THE CO-SEISMIC COULOMB STRESS CHANGES IN THE SOUTHEAST AND NORTHWEST OF IRANIAN PLATEAU

Behnam MALEKI ASAYESH1, Hamid ZAFARANI2, Majid MAHOOD3, Saeed ZAREI4

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).

1Ph.D Student, Department of Geophysics, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran, [email protected] 2Associate Professor, Earthquake Prediction Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran, [email protected] 3Assistant Professor, Earthquake Prediction Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran, [email protected] 4Ph.D student, Department of Geology, Faculty of Science, University of Birjand, Birjand, Iran, [email protected]

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 Apsheron–Balkhan sill, respectively, to the north (McKenzie, 1972; Copley and Jackson, 2006). The most dominant tectonic feature is the right-lateral, west-northwest–east-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. Magnitude Depth Length Width Mean Moment Main Plane Number Earthquake Date Longitude Latitude 18 (Mw) (km) (km) (km) Slip(m) (*10 )N.m Strike( °) Dip( ° ) Rake( ° ) Southeast of Iran events 57.680a 29.860a 6.6e 20e 14h,e 15h,e 1.4j 09.48e 169e 52e 156e 1 Golbaf 1981/06/11 57.360 29.690 6.6 20 ------09.82 172 37 171 57.790a 29.990a 7.1e 18e 60h,e 16h,e 2.7j 36.69e 177e 69e 184e 2 Sirch 1981/07/28 57.580 30.03 7.2 15.2 ------90.10 150 13 119 57.720a 29.900a 5.8e 10e 10.2h 6.1h 0.22j 00.70e 145e 69e 188e 3 S. Golbaf 1989/11/20 ------57.580a 30.080a 6.6e 5e 23h,e 12.4h,e 1.7j 09.09e 156e 54e 195e 4 Fandoqa 1998/03/14 57.600 29.950 6.6 15 ------09.43 154 57 174 ------30i 20i 0.08e 02.00e 149e 06e 095e 5 Shahdad 1998/03/14 ------58.268b 28.950b 6.6f 5.5f 20h,f 12h,f 2.14f 07.60f 354f 86f 182f 6 Bam 2003/12/26 58.240 29.100 6.6 15 ------09.31 172 59 167 56.736c 30.774c 6.5g 9g 18g 14g 1.4j 07.00g 260g 60g 104g 7 Zarand 2005/02/22 56.810 30.760 6.0 25.4 ------05.20 266 47 100 59.188d 28.325d 6.5d 5d 15d 13d 1.3d 07.10d 213d 85d 173d 8 Rigan1 2010/12/20 59.110 28.100 6.5 14.8 ------8.26 .36 87 180 59.044d 28.169d 6.2d 9d 7d 17d 0.63d 02.60d 311 86 003 9 Rigan2 2011/01/27 ------Northwest twin earthquake Ahar- 46.842k 38.399k 6.4k 6k 24.50h 10h 0.59h 05.04k 268l 86l 166k 10 2012/08/11 Varzaghan1 46.800 38.310 6.5 15 ------06.04 084 84 170 Aahar- 46.777k 38.425k 6.2k 12k 17.14h 9.40h 0.54h 02.58k 264l 80l 125k 11 2012/08/11 Varzaghan2 46.780 38.350 6.4 19.2 ------04.24 255 63 134 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

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.

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Table 2. . Minimum and maximum Coulomb stress changes for ruptured faults in south east of Iran. Coulomb Stress Changes (MPa) Number Earthquake Fault Name Date 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.

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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

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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.

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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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