Geodetic Constraints of the 2017 M_w7.3 Sarpol Zahab, Iran Earthquake, and Its Implications on the Structure and Mechanics of the Northwest Zagros Thrust‐Fold Belt

Abstract

We reveal transient surface deformation following the 2017 Mw7.3 Sarpol Zahab (Iran) earthquake using Interferometric Synthetic Aperture Radar (InSAR) measurements. Based on the coseismic interferograms derived from the Advanced Land Observing Satellite‐2 (ALOS‐2) data, the preferred slip model of the earthquake has a centroid depth of 14.5 ± 4 km and suggests that a basement fault is most likely responsible for the 2017 earthquake in the northwest Zagros fold‐thrust belt zone. Two slip asperities with a maximum slip of 6 m separated by 16 km are observed in the best fitting slip model. The accumulated afterslip in the first month after the mainshock determined from the Sentinel‐1 postseismic interferograms reveals a slip distribution that lies immediately updip of the coseismic slip, implying that frictional properties of the fault vary along the rupture patch. The Bamo Mount in the earthquake area was uplifted by approximately 1 m during the earthquake. We suggest that local topographic growth from both seismic and aseismic faulting behaviors is common within the Zagros fold‐thrust belt zone based on our findings regarding the 2017 earthquake and other significant events in the region.

Impulsive Source of the 2017 MW=7.3 Ezgeleh, Iran, Earthquake

“…The posterior mean model of the final cumulative slip is shown in Figure 4a. At first order, this solution is in agreement with previously published static models (Barnhart et al, 2018;Wanpeng et al, 2018). We infer a ∼50-km-long and ∼30-km-wide rupture, with a peak slip of 5.5 ± 0.5 m. One difference arises as previous models proposed that two distinct asperities ruptured during the earthquake.…”

“…Although the subvertical plane is compatible with a back thrust fault that may exist in the region (Tavani et al, 2018) or with the reactivation of steep normal faults (Jackson, 1980), the shallow-dipping plane is in better agreement with receiver functions analysis (Paul et al, 2010) and the tectonic setting (e.g., Berberian, 1995;Vergés et al, 2011). Our optimal plane (351 • strike, 14 • dip, 13-km depth) agrees well with other studies using a similar grid search approach (Barnhart et al, 2018;Wanpeng et al, 2018). In the following, we will consider that the Ezgeleh earthquake occurred on our optimum shallow dipping plane.…”

“…As suggested by previous studies (Barnhart et al, 2018;He et al, 2018;Wanpeng et al, 2018), the Ezgeleh earthquake likely occurred on the Mountain Flexure Fault (sometimes referred to as Main Front Fault, noted MFF in Figure 1). Along the major part of the Zagros belt, the MFF follows a NW-SE axis with a ∼ 120 • azimuth and is aligned with many topographic features (visible on the DEM in Figure 4).…”

“…We use a Bayesian framework to infer a population of coseismic slip models that fit available observations. While currently available studies were either limited to the static final distribution of slip on the fault (Barnhart et al, 2018;He et al, 2018;Vajedian et al, 2018;Wanpeng et al, 2018;Yang et al, 2018) or used far-field teleseismic data Nissen et al, 2019), we jointly invert interferometric synthetic aperture radar (InSAR) and near-field strong-motion data which provide a better resolution (Anderson, 2003) to propose a kinematic description of the earthquake source. We use a layered velocity model that is routinely used to locate earthquakes by the Iranian Seismological Center, which ensures modeling is performed to the best of our knowledge (supporting information Table S1).…”

The 2017 Mw 7.3 Sarpol Zahāb Earthquake, Iran: A compact blind shallow-dipping thrust event in the mountain front fault basement

“…3 on a shallow dipping within the Arabian crystaline basement, while afterslip concentrated up dip of the rupture zoned on the basal decollement. Using similar geodetic data, Feng et al, (2018) found two large-slip asperities with a maximum slip of 6 m at ~15 km depth. In this study, we use both C-band Sentinel-1 and L-band ALOS-2 data with multiple viewing geometries to generate the coseismic ground deformation maps.…”