Guided wave observations and evidence for the low-velocity subducting crust beneath Hokkaido, northern Japan

Abstract

At the western side of the Hidaka Mountain range in Hokkaido, we identify a clear later phase in seismograms for earthquakes occurring at the uppermost part of the Pacific slab beneath the eastern Hokkaido. The later phase is observed after P-wave arrivals and has a larger amplitude than the P wave. In this study, we investigate the origin of the later phase from seismic wave observations and two-dimensional numerical modeling of wave fields and interpret it as a guided P wave propagating in the low-velocity subducting crust of the Pacific plate. In addition, the results of our numerical modeling suggest that the low-velocity subducting crust is in contact with a low-velocity material beneath the Hidaka Mountain range. Based on our interpretation for the later phase, we estimate P-wave velocity in the subducting crust beneath the eastern part of Hokkaido by using the differences in the later phase travel times and obtain velocities of 6.8 to 7.5 km/s at depths of 50 to 80 km. The obtained P-wave velocity is lower than the expected value based on fully hydrated mid-ocean ridge basalt (MORB) materials, suggesting that hydrous minerals are hosted in the subducting crust and aqueous fluids may co-exist down to depths of at least 80 km.

Depth variations in seismic velocity in the subducting crust: Evidence for fluid‐related embrittlement for intermediate‐depth earthquakes

“…In addition to guided‐ P waves reported by Shiina et al . [], we detected another later phase in seismograms with a clear guided‐ P wave and interpreted it as a guided‐ S wave. Amplitudes of the later phases are dominant in horizontal components (Figures c and S1 in the supporting information), and the later phases arrive at stations 3–15 s after a theoretical arrival time of the direct S wave calculated for the JMA2001 1‐D velocity model [ Ueno et al ., ] (Figure ).…”

“…At the western side of the Hidaka mountain range in Hokkaido, guided waves propagating in the subducting crust [ Abers , ; Shiina et al ., ] are frequently recorded for earthquakes occurring near the upper surface of the Pacific slab (Figure a). In addition to guided‐ P waves reported by Shiina et al .…”

“…A guided wave that propagates in the subducting crust is often observed when earthquakes occur beneath eastern Hokkaido [ Abers , ; Shiina et al ., ]. Because guided waves propagate at long distances along the crust [ Shiina et al ., ], we can resolve the depth‐dependent seismic properties of the subducting crust by analyzing guided waves. In this study, we measured arrival times of guided‐ P and guided‐ S waves and then estimated P wave ( V p ) and S wave ( V s ) velocities in the crust beneath eastern Hokkaido.…”

Special issue ‘Geofluid processes in subduction zones and mantle dynamics’

“…He argues that the observed migration is associated with geofluids supplied from the uppermost mantle. Shiina et al (2014) show that hydrated mineralogy alone cannot sufficiently explain the low velocities observed in the subducting crust beneath Hokkaido, suggesting that fluids may coexist with hydrated rocks down to 80-km depth. Nakajima (2014) provides evidence of the presence of high-attenuation areas in a serpentinized mantle wedge using seismological tomography.…”

Velocity increase in the uppermost oceanic crust of subducting Philippine Sea plate beneath the Kanto region due to dehydration inferred from high-frequency trapped P waves

“…The seismic waves are effectively trapped in this low-velocity layer, and consequently, the trapped seismic energy is often identified as distinct later arrivals during intraslab earthquakes in the subduction zone (e.g., Fukao et al 1983;Hori et al 1985;Hori 1990;Abers et al 2003;Martin and Rietbrock 2006;Miyoshi et al 2012;Shiina et al 2014). Trapped P and S waves appear several seconds after the arrival of the relatively weak direct body waves.…”