Line of Sight Displacement from ALOS-2 ScanSAR Interferometry: Burst Alignment and the Mw 7.8 Gorkha, Nepal Earthquake

Abstract:

A challenge for traditional InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions exceed the typical swath width of 70 – 100 km. This problem is addressed by the ScanSAR mode of the ALOS-2 satellite’s PALSAR-2 instrument, which enables a repeat time of 2 weeks and a swath width of 350 km. We present InSAR line-of-sight displacement observations from ScanSAR images acquired within one week of both the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock. The Mw 7.3 aftershock extended the rupture area of the main shock toward the east, but also left a 20 km gap where the fault has little or no co-seismic slip. We estimate this un-slipped fault patch has the potential to generate a Mw 6.9 event. Our results are freely available online at http://topex.ucsd.edu/nepal, and we encourage their use in models of the fault slip and associated stress changes. We also discuss burst alignment of the PALSAR-2 ScanSAR images and show that while images acquired after February 8, 2015 are burst-aligned with each other, prior to that date the alignment drifted according to a sinusoidal function. We present an accurate formula for determining the burst offset between two scenes; the date ranges for burst alignment with post-February 8 images are July 22 – July 31, November 8 – November 17, December 16 – December 23 2014, and January 20 – January 29 2015.