Simulation of instrumental intensities in the Tokyo Metropolitan area using a 3D attenuation structure model.

Abstract:

In recent years the development of dense seismic networks in Japan has enabled high quality observations of instrumental intensities. However, the distribution of intensities of historical earthquakes can only be retrieved by the damage reports on historical documents. Their epicenter and magnitude can be roughly estimated from the intensity distribution, assuming that seismic intensity decays with distance. This approximation is not always accurate, since the amplitude of short period ground motion decays with focal distance and is affected by the 3D attenuation structure along the path and in addition displays frequency dependence. In order to estimate the location and size of a large historical earthquake, we need to accurately simulate the seismic intensity distribution, accounting for non linear attenuation of seismic waves along the path. The instrumental seismic intensities inside the Kanto basin observed at the Tokyo Metropolitan Seismic Observation network (MeSO-net) and Hi-net stations display unusual distribution patterns, with peak intensities observed several km away from the epicenter rather than at the stations closer to it. In order to understand the source of this intensity distribution, we estimated the theoretical instrumental intensities using a 3D attenuation structure and compare it to the observed intensity distribution. We first estimated a 3D attenuation structure using the spectral decay of seismic waves, by fitting the observed seismic wave spectrum to a theoretical spectrum using an ω² model. The obtained model suggests Q_s values of 50˜100 inside the Kanto basin and low Q_s values < 300 in the area where the Philippine Sea plate meets the upper part of the Pacific plate. We then use an ω² model in order to estimate the source acceleration spectrum of several earthquakes occurring below the Kanto basin at depths ranging 30~80 km. Our simulation shows that earthquakes occurring on the Pacific plate pass through the low Q_s area inside the Philippine Sea plate and are attenuated significantly. The estimated instrumental intensity distribution at the MeSO-net stations for these earthquakes largely coincides with the observed seismic wave intensity distribution. The implementation of our findings could help towards a better understanding of the damage area of historical earthquakes.