[S-SS07] Crustal Structure

[SSS07-P01] Local variations of seismic scattering-attenuation structure around the volcanic front in Hokkaido

*Kiyoshi Yomogida¹, Hidekatsu Masuda², Takahiro Shiina³, Kei Katsumata⁴ (1.Department of Earth and Planetary Dynamics, Graduate School of Science, Hokkaido University, 2.Earth and Planetary Dynamics, Faculty of Natural History Sciences, Graduate School of Science, Hokkaido University, 3.National Institute of Advanced Industrial Science and Technology, 4.Institute of Seismology and Volcanology, Hokkaido University)

Keywords:small-scale heterogeneities, scattering, attenuation, volcanic front

Inhomogeneous seismic structures of various scales exist in and above a subducting plate. Especially for S waves in a frequency range of higher than 1 Hz, the observed waveforms are affected by scattering and attenuation by small-scale inhomogeneous structures. As a result, a wavepacket, generally called coda wave, is formed after the direct S wave, but their features are far from the standard of monotonically decaying in time. The envelope of an S waveform passing through the strong heterogeneous region in the subduction zone has its peak time delayed relative to the expected time of the direct wave, and its duration is generally long, forming a spindle-shaped waveform. Peculiar waveforms are observed especially near the volcanic front. In Kanto and Tohoku regions of Japan, large difference in waveform characteristics has been clearly detected at their volcanic front as a sharp vertical boundary. The characteristics of their fine heterogeneous structures have been studied. For example, characteristic S coda waves are observed with a high frequency component of 8 Hz or higher (Shiina et al., 2021).

In this study, we analyzed S waveforms recorded at stations in Hokkaido, Japan, for deep intraplate earthquakes. The frequency dependency of their S wave envelopes is strong on the north side of volcanic front but weak on its vicinity. We also investigated the spatial variation of S wave envelopes along the volcanic front, in addition to across the front. The arrival delays of envelope peaks are generally small in the central Hokkaido area including the Hidaka mountains while those in the eastern area of Shiretoko Peninsula shows similar characteristics to the Tohoku region. In contrast, the volcanic front near Lake Akan appears to be a sharper discontinuity than in the other regions, that is, there exist highly complex structures with small scales together with a distinctive volumetric body of intensive seismic attenuation.