[S03-P] Poster

[S03-P-04] Amplitude fluctuation of seismic waves in the crust

^○Kazuo Yoshimoto¹, Shunsuke Takemura², Manabu Kobayashi¹ (1.Yokohama City University, Yokohama, Japan, 2.NIED, Tsukuba, Japan)

The amplitudes of high-frequency P- and S-waves generated by local earthquakes vary from site to site, even at similar hypocentral distances. It had been suggested that, in addition to non-isotropic source radiation and local site effects, complex wave propagation in inhomogeneous crust is responsible for this observation. To quantitatively investigate this effect, we performed observational and numerical studies on the amplitude fluctuation of P- and S-waves in inhomogeneous crust. Using seismic data from 31 small-to-moderate crustal earthquakes that occurred in western Honshu, Japan, we measured the fluctuation in maximum velocity amplitudes observed by the borehole seismometers of the NIED Hi-net. Observed maximum velocity amplitudes of P- and S-waves in the frequency bands of 1–2 and 2–4 Hz were normalized on the basis of the coda normalization method. Other technical details are same as in Yoshimoto et al. (2015). Our observations of P- and S-wave amplitudes (coda-normalized maximum velocity amplitudes) revealed that fluctuations in P- and S-wave amplitude increase with increasing frequency and hypocentral distance, with large fluctuations showing up to ten-times difference between the largest and the smallest amplitudes. It was found that the standard deviation of amplitude fluctuation of P- and S-waves increases monotonically in small hypocentral distances; however, the increase tends to become saturated at a hypocentral distance of tens of kilometers. Our numerical simulation via finite-difference method indicated that the observed characteristics of amplitude fluctuation can be explained by crustal inhomogeneity stochastically characterized by an exponential random model with a correlation length of about one kilometer and standard deviation in seismic velocity of a few percent.