Observation of the Gorkha, Nepal earthquake of April 25, 2015 with ALOS-2/PALSAR-2

Abstract:

The Gorkha, Nepal earthquake (Mw7.8) of April 25, 2015 is the first event in the Himalaya region that occurred after the development of modern space geodetic technology. Since the epicenter of this earthquake is located in a mountainous and rural area where few continuous GNSS observation is made, satellite remote sensing is required to reveal crustal deformation and damage. The Japan Aerospace Exploration Agency conducted emergent observations of ALOS-2/PALSAR-2 that has been in operation since summer in 2014.

We analyzed acquired images including ScanSAR images whose swath width is about 350 km using Gamma software with a digital ellipsoidal height model based on SRTM DEM. Coherence of all interferograms is very high. However, we found two spots of low coherence in Kathmandu, including the Durbar Square. These spots indicate structural damages or liquefaction.

ScanSAR-ScanSAR interferograms contain large phase ramp, possibly due to ionospheric disturbance or inaccurate orbit. We corrected phase ramp using quadratic function of latitude, longitude and altitude. Corrected interferograms show an elliptic zone of LOS decrease extending from the epicenter ESE-ward with a peak of > 1 m. North of the uplift zone, we found LOS increase of ~0.8 m. This result is consistent with thrust motion on a low-angled fault plane. We inverted corrected interferogram to estimate slip distribution and dip angle of fault. Preliminary results imply that the thrust motion is concentrated at depth between 10 and 20 km and the maximum slip is ~ 4.4 m. Geodetic moment is estimated to be 1.47 x 10^21 Nm (Mw8.0).

We also processed a postseismic pair and detected coseismic deformation of the May 12 aftershock (Mw7.3). Area of uplift with a diameter of 30 km is found and that of subsidence on its north. It also is consistent with low-angled thrust. It is hard to recognize postseismic deformation of the mainshock possibly due to the correction of phase ramp.

ALOS-2/PALSAR-2 images were provided by JAXA through EQWG and ALOS-2 CVST. Copyright and ownership of ALOS-2/PALSAR-2 images belong to JAXA.