fault, southern
end of the Shahdad fault, and entire of the Lakarkuh fault received positive Coulomb stress changes
and the 2017 December Hojedk earthquakes with moment magnitude 6.1, 6.2, and 6.1 received
positive Coulomb stress changes on their hypocenters.
Coulomb stress changes due to 2012 twine Ahar-Varzaghan earthquake showed positive stress
changes on the southeast end of NTF and northwest of NTF (Mishu fault). Positive stress changes that
were imparted to the locations of the December 23, 2012 and the April 8, 2013 earthquakes, which
occurred close to west Mishu and east Mishu segments, 4 and 8 months after the twin 2012 Ahar-
Varzaghan earthquakes, respectively could trigger these events. On the other hand, negative Coulomb
stress changes were observed at the central part of NTF, which the Tabriz city is located. Imparted
Coulomb stress changes by the twin Ahar- Varzaghan earthquakes on reverse faults (north and south
Bozqosh faults) are positive.
7. ACKNOWLEDGEMENT
We are thankful to International Institute of Earthquake Engineering and Seismology (IIEES) and
Geological survey of Iran, Tabriz branch for supporting this research work.
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Conference Paper
Full-text available
On 11th august 2012 two destructive earthquakes(Mw=6.5, 12:23:15 GMT &Mw=6.3, 12:34:34 GMT) happened in northwestern Iran close to Ahar and Varzaghan cities. Their hypocenter situated just 4 kilometers far from each other. Since both mainshocks were close to each other, there was a doubt on their causative faults geometries and their seismotectonic features. On the other hand, seismic cross sections showed a big gap in the middle of aftershocks cloud. We study the source kinematics using elliptical subfault approximation method to find the best fault(s) geometry and we correlate the slip history of both events with the spatial distribution of aftershocks. We inverted 11 strong motion waveforms within a distance range of 7 to 38 km and used a local velocity model we retrieved from the aftershock sequence. We inverted each ellipse for slip, rupture speed, rake and rise time. The preferred model for the first event is almost vertical(86 degrees and south dipping) and has strike of 268 degrees which is consistent with the E-W striking 13 km observed surface ruptures. Using this geometry, our calculated kinematic model for event-1 contains 3 ellipses. The largest one has semi-major and semi-minor axis of about 7.5 and 6.0 kilometers respectively. This patch showed maximum slip of about 3.5 meters on its center coinciding with a sizeable gap in the aftershock distribution within the central aftershock cloud. The rupture started from a depth of 10 km beneath this patch with a speed of 2.28 km/s. Two other small patches with lesser displacements on east and west of the main patch were surrounded by aftershocks. The first event occurred in about 10 seconds and exhibited unilateral rupture propagation towards the west. The rupture started from the eastern end of the reported surface ruptures and grown towards the western end of the aftershock sequence. On the other hand, the second event occurred on distinct fault situated just 2 km to the north of the causative fault of the first event. The fault dips 80 degrees to the north and its strike is about 264 degrees, almost parallel to the causative fault of the first event. The best kinematic model for the second event has 3 patches. The maximum slip is 3.22 meters and the rupture lasted for about 6 seconds. The rupture speed changed from 1.8 to 3.0 km/s and started from a depth of 12 km towards the ground surface. The earthquake doublet is 50 km far from the city of Tabriz with more than 2 million population. The closeness of the causative faults to the North Tabriz fault system characterized by a great history of destructive earthquakes, confirms the importance of a detailed study of the sources of this earthquake doublet. The distinct rupture characteristics of these two earthquakes illuminate differences in seismogenic properties and seismic hazard of such an important fault system.
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