Seismic evidence for the low-velocity subducting crust of the Pacific slab beneath Hokkaido, northern Japan
Abstract
It is considered that the existence of water in the subducting crust contributes to genesis of the intraslab earthquakes (e.g., Kirby et al., 1996). Therefore, the investigation of seismic velocity structure and water distribution in the crust is important to understand water transportation and seismogenesis in subduction zones. Later phases, such as mode-converted waves and guided waves, are often observed from intermediate-depth earthquakes (e.g., Hori et al., 1985). It is known that some later phases are generated associated with the top of the slab and the low-velocity subducting crust (e.g., Matsuzawa et al., 1980; Martin and Rietbrock, 2006). Because of their propagation paths, later phases are much more sensitive to seismic velocity in the subducting crust. Shiina et al. (2013) resolved the detailed P-wave velocity structure in the subducting crust of the Pacific slab beneath Tohoku from travel times of PS-converted wave which converted from P to S waves at the top of the slab (Matsuzawa et al., 1986). In this study, we estimate seismic velocity of the subducting crust of the Pacific slab beneath Hokkaido from travel times of PS-converted waves that are frequently observed in the forearc region of Hokkaido. We identified PS-converted waves from earthquakes which occurred in subducting crust and manually picked arrival time of the wave. The observed travel times of PS-converted waves are explained by the existence of the subducting crust with P-wave velocities of 10-15% lower than those of the surrounding mantle. Moreover, we found other later phase (X phase) that arrives 2-3 s after the P-wave arrival and has large amplitudes in vertical component. X phases are frequently observed at the southern part of the Hokkaido from earthquakes that are distributed near the upper surface of the Pacific slab. We simulate wave propagation from intraslab earthquakes by using a 2-D finite different method (e.g., Virieux, 1986) to identify the origin of X phase. Then, X phase is interpreted as a guided wave which is trapped in the low-velocity subducting crust of the Pacific slab. These observations of PS-converted wave and guided waves at Hokkaido indicate that the low-velocity subducting crust is located in the uppermost part of the Pacific slab beneath Hokkaido. In addition, guided waves associated with the subducting crust are considered to be observed in areas where the low-velocity subducting crust is in contact with the continental lower crust (e.g., Abers et al., 2005; Miyoshi et al., 2012). Therefore, our observations on guided waves confirm the presence of continental crustal materials above the Pacific slab at depths of 60-80 km beneath the southern part of the Hokkaido, as observed in Kita et al. (2010).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T43E2711S
- Keywords:
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- 8170 TECTONOPHYSICS Subduction zone processes;
- 7220 SEISMOLOGY Oceanic crust;
- 7230 SEISMOLOGY Seismicity and tectonics