Distortion of the apparentS-wave radiation pattern in the high-frequency wavefield: Tottori-Ken Seibu, Japan, earthquake of 2000

Abstract

S U M M A R YThe distortion properties of the apparent S-wave radiation pattern in the high-frequency seismic wavefield of over f > 2 Hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the Tottori-Ken Seibu, Japan, magnitude (M w ) 6.6 earthquake in 2000. The dense seismic records from the KiK-net strong motion network show a clear four-lobe pattern in the apparent S-wave radiation pattern in the low-frequency wavefield of f < 2 Hz, and shows an almost isotropic distribution in all directions as the frequency increases above 5 Hz. The distortion of the apparent S-wave radiation pattern in the high-frequency wavefield increases as travel distance increases. Therefore, the path effect caused by the scattering of seismic waves due to small-scale heterogeneities in the crust is a major cause of distortion of the radiation pattern. This hypothesis is examined by a 2-D finite-difference method simulation of seismic waves in heterogeneous structure models. The results of simulations clearly demonstrate the collapse of the S-wave front due to seismic wave scattering in the heterogeneous structure. By comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models, the most preferable model that can explain the observation is described by a von Karman autocorrelation function with correlation distance of a = 3-5 km, order of κ = 0.5 and rms value of ε = 0.07. However, our simple stochastic random heterogeneity model proposed, herein, somewhat overestimates the scattering of low-frequency signals below 2 Hz.

Is Ground‐Motion Variability Distance Dependent? Insight from Finite‐Source Rupture Simulations

“…Nevertheless the radiation pattern effect, which is clearly observed in our synthetics, might be limited to lower frequency range (<∼1 Hz) in real velocity structures. The theoretical four-lobe S-wave radiation pattern may be limited to low frequencies (< 1 Hz), with an isotropic pattern at high frequency due to the scattering of seismic waves (e.g., Liu and Helmberger, 1985;Takenaka et al, 2003;Takemura et al, 2009). In addition, according to Cho et al (2010), observations suggest that far-field radiation patterns change from a distinct doublecouple pattern, with strong directivity effects at low frequencies (< 1 Hz), to a more isotropic pattern with diminished directivity effects at high frequencies, putting forward the fact that directivity effects are also frequency dependent.…”

High-frequency directivity effects: evidence from analysis of the Les Saintes records

“…Indeed, Takenaka et al (2003) suggested that the frequencydependent distortion of the S-wave radiation pattern might be caused by the mixing and coupling of the horizontal (S H ) and vertical (S V ) S-waves in the heterogeneous structure near the source region. By analyzing dense KiK-net array observations from the Tottori-Ken Seibu earthquake, Mw 6.6, and its aftershocks, Takemura et al (2009) demonstrated the collapse of the S-wave front due to seismic-wave scattering in a heterogeneous structure and showed that the radiation pattern is more isotropic at high frequencies (>2 Hz). Castro et al (2006) suggested that the S H -wave radiation approaches the theoretical radiation only for frequencies below 0.5 Hz.…”

Site effect determination using seismic noise from Tungurahua volcano (Ecuador): implications for seismo-acoustic analysis

“…Studying the Tottori-Ken Seibu earthquake (6.6 M w ), that occurred in 2000 in Japan, Takemura et al (2009) demonstrated that the S-wave radiation pattern could be affected by medium heterogeneities that scatter seismic waves, generating an isotropic pattern. This physical process is frequency dependent.…”