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This reconnaissance report is based on a joint effort by the International Institute of Earthquake Engineering and Seismology (IIEES) in Tehran, and the Persian Gulf University in Bushehr. The publication of this report is supported by IIEES and the Earthquake Engineering Research Institute (EERI).

The members of this joint team started their study on April 9, 2013, and made a visit to the epicentral region on April 14, 2013. The team was made up of Mehdi Zare, M. EERI, and Anooshirvan Ansarai, IIEES, Tehran; Shobair Ashkpour-Motlagh, Persian Gulf University, Bushehr; Bahman Esmaeili, Kayson Company, Tehran; and Mohammad Shahvar, IIEES, Tehran.

Mehdi Zare, IIEES Anooshirvan Ansarai, IIEES Shobair Ashkpour-Motlagh, Persian Gulf University Bahman Esmaeili, Kayson Company Mohammad Shahvar, IIEES

Introduction
At 11:52 UTC (16:22 local time), April 9, 2013, a Mw 6.4 earthquake struck Shonbeh in the sparsely populated Borazjan region in South Iran, 89 km southeast of Bandar (Figure 1). Shonbeh has a population of about 6,000 people. The earthquake killed at least 37 and injured more than 950 people. Since the earthquake happened in the afternoon, most of the people were outdoors, so about 60% of the victims were either women or elderly family members. The event destroyed 100 houses, left 1000 homeless, and damaged 500 more houses in nearby villages. The early financial assessment of the earthquake losses by the governor general of Bushehr Province was about $50 million USD (Mehr News Agency, April 10, 2013).

Figure 1. Location of the April 9, 2013, Mw6.4 Shonbeh, Bushehr, SW Iran earthquake (adopted from Google map, 2013). Figure 2. The quick moment tensor solutions (EMSC).

According to the Euro-Mediterranean Seismological Center (EMSC), the earthquake exhibited an almost pure thrust focal mechanism, with a NW–SE fault plane and a focal depth of 10 km (Figure 2). Other reports on focal mechanisms (i.e., by the USGS) show a mostly compressional strike-slip component. The most important fault in the epicentral region is the Borazjan fault, having a north-south strike with a NNW-SSE trend. The fault segment is known to be about 100km long, and it is located in the east of the Mond anticline.

The fault passes from the western part of the Jashk Salt Diapir, one of the largest and most active salt domes of Iran, about 144 km southeast of Bushehr. The dome is 1350 m above sea level. Some NNW-SSE surface fissures (N300-320) were observed in the foothills of this mountain after the earthquake.

Seismicity
The earthquake struck within the Zagros fold-and-thrust belt, the portion of the Zagros Mountains that border the north shore of the Persian Gulf. The seismotectonics of southern Iran are controlled by convergence between the Arabian and Eurasian tectonic plates, with the being seen in 1) subduction in the Makran which stretches from the mouth of the Persian Gulf eastward into Pakistan, and 2) continental collision in the Zagros Mountains, which stretch from the Makran northward into Iraq and Turkey (USGS, 2013).

Figure 3. Seismicity map of Shonbeh epicentral region. Figure 4. Intensity ShakeMap of the Shonbeh event generated by the IIEES. Figure 5. PGA ShakeMaps of the Shonbeh event generated by the IIEES. Figure 6. The isoseismal Intensity map of the Shoneh Bushehr earthquake.

The historical seismicity of the region highlights the 978 CE and 1008 CE earthquakes in Siraf (170 km SE of Shonbeh) on the coastline of the Persian Gulf. The magnitudes assigned by Ambraseys and Melville (1982) were 5.3 and 6.5, respectively. Another historical earthquake struck Siraf in 1883, having an estimated magnitude of 5.8. More recently, on February 17, 2002, a mb5,6 earthquake struck north of Daiyer (70 km SE of Shonbeh) with some local damages to nearby villages. Figure 3 shows historical earthquakes in the region. In the first 20 hours after the main shock, 62 magnitude >3.6 aftershocks hit the epicentral region, mostly east and north of the epicenter.

