JpGU-AGU Joint Meeting 2017

Presentation information

[EJ] Oral

S (Solid Earth Sciences) » S-SS Seismology

[S-SS11] [EJ] Seismic wave propagation: Theory and Application

Thu. May 25, 2017 9:00 AM - 10:30 AM A08 (Tokyo Bay Makuhari Hall)

convener:Kiwamu Nishida(Earthquake Research Institute, University of Tokyo), Hisashi Nakahara(Solid Earth Physics Laboratory, Department of Geophysics, Graduate School of Science, Tohoku University), Kazuya Shiraishi(Japan Agency for Marine-Earth Science and Technology), Jun Matsushima(School of Engineering, The University of Tokyo), Chairperson:Kentaro Emoto(Graduate School of Science, Tohokuk University), Chairperson:Shunsuke Takemura(National Research Institute for Earth Science and Disaster Resilience)

9:25 AM - 9:40 AM

[SSS11-02] Significant anomalies in high-frequency seismograms for intra-slab earthquakes observed in Kanto area, Japan

*Nozomi Kanaya1, Takuto Maeda1, Kazushige Obara1, Akiko Takeo1 (1.Earthquake Research Institute, The University of Tokyo)

Keywords:seismic wave propagation, scattering, seismogram envelope, subduction zone, numerical simulation

In Kanto-Tokai area in Japan, very complicated high frequency seismograms are frequently observed as it is located above a complicated plate boundary zone between the Pacific plate and the Philippine Sea plate. In this study, we found characteristic high-frequency seismogram anomalies potentially being related to characteristic inhomogeneity in this area.

We analyzed 20 intra-slab earthquakes (M4.4−6.9) occurred within the Pacific slab from October 2004 to April 2016, 227−453 km in focal depth. The datasets are velocity seismograms recorded by 258 NIED Hi-net stations in Kanto-Tokai area. After applying bandpass filters of octave bandwidth ranging from 1 to 16 Hz, characteristic wave packets and their frequency dependence are 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 10−20 s. The wave packets lasted about 10 s and their amplitude was obviously larger than that of the P-coda waves. At lower frequency of 1−2 Hz, we did not find similar packets. These wave packets were not observed for intra-slab shallow earthquakes, less than 200 km in focal depth. The amplitude of the packet at high frequency was always predominant in the vertical component. No strong polarization in the horizontal component RMS envelopes is observed.

Complicated-shape wave packets lasting about ten seconds, without significant pulse of boundary conversion, suggest that a sort of scattered wave packets are generated by the small-scale inhomogeneities. In addition, this cannot be explained by a simple S-to-S scattering because the arrival time of the packet precedes that of the direct S wave. Based on the systematic detection, we found that the wave packet propagated almost along the radial direction from the epicenter with almost 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, by characteristic inhomogeneity between the epicenter and seismic stations.

We conducted preliminary numerical simulations of seismic wave propagation with a community velocity model in this area superimposed onto a small-scale stochastic velocity fluctuation. The resultant synthetic waveforms did not explain these peculiar wave packets. Considerable updates especially for small-scale inhomogeneities in the velocity model are expected to contribute to further understandings of seismic wave propagation in the subduction zone.