Complex co- and postseismic faulting of the 2017-2018 seismic sequence in western Iran revealed by InSAR and seismic data

The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran-Iraq border on 12 November 2017. This M-w 7.3 earthquake was followed by an intense seismic sequence. Implementing the double-difference earthquake location technique, we relocate 1069 events recorded by our local seismic network, deployed after the mainshock. The spatial distribution of the epicenters indicates linear alignments of the events nucleated along at least four notable clusters. The clusters are characterized by at least one significant earthquake, such as the Tazehabad earthquake of 25 August 2018 (M-w 5.9) along a dense, east-west trending cluster and the Sarpol-e Zahab earthquake of 25 November 2018 (M-w 6.3) along the cluster with a northeast-southwest trend. We use two-pass differential SAR interferometry (DInSAR) and Small BAseline Subset (SBAS) methods to study the coseismic permanent displacements of the Azgeleh, Tazehabad and Sarpol-e Zahab events as well as the one-year postseismic deformation field of the 2017-2018 seismic sequence, respectively. We use non-linear and linear optimization algorithms to derive the source geometry and the slip distribution along the fault planes. The inversion is conducted by introducing also seismological constraints, leading to the definition of a listric geometry for the Azgeleh mainshock rupture that accommodates the slip area at depth of 10-16 km along a sub-horizontal plane (dipping similar to 3 degrees) and a low-angle (similar to 16 degrees) ramp. The thrust and dextral movements along this NNW-striking (similar to 345 degrees) fault have triggered a tear fault responsible for the Tazehabad event ruptured an east-west trending (similar to 267 degrees), north-dipping (similar to 78 degrees) sinistral shear fault. We present the dextral slip distribution of the Sarpol-e Zahab event along a NE-striking (similar to 34 degrees) fault, as a synthetic Riedel structure for the southern segment of the Khanaqin fault, dipping 63 degrees to the southeast. We find the postseismic deformation field associated with the seismic sequence is not confined only to the mainshock source (the Azgeleh fault), but also develops along the Tazehabad and Sarpol-e Zahab faults. We additionally propose afterslip along a duplex, flat-ramp-flat structure down-dip and up-dip of the Azgeleh coseismic slip area. The up-dip afterslip develops onto the shallow detachment (similar to 3 degrees) at depth of similar to 8 km and the down-dip afterslip propagate onto the mid-crustal decollement level within the PanAfrican basement. The Azgeleh, Tazehabad, Sarpol-e Zahab and Khanaqin faults mark the Lurestan Arc-Kirkuk Embayment sharp margin in the Northwest Zagros and play a key role in the lateral escape of the Lurestan Salient and vertical strain partitioning in the Zagros front.

Automated summary

The largest earthquake in the Zagros Mountains struck the city of Azgeleh on the Iran-Iraq border on 12 November 2017, followed by intense seismic activity. A total of 1069 events were relocated by utilizing the double-difference earthquake location technique, revealing linear alignments of events in at least four notable clusters. The seismic sequence from 2017-2018 involved significant earthquakes in various clusters, reflecting the complex fault structures in the region.