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 M-w 6.9 Iwate-Miyagi Nairiku earthquake, which occurred in the Tohoku region, Japan. We extract Green's functions between pairs of stations by regarding seismometers located in the vicinity of the main shock fault plane as virtual sources and others as receivers. We calibrate the amplitude of the extracted Green's functions using records of a M-w 5.0 aftershock with a reverse-faulting focal mechanism similar to that of the main shock. We use scaling relations between small and large earthquakes to construct several finite fault models that are used together with the extracted Green's functions to simulate the ground motions of the main shock. Among the different models, a uniform finite source model with a rupture velocity of 1.95 km/s combined with the Green's 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.