Using P-wave tomography to image the crustal structure of the Main Himalayan Thrust in central Nepal

Abstract:

In this study, we use arrival time data from local aftershocks occurring between June 2015 and April 2016 following the M7.8 Gorkha earthquake to determine a 3D Vp velocity structure and hypocenter location in the upper crust of the Main Himalaya Thrust (MHT). We consider a subset of well located events with at least six P arrivals and with an azimuthal gap less than 180° that were recorded in central Nepal on a network of 45 seismic stations, called the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake (NAMASTE). Several synthetic tests were implemented to assess the resolution of the data and clear structure can be recovered up to a depth of 20 km. Preliminary results from the tomographic images reveal that the 3D velocity structure beneath this region is heterogeneous with several regions of high and low velocity zones. At 5 km depth, a pronounce low velocity anomaly sits along the Main Boundary Thrust (MBT) and this low velocity zone appears to migrate to the northeast as we go deeper. In addition, cross section of the Vp model perpendicular to the MHT reveals two parallel sub-horizontal features that are 10 km apart with velocities 4 per cent slower than average. Overall, the velocity contrast obtained from the aftershock arrival times is in agreement with surface geological studies and a duplex along the MHT (Mendoza et al., 2019).