[S07-1] Amplification of ground motions and GMPEs

[S07-1-04] Preparation of 1D velocity structure using records from moderate sized earthquakes

^○Subeg Bijukchhen¹, Nobuo Takai¹, Michiko Shigefuji², Masayoshi Ichiyanagi¹, Tsutomu Sasatani¹ (1.Hokkaido University, 2.Kyushu University)

Kathmandu Valley is a tectonic basin filled with thick (~600 m) fluvio-lacustrine sediments. It carries a history of many damaging earthquakes occurring in and around Nepal Himalaya; 2015 Gorkha Earthquake being the latest one. Its thick unconsolidated sediment and location in seismically active region has made a large population vulnerable to earthquakes. An understanding of underground structure is necessary for mitigating the earthquake damage. We've used earthquake records from four strong motion seismometers to estimate the 1D velocity structure of the valley. Moreover, records from six additional temporary seismic stations are also used for the purpose. This is a step towards preparation of 3D basin structure of the valley. We considered the records from station KTP, installed over rock site, as input motion and simulated the observed waveform of sediment sites by Propagator Matrix Method. The initial velocity model for the simulation was prepared by consultation of available borehole logs, geological map, and geological cross-sections of the Kathmandu Valley. We fixed the average velocity and density of the geological layers based on previous studies. The models were then adjusted to fit the simulated and observed waveform of a moderate earthquake (mb4.9) occurred in 2013 August 30. We then checked the adjusted models with two Mw5.1 aftershocks of the Gorkha Earthquake with encouraging agreement between observed and simulated waveform. We also used the Diffused Field Theory Method to cross-check the 1D velocity models. Since, the temporary sites were installed after the Gorkha Earthquake, we tried adjusting the velocity models using the DFT method and checking them with Propagator Matrix Method for another moderate sized (mb5.5) earthquake resulting in good agreement. The adjusted velocity models show bedrock depth at 150-480 m indicating an uneven basin topography of the valley.