InSAR and seismic analyses of the 2014-15 earthquake sequence near Bushkan, Iran: Shallow faulting in the core of an anticline fold

From 2014 December 30 to 2015 January 10, a moderate earthquake sequence shook the region 25 km southeast of the village of Bushkan within the central Zagros Mountains, Iran. The Bushkan earthquakes produced only light damage and had teleseismic magnitudes in the range of m b 3.9-to-5.3 (United States Geologic Survey, USGS). The first and largest event produced a clear interferometric synthetic aperture radar (InSAR) signal and several other events excited seismic signals that were well observed at regional and teleseismic distances. Sentinel-1 InSAR observations sampled the region several times during the sequence and indicate that a majority of the deformation was caused by the M W 5.1 (Global Centroid Moment Tensor catalogue, GCMT) event that occurred on 2014 December 30. We resolve an oblique-slip reverse fault plane dipping steeply to the northeast that is consistent with GCMT and Iranian Seismological Centre (IRSC) moment tensor solutions. Although the depth extent of the slip is difficult to resolve from InSAR data alone, weak but clear teleseismic P waveforms suggest a shallow centroid depth that helps bound the InSAR constraints. Combined, the observations suggest that the Bushkan earthquakes result from high-angle reverse faulting with a small right-lateral strike-slip component within the sedimentary section of the central Zagros. The shallow depth of roughly 5 km and steep fault reveal seismogenic deformation near the northwest tip of a large, well-developed anticline, possibly indicating a response to deformation within the core of this feature. More generally, the observations illuminate the opportunities provided by the joint analysis of satellite and seismic observations from even modest-magnitude, shallow (less than 10 km depth) earthquakes. Future, routine modelling of such events could help integrate geophysical analyses with smaller-scale tectonic investigations, which are important to understanding the development and evolution of significant geologic structures.