[SSS14-P22] Shallow soil effects for contrasting PGAs at nearby strong-motion stations dring the Mj 5.2, 2017, Southern Akita Prefecture earthquake
Keywords:Site amplification, Shallow soils, Strong-motions
The shallow velocity structure model at the recording sites provided by National Research Institute for Earth Science and Disaster Resilience (NIED) shows that the S-wave velocities of the top five meters of the soil columns are significantly different between the two stations of each pair. In contrast, the deeper subsurface velocity model provided also by NIED shows that the deeper velocity models at each pair of stations are identical. We calculated the theoretical 1D site amplifications using the shallow velocity models at the stations, and found that the amplifications are much larger at frequencies between about 5 Hz and 10 Hz at the sites having larger PGAs. We compared the PGAs from bandpass filtered waveforms, and found that the PGAs were controlled by the frequency components at 5 Hz to 10 Hz, which corespond to the frequencies of maximum site amplifications at the sites having larger PGAs. The values of ratios of PGVs at the corresponding stations are nearly half of the values obatained for the PGAs, thus suggesting that the components of relatively longer period ground motions are similar. We also analyzed the PGAs and PGVs from several well recorded events, and found that the results are similar. Thus, we conclude that the shallow soil amplification effects greatly contributed to the variation of PGAs at the sites mentioned above.