Inversion Analysis of Site Responses in the Kanto Basin Using Data from a Dense Strong Motion Seismograph Array

We applied nonlinear inversion analysis to estimate site response in the Kanto area using data from 19 moderate-sized earthquakes observed at the 843 stations constituting strong motion accelerograph networks of the Seismic Kanto strong motion network (SK-net), K-NET, and KiK-net. We determined the source parameters (seismic moment, M(0); corner frequency, f(c)) for each event and the quality factor (Q(f)) for the propagation path based on borehole records. We also determined the frequency-independent and reference-independent site responses. The resulting seismic moments are in good agreement with those determined by the National Research Institute for Earth Science and Disaster Prevention (NIED) and are proportional to f(c)(-3). We found a frequency-dependent quality factor for the propagation path: Q(f) = 107f(0.52). The site responses, averaged over 0.5-1.0 Hz, correlate well with shear-wave velocities for the upper 30 m (AVS30; correlation coefficient of -0.63) and with depth to the seismic basement, the shear-wave velocity of which is around 3.0 km/sec. Large site responses are found in areas of unconsolidated sediments, such as swampy land. The predominant frequencies of our site responses roughly agree with those of the horizontal to vertical (H/V) spectral ratio. This indicates that the H/V ratio is valid for estimating predominant frequencies of S-wave amplification.