Imaging Ground Motions in the Tokyo Metropolitan Area Based on MeSO-net: Azimuth Verification for Seismometers and Transfer Function Estimation for Site Effects

Abstract:

Prediction of structural motions during large earthquakes is important to prevent secondary disasters. To evaluate such strong motion as accurately as possible, it is essential to infer the image of ground motion in the target area based on densely installed seismological networks. In the Tokyo metropolitan area of Japan, the dense seismological array “MeSO-net” was established in 2007, and has approximately 300 stations with several kilometer intervals. Mizusako et al. (2014, AGU) applies lasso, which is a linear regression modeling method using the L1 regularization, to the MeSO-net data during the 2011 off the Pacific coast of Tohoku Earthquake to infer the spatially-high-resolution strong motions in the metropolitan area. Their method succeeds in reproducing the waveforms up to much higher frequency component than previous studies. However, there are two topics to deal with before practical use of their study. The first topic is that real azimuths of MeSO-net seismometers installed after 2009 have not been verified, while those installed in 2007 and 2008 were already verified based on cross correlation with nearby tiltmeters of Hi-net and/or seismometers of F-net (Shiomi et al., 2009). Since azimuths of seismometers obviously affect the data processing, we evaluate the azimuths of seismometers following Shiomi et al. (2009). The second topic is that we cannot directly obtain the ground motion data on surface since MeSO-net seismometers are installed at 20m depth. We have been also developing the method to estimate transfer functions that convert strong motion at 20m depth to that on surface, by utilizing continuous observations obtained both on surface and at 20m depth at two stations, and short-term observations obtained above the boreholes at more than 100 stations. A combination of this vertical transformation method and the horizontal estimation method (Mizusako et al., 2014), enables us to infer an image of ground motions in the whole Tokyo area.