The intensity and PGA ShakeMaps generated by IIEES are shown in Figures 4 and 5. The shock was felt strongly in Shonbeh (assessed at VII), 18 km SW of the epicenter; Khormoj (assessed at VI+), 22 km NE; and Bushehr (assessed at V), 86 km NW (see Figure 6). The maximum recorded acceleration in this event was obtained in Khormoj (70 gals); the mainshock was recorded in Khormoj (the records are available at the BHRC web site: https://www.bhrc.ac.ir). In Bushehr (80 km NW of the epicenter), where the maximum acceleration was estimated at around 50 gals, there was no damage reported to a nuclear power plant.

Geotechnical Features
Near Shonbeh (4km southeast of the city) there is clear evidence of liquefaction along the Mond River in the Dashte-Zaal ("white plain”), (Figure 7). Some surface tension cracks were induced by liquefaction in the Mond River terrace (Figure 8). These features could be seen along about 3 km of the terrace.

Figure 7. Close-up view of liquefaction in the Dashte Zaal area. Figure 8. Extension cracks and joints in young alluvium river bank deposit in the Dashte Zaal area. Figure 9. Surface fissure (N300-320) in young deposits, east area of Jashk Salt-Diapir. Eight km of this fissure can be seen in the field. Figure 10. Closer view of surface fissures (N300-320) in young deposits.

Surface fissures along the reported causative fault could be observed south of Shonbeh, on the road to Baghan and Bandar-Daiyer (Figures 9 and 10). These en-echelon, non-continuous segmented fissures do not show the displacement consistent with the fault mechanism (compressional with a strike slip movement); however, it is mostly a vertical displacement in most of segment, with some surface strike slip movements (not systematic in the same direction). The segments were located about 1 km distant from the Jashak Salt Diapir and it may be that the salt layers are effective in conducting the rupture to the alluvium surface.

Structural Damage
Most of the buildings damaged in this earthquake are adobe buildings, made of heavy material and constructed in the last 50 years (Figures 11 and 12). The walls collapsed mostly in east-west or NNW-SSE directions. An old abandoned castle (Ghale Daaraab Khaan) located east of Shonbeh (Figure 13) suffered some damages in the main shock, but more damages resulted from an aftershock followed by a heavy rain (100 mm over 12 hours on May 2). A windmill in the center of Shonbeh, evidently the highest structure in the epicentral region, was not damaged in the earthquake or its aftershocks (Figure 14).

Figure 11. Complete destruction in Shonbeh. Figure 12. Destroyed buildings in Shonbeh. Figure 13. The abandoned castle (Ghale Daaraab Khaan) in Shonbeh. Figure 14. The tallest structure in Shonbeh withstood the earthquake and its aftershocks.

Disaster Management
First aid was delivered to the approximately 10,000 inhabitants in the epicentral area on the first night by Iranian Navy forces and by the Iranian Red Crescent. Food (cold, packed and cooked) was delivered on second day after the earthquake. Temporary shelters were put up in the first week after the quake; the removal of debris using heavy machines was ongoing (Figures 15 and 16).

Figure 15. Prefabricated offices to manage the installation of housing units. Figure 16. Heavy machinery removes debris in Shonbeh.

Acknowledgments
Local logistical support was provided by colleagues at the Persian Gulf University.

References
Ambraseys, N. and C. Melville, 1982. A History of Persian Earthquakes. Cambridge University Press.
Euro-Mediterranean Seismological Centre, 2013; www.emsc-csem.org/Earthquake/earthquake.php?id=311828
National Broad Band Network of Iran (BIN, 2013. www.iiees.ac.ir/iiees/English/bank/eng_databank.html#link International Institute of Earthquake Engineering and Seismology.
US Geological Survey, 2013. M6.4-89km SE of Bandar Bushehr, Iran. http://comcat.cr.usgs.gov/earthquakes/eventpage/usb000g2y5