LONG PERIOD GROUND MOTION SIMULATION OF THE 2011 OFF THE PACIFIC COAST OF TOHOKU EARTHQUAKE USING THEORETICAL GREEN'S FUNCTION

Examination based on Inverted Pseudo Point Source Model

Abstract

 We simulated strong ground motions of the 2011 off the Pacific coast of Tohoku Earthquake at periods from 2 to 10 s based on the 3D finite difference method. For the source model, we used the space and time distribution of pseudo point sources by Nozu (2012), and the seismic moment, rake angle, and rise time of 9 point sources were estimated by inverting observation records. The theoretical Green's functions were evaluated from the finite difference method using a 3D deep subsurface model by the Headquarters for Earthquake Research Promotion (2012).

 In order to validate the constructed model, the reproducibility of observation records of rock sites not used in the inversion was examined. It was confirmed that Fourier amplitudes and predominant periods of 2 to 10 s of observed records were generally reproduced. Furthermore, we investigated the reproducibility of 289 observation records from Tohoku to Kanto regions including the area where the sedimentary layers are thick. Although the amplitudes at periods from 5 to 10 s at northern Iwate prefecture are underestimated by 0.5 to 1 times, it was found that the observation records at others were well reproduced. In particular, the maximum Fourier amplitudes over 80 cm/s*s observed in the Kanto plain were satisfactorily simulated. It is thought that evaluating the theoretical Green's function in consideration of the complicated 3D deep subsurface structure led the result of well reproducibility.

 Since the pseudo point source model cannot consider a rupture directivity effect, we performed the numerical experiment on the effect by comparing the simulated ground motions between pseudo point source model and SMGA source model. It was confirmed that the rupture directivity effects are limited at the observation located in the forward direction of the rupture. Because the amplitude at periods from 5 to 10 s in the northern area of Iwate Prefecture is underestimated, it might be necessary to add the point source off the coast of the area. This is a future issue for upgrading the source model. Using the current pseudo point source model, observation records over the wide area from Tohoku to Kanto regions are well reproduced. In particular, in the Kanto plain where thick sedimentary layers are distributed, we were able to simulate the characteristics such as large-amplitude ground motions propagated for a long time after S wave arrival.

 In this mega thrust earthquake, large amplitude with long-period ground motions were observed in Osaka bay area seven hundred kilometers away from the hypocenter (e.g. Sato et al., 2012). In the next step, we are going to apply our pseudo point source model to the wider area from Tohoku to Kansai and try to reproduce long-period ground motions.