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[SSS04-P01] Estimation of source radiation amplitude from seismic coda waves considering the heterogeneous seismic structure
Keywords:Seismic coda waves, Source spectra, Seismogram envelopes
Seismic coda waves have widely been used to estimate seismic source spectra. Coda waves, especially late coda waves (lapse time is larger than twice of S-wave arrival times) have usually been assumed to reflect source and site amplification terms. However, recent dense seismic networks have enabled us to estimate 3-D heterogeneities in a seismic structure such as velocity, intrinsic attenuation, and scattering parameters. Hence, we should consider medium heterogeneities when we use coda waves to analyze source and site amplification characteristics.
In this study, we propose a method to estimate source radiation amplitude using full-wave seismogram envelopes. Observed narrow-band band-pass filtered seismogram envelopes are modeled as the combination of source, site amplification terms, and Green’s function of seismogram envelopes. We adopt a Monte Carlo simulation based on the radiative transfer theory with Born approximation to calculate Green’s function of seismogram envelopes in 3-D heterogeneous velocity structure. We search appropriate intrinsic attenuation factor and scattering parameter numerically for each source-station pair, thus we implicitly consider 3-D heterogeneities of intrinsic attenuation factors and scattering parameters. After finding the appropriate seismogram envelopes, we invert source and site amplification factors simultaneously with appropriate constraints.
We applied the proposed method to earthquakes that occurred in the Kanto-Chubu area, Japan. The ranges of depth and magnitude (in the JMA scale) were less than 100 km and 3.0 to 4.5, respectively. Frequency ranges were 1-2 and 2-4 Hz because of the limitation of Born approximation. The derived source radiation terms are proportional to the JMA magnitude in both frequency ranges. The source radiation terms of the earthquakes deeper than 60 km tended to be larger than those of shallower than 30 km regardless of the same magnitude. We further plan to investigate regional heterogeneities in the source radiation terms.
Acknowledgments
We used the seismograms recorded by the Hi-net seismic network operated by the NIED. This study was partially supported by JSPS KAKENHI Grant No. JP18K13622 and JP21K14002.
In this study, we propose a method to estimate source radiation amplitude using full-wave seismogram envelopes. Observed narrow-band band-pass filtered seismogram envelopes are modeled as the combination of source, site amplification terms, and Green’s function of seismogram envelopes. We adopt a Monte Carlo simulation based on the radiative transfer theory with Born approximation to calculate Green’s function of seismogram envelopes in 3-D heterogeneous velocity structure. We search appropriate intrinsic attenuation factor and scattering parameter numerically for each source-station pair, thus we implicitly consider 3-D heterogeneities of intrinsic attenuation factors and scattering parameters. After finding the appropriate seismogram envelopes, we invert source and site amplification factors simultaneously with appropriate constraints.
We applied the proposed method to earthquakes that occurred in the Kanto-Chubu area, Japan. The ranges of depth and magnitude (in the JMA scale) were less than 100 km and 3.0 to 4.5, respectively. Frequency ranges were 1-2 and 2-4 Hz because of the limitation of Born approximation. The derived source radiation terms are proportional to the JMA magnitude in both frequency ranges. The source radiation terms of the earthquakes deeper than 60 km tended to be larger than those of shallower than 30 km regardless of the same magnitude. We further plan to investigate regional heterogeneities in the source radiation terms.
Acknowledgments
We used the seismograms recorded by the Hi-net seismic network operated by the NIED. This study was partially supported by JSPS KAKENHI Grant No. JP18K13622 and JP21K14002.