Measuring Crustal Deformation Caused by the Nepal (Gorkha) Earthquake Using ALOS-2 SAR Interferometry

Abstract:

A huge earthquake (Mw 7.8, USGS) occurred on 25 April, 2015 in Nepal, followed by the largest aftershock (Mw 7.3, USGS) on 12 May. We applied an InSAR technique to detect crustal deformation caused by the earthquakes using L-band SAR data acquired by ALOS-2.

One of the advantages of ALOS-2 over ALOS is ScanSAR interferometry. Beam synchronization, which is a critical factor for ScanSAR interferometry, is always tuned among ALOS-2 observations. While a width of 350 km is covered by a ScanSAR acquisition, the provided data is divided into five swaths with each width of 70 km. Adjacent swaths have an overlapping area with a width of several kilometers, where interferometric phases for two swaths are basically comparable. We processed each swath independently.

Preliminary interferograms are contaminated by noises with long wavelength, which makes it difficult to measure the amount of the crustal deformation accurately. We reduce the noises by following steps. First, pseudo reference points with no displacement are put at intervals of 40-100 km outside of the possible deforming area. The differential phase at the points are assumed to be due to the noises. The curved surface of the noises is estimated by smoothly interpolating the differential phase at the points. Note that the pseudo reference points should be located in a high coherence area where the phases can be unwrapped. Furthermore the points in the overlapping area between adjacent swaths allow to preserve consistency of the phases between adjacent swaths.

The interferograms show the clear and detailed crustal deformation (published on http://www.gsi.go.jp/cais/topic150429-index-e.html). The maximum of a quasi up-down component of the deformation estimated from the interferograms with different beam directions reaches over 1.4 m uplift at 20 km northeast from Kathmandu and 0.6 m subsidence at a northern part of the deforming area.