Relocation of the 2008 and 2012 earthquake sequences on the Zagros Foredeep fault (SE Iran/Iraq border).

Abstract:

In 2008 and 2012 two earthquake aftershock sequences occurred on the SE border between Iran and Iraq. Initial locations placed these events on a portion of the Zagros Foredeep Fault that is thought to be largely aseismic. The Mw 5.7 main shock in 2008 had a strike slip mechanism while the 2012 Mw 4.7 main shock had a thrust mechanism. Ground deformation due to the 2008 event was observed using Interferometric Synthetic Aperture Radar. We inverted these InSAR data to find the geometry of the fault and the distribution of slip on the fault plane. Whilst the inversions provide a well-determined fault plane geometry that is consistent with seismological solutions, the geodetic moment is larger than estimates of seismic moment, likely due to significant triggered asesimic slip. We estimated depths for 36 of the 89 earthquakes in the two aftershock sequences either by inverting the surface wave amplitude spectra or by picking the pP depth phase. The estimated depths ranged from 4 -13km. Using these estimated depths and the InSAR inversion results to fix the location of the 2008 main shock we relocated the aftershocks using a joint epicentral determination method. This relocation splits the seismicity into two distinct clusters, with the 2008 seismicity ~20km to the SE of the 2012 cluster. The location of the 2012 cluster suggests it occurred on the Zagros Foredeep Fault. Linear structures can be seen in the relocated 2012 seismicity, which show some time dependence. The majority of the 2012 events all have similar thrust focal mechanisms, and our estimated depths suggest that the 2012 aftershock sequence occurred on a thrust within the sediment cover that does not cut the basement. High resolution satellite imagery has also revealed a right-lateral stream offset in the region of the 2008 earthquake sequence, providing evidence that strike-slip faulting in this area is not blind, in contrast to the majority of faulting in the Simply Folded Belt.