Seismic Investigations of the Central Nepal Region Using Teleseismic and Local Earthquakes

Abstract:

Central Nepal encompasses a tectonically interesting and hazardous region. The general structure of the Himalayan orogen is known, but the details are not. These details are important when it comes to understanding the large thrust earthquakes that can occur in the region, such as the Mw7.8 25 April 2015 event. We take two approaches to add to the community knowledge about the structure of this area. First, we make new measurements on the existing IRIS Passcal Himalayan Nepal Tibet (HIMNT) data set. This array was operated from late 2001 through 2002. We collect two-station phase measurements for Rayleigh waves from teleseismic events at periods of 25 to 125 s. We estimate the uncertainty in these measurements, and invert the weighted measurements to produce phase-velocity models covering an area from 85˚ to 88˚ longitude, and 26˚ to 30˚ latitude. We evaluate the models on several grid sizes, and select the preferred model based on data coverage and uncertainty estimates.

The second approach is to better constrain the focal mechanisms and depths of aftershocks of the 25 April 2015 event. We use the Cut and Paste (CAP) method with locally and regionally recorded waveforms (<30˚ distance), as well as teleseismic body waves. We use a 1-D velocity model based on the phase-velocity results from HIMNT, and experiment with varying this 1-D model for different event-station paths. We use both body waves and surface waves, where possible, and perform a grid search over depth. The dip and depth of the focal mechanisms provide important information on the geometry of the plate interface, and potentially of smaller thrusts, and add constraints on the interpretation of the phase-velocity models.