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Seismic Site Effects During 2016 Kumamoto Sequence Using Source-to-Site 3D Physics-Based Simulations

Session: Physics-Based Earthquake Rupture Modeling and Strong Motion Simulations I

Type: Oral

Date: 4/22/2021

Presentation Time: 02:45 PM Pacific

Description: 

Local site effects are evident in near-field ground motion records during the 2016 Kumamoto (Japan) seismic sequence. Micro-tremor studies reveal that fundamental peak frequencies for amplification range from 1-4 Hz and do not fit a specific pattern (Kawase et al., 2017). This can be related to fact that shallow layers consist of thin, soft sediments (shear wave velocity, Vs, of 100-500m/s) with varying thickness (0-60m). In this study, we carry out 3D physics-based simulated ground motions for three selected earthquakes of the Kumamoto seismic sequence, namely Mw7.0 mainshock, Mw6.2 foreshock an Mw5.5 aftershock, respectively, and considering a 1D velocity model (minimum Vs of 500 m/s) along with surface topography. The open-source spectral element code SPEED (http://speed.mox.polimi.it/) is used for the numerical simulations. For the MW7.0 mainshock, the seismic source is modelled kinematically as a complex fault system consisting of 6 segments with variable strike and dip angles (Kobayashi et al., 2017). Fling step-like pulses found in displacement waveforms derived from GNSS records are accurately captured in simulations, proving the reliability of source model. Broadband ground motions are then generated (Paolucci et al., 2018) by enriching the low-frequency waveforms simulated by SPEED in the high-frequency part using an artificial neural network, trained on earthquake recordings, and then, validated against near-source strong motion data.

We further attempt to perform detailed site-response analysis at KiKNet stations, where seismo-stratigraphic information is available, using simulated broadband signals as input at stiff soil outcrop and considering non-linearity of soil. This coupling of broadband simulations with soil response analysis is useful to study the spatial variability of local site effects and explain the uneven pattern of building damage (Yamada et al.,) near Kumamoto source region.

Presenting Author: Srihari Sangaraju

Student Presenter: Yes

Authors