Ground motion simulation of an Mw 5.5 earthquake in the Osaka Basin, Japan

Abstract:

An Mw 5.5 earthquake occurred at 13 km depth under the Osaka Basin on June 18, 2018. PGA and PGV reached 900 cm s^-2 and 44 cm s^-1 in the epicentral area. Observed ground motions in the Osaka Basin exhibited a complex wavefield due to the 3D structure of the sedimentary layers of the basin. The basin is filled with the Quaternary deposits that make a large velocity contrast with the bedrock. We conduct a ground motion simulation with a finite source model for this earthquake using 3D velocity structure models of the Osaka basin to study the strong motion generation mechanism. We separate the wavefield into P, SV and SH-wavefields to see their spatio-temporal variation.

The finite source model comprising of two fault planes has been obtained from the inversion of strong motion records up to 2 Hz. The 3D Osaka basin model has been constructed following the strategy of constructing a depth model of isochronous surfaces and converting the depositional age structure into the seismic wave–velocity structure, summarizing depth data from geophysical exploration and seismic observations. Ground motion is calculated up to 2 Hz using the 3D finite difference method for the 3D sedimentary structure for V_S > 350 m s^-1 and the equivalent linear site response method for the Holocene and Pleistocene sedimentary layers.

PGV distribution of the simulation shows large values distributed southwest of the epicenter, which agrees with the observed one. This feature is due to the S-wave radiation pattern of the northeast-to-southwest striking fault plane enhanced by the rupture propagation toward the southwest-up-dip on this fault plane, and further raised by the amplification effect of the sedimentary layers. The calculated ground motion shows a complex wavefield. The later phases are the multiple reflections between the surface and the basin bedrock, and the surface waves generated at the basin edge or the step-like bedrock topography under the sediment due to the active faults. Separation of the wavefield reveals that the ratios of SH and SV-wave components in the wavefield vary by region, which may be due to the relative location of the hypocenter to the sources of the surface waves like the basin edge. Semblance analysis shows that SH-waves tend to shift propagation direction so to cross contours of bedrock depth with a high angle.