INSAR MEASUREMENTS OF SUBSIDENCE AND EARTHQUAKE SWARMS IN THE WESTERN PAKISTAN

In recent years several cities in western Pakistan have experienced unprecedented level of subsidence attributed mainly to groundwater withdrawal. But this region is tectonically also very active and is home to several regional strike-slip faults including the north-south striking left-lateral Chaman fault system. Several large earthquakes, including one fatal 6.4 magnitude (Mw) earthquake have caused major damages in this region. This study integrated InSAR results with several 2D seismic and gravity datasets to quantify the impact of the tectonic and anthropogenic processes on subsidence and earthquake pattern in this region.

The combined influence of anthropogenic and tectonics on surface deformation is rarely studied, due to complexity involved in combining the different datasets. For this research, twelve Phased Array L-band Synthetic Aperture Radar (PALSAR) images acquired by Advanced Land Observing Satellite (ALOS) from 2007 – 2010 and forty ENVISAT Advanced Synthetic Aperture Radar (ASAR) images spanning from 2003 – 2010 were utilized. Small Baseline Subset (SBAS) techniques were used to investigate surface deformation. Five seismic lines totaling ~60 km, acquired in 2003, were used to map the several blind thrusts beneath Holocene alluvium. Presence of these blind thrusts is confirmed by gravity data.

The InSAR SBAS multi-temporal analysis results support groundwater withdrawal to be the dominant source of subsidence with some contribution from faulting. InSAR data also shed light on pre-, co-, and post-seismic displacement pattern for the earthquake swarms of 2008 across two strike-slip faults. Three zones were chosen at the locations of earthquake swarms, all of these three zones display significant increase in post-seismic slip rates. Results of this work are helpful for future land-use planning and earthquake risk mitigation efforts in western Pakistan.