Preliminary report of April 16 2013 North Saravan strong Earthquake (Mw=7.8), SE Iran

Description

  Preliminary report of April 16 2013 North Saravan strong Earthquake (Mw=7.8), SE Iran Seismotectonic and Seismology Department (SSD), Zahedan and Kerman branches of Geological survey of Iran The North Saravan seismic event of April 16 2013 (Mw= 7.8: USGS) occurred at 10:44:17 UTC (15:14:17 local time on 27 Farvardin 1392) within Sistan-Baluchestan province, SE Iran. The instrumental epicenter of the event given by IRSC is 28.04° N, 62.03° E; located north of Saravan city with 82 km in depth (USGS) (Figure 1).Generally, this earthquake occurred in a sparsely populated area. The epicenter of this event is located within a fault zone trending mainly NW-SE, north of Makran E-W trending subduction zone. The fault zone contains however some secondary ~E-W and N-S structural trends. Generally, the NW-SE Saravan fault system seems that formed as a resultant of    N-S major dextral faults of East Iran and E-W, mainly thrust faults of Makran subduction zone.   Figure 1: Epicenter of April 16 2013 North Saravan earthquake (Mw= 7.8: USGS) from IRSC (crossed green rectangle). According to the location of epicenter, the seismic event occurred potentially within a fault zone trending NW-SE; Saravan fault zone (violet rectangle). This earthquake felt in a far distance to the epicenter because of deep focus. Considering the epicenteral location of the earthquake (e.g. IRSC), the seismic event could be caused beneath the Saravan fault system.  The fault is a major NW-SE active fault zone (length ~ 250km) in SE Iran (e.g. Hessami et al., 2003; IIEES) (Figures 1 and 2). Figure 2: NW-SE trending Saravan fault, south of Zahedan City (length~ 250 km) in Active faults map of Iran (Hessami et al., 2003; IIEES). According to Normal mechanism of ~ENE-WSW faulting (N 60°-70° E) presented in focal solutions (e.g. USGS, 2013: GFZ, 2013) (Figure 3) and depth of the event (more than 50 km), it could be potentially caused by extension-brittle deformation concentrated in slab zone of Makran subduction zone (Figures 4 and 5).     Figure 3: Focal solution of the April 16 2013 North Saravan strong earthquake from deferent sources.   Makran subduction zone contains one of the largest accretionary wedges on the globe, formed by the convergence between the Eurasian and the Arabian Plates. It is characterized generally by a shallow subduction angle (e.g. Kopp et al., 2000; Tectonophysics) (Figure 4). According historical (e.g. events of; 1765, 1851 AD) and instrumental seismicities (e.g. events of; 1945, 1983), Makran region is known as an active seismic zone. In General view, the western and eastern portions of the zone are located in Iran and Pakistan territories, respectively. Seismologically, strong earthquakes within Makran have long return periods, especially in Iranian portion. Review of earthquakes mechanism’s occurred within Makran could be show two different types of reverse-shallow (in coastal portion, to the south) and mainly Normal-deep seismic events (in northern parts).            Figure 4: Makran subduction zone, formed by the convergence between the Eurasian and the Arabian Plates                                   (Shah-hosseinia et al., 2011; Marine Geology)   Figure 5: The location of April 16 2013 North Saravan strong earthquake in a cross section of Makran subduction zone                        (The two possible hypocenters are represented (50 km or 82 km deep) By M.-A. Gutscher; EMSC website, 2013). Based on preliminary field observations, we found however just a short (~ 200m) systematic fissure trending NW-SE along the Saravan fault as a reactivated structural trend. The pattern of this secondary surface feature is En-echelon left-stepped with a steep normal vertical component (Figures 6 and 7). During the seismic event it is however southern block which has been uplifted (Figure 7).   Figure 6; The En-echelon left-stepped secondary surface feature triggered by the strong seismic event of April 16, 2013 (Mw=7.8, depth=82 km) found along the NW-SE Saravan fault trace. Dip-direction of the normal vertical displacement seen on this structural trend is however contrary to the main kinematics of the Saravan fault marked in topography. Photo; GSI Zahedan branch. Figure 7; Normal striations and small displacement observed on steep fracturing along the triggered structural trend of Saravan fault (view to the SW). Photo; GSI Kerman branch. .