Loïc Viens

Long-period ground motion simulation of a subduction earthquake using the offshore-onshore ambient seismic field

Large earthquakes that occur in subduction zones are likely to generate long-period ground motions that can cause severe damage even at great distances from the epicenter. We extracted surface-to-surface impulse response functions from the ambient seismic field recorded by offshore ocean bottom seismometers located atop the Nankai subduction zone and onshore stations. We showed that these offshore-onshore impulse response functions can be used to accurately simulate the long-period ground motions generated by an offshore moderate subduction earthquake. Moreover, we also found that the distributions of the earthquake and impulse response function pseudovelocity response spectra have similar maximum amplifications in the same area close to the earthquake epicenter. This suggests that the ambient seismic field recorded by the increasing number of ocean bottom seismometers around the world can be used to assess seismic hazard related to offshore subduction earthquakes without prior knowledge of the velocity structure.

Simulations of long‐period ground motions from a large earthquake using finite rupture modeling and the ambient seismic field

Long-period ground motions generated by large earthquakes slowly attenuate with distance and can be significantly amplified by local velocity structures even at large distances. We take advantage of the wave propagation information carried by the ambient seismic field to simulate the long-period ground motions of the 2008 Mw 6.9 Iwate-Miyagi Nairiku earthquake, which occurred in the Tohoku region, Japan. We extract Greens functions between pairs of stations by regarding seismometers located in the vicinity of the mainshock fault plane as virtual sources and others as receivers. We calibrate the amplitude of the extracted Greens functions using records of a Mw 5.0 aftershock with a reverse-faulting focal mechanism similar to that of the mainshock. We use scaling relations between small and large earthquakes to construct several finite fault models that are used together with the extracted Greens functions to simulate the ground motions of the mainshock. Among the different models, a uniform finite source model with a rupture velocity of 1.95 km/s combined with the Greens functions extracted using one virtual source station located at the center of the fault plane yields the best fit between the simulated and observed long-period ground motions. This study demonstrates the potential of the ambient seismic field combined with finite source modeling to assess the seismic hazard related to long-period ground motions that could be generated by forthcoming large earthquakes.