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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.

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