The 2008 and 2012 Moosiyan earthquake sequences; rare insights into the role of strike slip and thrust faulting within the simply folded belt (Iran)

The Zagros mountain belt has an unusually large discrepancy between seismic and geodetic strain rates, implying very large aseismic release of strain. However, the spatial and depth relationship between seismic and aseismic deformation is poorly understood and controversial, with important implications both for understanding the role of aseismic deformation in regions of continental convergence and for characterizing seismic hazard for large urban populations in this region. Two recent earthquake sequences in 2008 and 2012 provide us with an ideal opportunity to use geodetic and seismological data to address this topic, not only for thrust faulting but also for rarer strike-slip faulting in the Zagros. These aftershock sequences occurred on the southeast border between Iran and Iraq. We use Interferometric Synthetic Aperture Radar (InSAR) to obtain a mechanism for the 2008 mainshock and observe significant aseismic slip accompanying the earthquake. This aseismic slip occurred along strike from the seismic asperity and approximately doubled the seismic moment release, demonstrating that aseismic slip plays an important role for strike slip as well as thrust faulting in the Zagros. Depths are calculated by inverting surface-wave amplitude spectra and depth phase observations, demonstrating that all events occur at depths less than 12 km, supporting the view that seismicity predominately occurs in the sedimentary cover in the Zagros. Using both the calculated depths and InSAR location, we relocate the aftershocks to reveal two distinct clusters, with the 2012 cluster occurring in the vicinity of the Zagros foredeep fault, challenging the traditional view that this region is aseismic. Electronic Supplement: Figures of Interferometric Synthetic Aperture Radar (InSAR) data, model, and resolution.