17:15 〜 18:30
[SCG39-P25] Along-trench structural variation of the Philippine Sea plate subducting in the Hyuga-nada region and its relation to slow earthquakes
キーワード：日向灘、九州パラオ海嶺、地震探査
The Hyuga-nada subduction zone east off the Kyushu Island, southwest Japan, host active tectonic tremor and very low-frequency earthquakes (VLFEs) that presumably occur at the shallow plate boundary (e.g., Yamashita et al., 2015). The source region of the slow earthquakes is known to coincide with the subduction of the Kyushu-Palau Ridge (Yamamoto et al., 2013; Tonegawa et al., 2020). These seismogenic and tectonic environments allow us to examine physical controls on shallow slow earthquakes associated with the ridge subduction. In order to better understand structural properties controlling the seismogenic behaviors in the Hyuga-nada region, we conducted high-resolution seismic refraction experiments along two seismic lines and multi-channel seismic (MCS) reflection surveys in August-September 2020. In this presentation, we report preliminary results of P-wave velocity (Vp) modeling along one of the refraction profiles, trench-parallel HYU01 line, and discuss the subduction structure of the Philippine Sea plate and its relation to seismic activity in the region.
The HYU01 profile is located above the subducted part of the Kyushu-Palau Ridge and crosses the active area of tremor and VLFEs in the NE-SW direction. For acquisition of seismic refraction data, 50 ocean bottom seismographs (OBS) were deployed at 2 km interval during the research cruise KM20-05 and recorded over 3,800 signals generated at 50- and 200-m intervals by the 10,600-cubic-inch airgun arrays of the R/V Kaimei. We also acquired multichannel seismic reflection data on the coincident line using the 5.6-km-long, 444-channel streamer cable. Together with the preexisting MCS reflection data nearby (KR98-10-4), these data sets provide seismic constraints on the along-trench variation of the overriding and subducting Philippine Sea plates.
We applied traveltime tomography analysis to this dense OBS-airgun data and constructed a Vp model along the HYU01 profile. The preliminary result reveals that the overriding plate (forearc region) exhibits low-velocity (Vp < 5.0 km/s) on the whole without significant along-trench variation in velocity. On the other hand, the subducting plate shows a rapid lateral velocity change suggesting a structural transition from a typical oceanic crust to more arc-like feature indicating the subducted part of the Kyushu-Palau Ridge. This transition roughly corresponds to the location with active tectonic tremor. It is also indicated that tremor and VLFE activities are limited to the eastern part of the profile where the plate boundary is shallower than ~15 km depth.
The HYU01 profile is located above the subducted part of the Kyushu-Palau Ridge and crosses the active area of tremor and VLFEs in the NE-SW direction. For acquisition of seismic refraction data, 50 ocean bottom seismographs (OBS) were deployed at 2 km interval during the research cruise KM20-05 and recorded over 3,800 signals generated at 50- and 200-m intervals by the 10,600-cubic-inch airgun arrays of the R/V Kaimei. We also acquired multichannel seismic reflection data on the coincident line using the 5.6-km-long, 444-channel streamer cable. Together with the preexisting MCS reflection data nearby (KR98-10-4), these data sets provide seismic constraints on the along-trench variation of the overriding and subducting Philippine Sea plates.
We applied traveltime tomography analysis to this dense OBS-airgun data and constructed a Vp model along the HYU01 profile. The preliminary result reveals that the overriding plate (forearc region) exhibits low-velocity (Vp < 5.0 km/s) on the whole without significant along-trench variation in velocity. On the other hand, the subducting plate shows a rapid lateral velocity change suggesting a structural transition from a typical oceanic crust to more arc-like feature indicating the subducted part of the Kyushu-Palau Ridge. This transition roughly corresponds to the location with active tectonic tremor. It is also indicated that tremor and VLFE activities are limited to the eastern part of the profile where the plate boundary is shallower than ~15 km depth.