Estimation of Bedrock Depth in the Kathmandu Valley, Nepal, Using Long-Term Ambient Noise and Teleseismic Data

Date: 4/26/2019

Time: 11:30 AM

Room: Pine

Long-period strong ground motions were predominantly observed in the Kathmandu Valley, Nepal during the 2015 Gorkha earthquake (Dixit et al., 2015; Bhattarai et al., 2015; Takai et al, 2016). Analysis of aftershocks and gravity data indicated that there is a strong S-wave velocity contrast between sedimentary layers and bedrock in the valley (e.g., Dhakal et al., 2016; Bijukchhen et al., 2017; Pradhan et al., 2018). We have conducted microtremor array explorations with broadband seismometers and determined phase velocities of Rayleigh wave in the lower frequency range (< 1 Hz) to estimate the depth to bedrock (Bhattarai et al., 2017; Yokoi et al., 2018), but the dispersion characteristics below 0.2 Hz were unclear due to weak power of ambient noise. In order to explore the surface-wave phase velocity below 0.2 Hz for better estimation of bedrock depth, we deployed temporary continuous seismic observation stations (totally eight broadband stations) in the valley from February 2018 to present. The station-to-station intervals are 4.6 - 14.7 km. During the observation period, signals from shallow local (M > 4) and teleseismic events (M > 6) were clearly detected and we used surface-wave component of the recordings for the estimation of Rayleigh-wave phase velocities in the lower frequencies. The estimated phase velocities show dispersive characteristics between 0.01 and 0.13 Hz and the phase velocity at higher frequency side can be connected with the estimated ones from our microtremor explorations (> 0.2 Hz). We confirmed that the estimated phase velocities are comparable with those derived from the newly proposed seismic velocity structure model of Nepal (Yamada et al., 2018). We also examined dominant frequencies of ambient noise H/V peaks as well as site amplifications at our target sites, and estimated surface-wave group velocities inside Kathmandu Valley using seismic interferometry. Our results indicate that the maximum depth to the bedrock could be 700 - 800 m.

Presenting Author: Takumi Hayashida

Authors