Postseismic to coseismic moment ratios of the 2016 moderate earthquakes along the Chaman Fault, Pakistan

Abstract:

The Chaman Fault System is a left-lateral transform boundary between the India and Eurasian plates and is known for its low seismicity, which has been attributed to either a long recurrent interval of large earthquakes or aseismic creep (Ambrasseys and Bilham, 2003).

Few geodetic measurements have been performed along the fault, but recent developments of InSAR have allowed to examine the crustal deformation around the fault system. Furuya and Satyabala (2008, we hereafter call FS2008) detected a long-lasting afterslip due to a M5.0 earthquake in 2005, whereas Fattahi and Amelung (2016) and Barnhart (2016) proposed the creeping zone and locked zone, by analyzing much wider and temporally longer coverage.

Since the 2005 M5.0 event, no larger earthquakes have indeed been reported. On May 13 and July 10, 2016, however, moderate earthquakes occurred along the Chaman fault, which were the largest earthquakes since the 2005 M5.0 event but located ~150 km further to the south. According to USGS report, the 1st event on May 13 consists of triplet at almost the same hypocenter with Mw 5.2, 4.7 and 5.5, whereas the 2nd event ~20 km to the north-east was mb 4.7.

The purpose of this study is to examine if the earthquakes accompany with afterslip like what reported by FS2008. Moreover, we estimate the post-seismic and coseismic moment ratios for these moderate earthquakes, which have been estimated for only a few cases and have implications for a size dependence in the dynamics of earthquakes or in the frictional properties of fault areas (Alwahedi and Hawthorne, 2019).

Based on Sentinel-1 images, we generated 55 interferograms and performed SBAS-type time-series analysis (Berardino et al., 2002; Schmidt and Bürgmann, 2003) to reveal the pre-, co- and post-seismic deformation. Based on our preliminary estimated slip distributions, the post-seismic to co-seismic moment ratio for the event 1 was derived to be 0.52±0.51, whose large uncertainties are due to the significant overlap of the deformation signals with uncorrected atmospheric signals. On the other hand, the evolution of the post-seismic moment for the smaller event 2 is clearly systematic, and the post-seismic to co-seismic moment ratio turned out to be 7.33±2.33. This surprisingly large record-breaking value needs more careful error analyses that will be presented at the meeting.