Depth segmentation of fault slip: deep rupture in the 2011 Van Earthquake leaves shallow hazard

We use InSAR, body-wave seismology, satellite imagery and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse-slip earthquake, in the Turkish-Iranian Plateau. Distributed slip models from elastic dislocation modelling of the InSAR surface displacements from ENVISAT and COSMO-SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40-54 degree dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8--9 km of the surface, consistent with the lack of significant ground rupture. The up-dip extension of this modelled WSW-striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments, and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent. This leaves an unruptured up-dip fault width of 8-11. Given that the surface trace of the fault is clearly visible in the geomorphology of the mountain range to the north of Van, and that fault gouge was found in Quaternary sediments at the surface, it is very likely that the upper portion of the crust is seismogenic. A rupture along a similar fault length of 30 km across the remaining unruptured fault width of 10 km, with a similar average slip of 3 m, would yield a Mw 6.9 earthquake. Vertical separation of slip within the seismogenic crust and a delay in rupture between two along-dip portions of a fault is known to have happened recently in the case of a pair of Mw 6.2 reverse faulting events in the Qaidam basin, NE Tibet (Elliott et al. (2011). The first earthquake occurred in November 2008 with a centroid depth of 18 km. Just ten months later another Mw 6.2 event occurred at almost the same epicentral location, but with a centroid depth of 5 km. The InSAR data collected for this pair of events showed that they were nearly co-planar, occurring up and down dip of each other. The delay was attributed to structural depth segmentation of the seismogenic crust likely resulting from the intersection of an opposing NE-dipping reverse fault plane that intersected the break in slip between these two SW-dipping events. Elliott, J. R., A. C. Copley, R. Holley, K. Scharer, and B. Parsons (2013), The 2011 Mw 7.1 Van (Eastern Turkey) Earthquake, Journal of Geophysical Research, doi:10.1029/2012JB009569 Elliott, J. R., B. Parsons, J. A. Jackson, X. Shan, R. A. Sloan, and R. T. Walker (2011), Depth segmentation of the seismogenic continental crust: The 2008 and 2009 Qaidam earthquakes, Geophysical Research Letters, 38, L06305, doi:10.1029/2011GL046897