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[S03-P-03] Significant anomalies in high-frequency seismograms for intra-slab earthquakes observed in Kanto area, Japan: Importance of mode-conversion scattering
In Kanto-Tokai area in Japan, very complicated high frequency seismograms are frequently observed as it is located above a complex subducting zone. In this study, we found characteristic high-frequency seismogram anomalies potentially being related to characteristic inhomogeneity in this area.
We analyzed Hi-net (NIED) records in Kanto-Tokai area for 20 intra-slab earthquakes (M4.4−6.9) occurred within the Pacific slab from October 2004 to April 2016, 227−453 km in depth. After applying bandpass filters of octave bandwidth of 1−16 Hz, characteristic wave packets were identified from root mean squared (RMS) envelope seismograms. At a frequency range of 8−16 Hz in Kanto area, we found wave packets preceding the arrival of S waves by about 10 s. At lower frequency of 1−2 Hz, we did not find similar packets. The amplitude of the packet was always predominant in the vertical component. No strong polarization in the horizontal component RMS envelopes is observed.
Complicated-shape wave packets lasting about 10 s, without significant pulse of boundary conversion, suggest that scattered wave packets are generated by the small-scale inhomogeneities. In addition, this cannot be explained by a simple S-to-S scattering because of its arrival time. Based on the systematic detection, we found that the wave packet propagated almost along the radial direction from the epicenter with the same apparent velocity as that of the S waves. Considering the wave packets arrived earlier than S waves, they are expected to involve mode-conversion such as P-to-S or S-to-P scattering generated by characteristic inhomogeneity between the epicenter and seismic stations.
Preliminary numerical simulations of seismic wave propagation in this area with superimposed onto a small-scale stochastic velocity fluctuation did not explain these peculiar wave packets. Considerable updates especially for small-scale inhomogeneities are expected to contribute to further understandings of the subduction zone.
We analyzed Hi-net (NIED) records in Kanto-Tokai area for 20 intra-slab earthquakes (M4.4−6.9) occurred within the Pacific slab from October 2004 to April 2016, 227−453 km in depth. After applying bandpass filters of octave bandwidth of 1−16 Hz, characteristic wave packets were identified from root mean squared (RMS) envelope seismograms. At a frequency range of 8−16 Hz in Kanto area, we found wave packets preceding the arrival of S waves by about 10 s. At lower frequency of 1−2 Hz, we did not find similar packets. The amplitude of the packet was always predominant in the vertical component. No strong polarization in the horizontal component RMS envelopes is observed.
Complicated-shape wave packets lasting about 10 s, without significant pulse of boundary conversion, suggest that scattered wave packets are generated by the small-scale inhomogeneities. In addition, this cannot be explained by a simple S-to-S scattering because of its arrival time. Based on the systematic detection, we found that the wave packet propagated almost along the radial direction from the epicenter with the same apparent velocity as that of the S waves. Considering the wave packets arrived earlier than S waves, they are expected to involve mode-conversion such as P-to-S or S-to-P scattering generated by characteristic inhomogeneity between the epicenter and seismic stations.
Preliminary numerical simulations of seismic wave propagation in this area with superimposed onto a small-scale stochastic velocity fluctuation did not explain these peculiar wave packets. Considerable updates especially for small-scale inhomogeneities are expected to contribute to further understandings of the subduction zone.