Abstract
Distinct later phases of waves with rich high-frequency (> 8 Hz) components were observed for intraslab earthquakes that occurred at intermediate depths, particularly at depths exceeding 100 km, in the northeastern (NE) Japan subduction zone. These high-frequency later phases (HFLPs) showed anomalously large peak-amplitude delays, up to ~ 50 s after direct S-wave arrivals at stations in the backarc region. Using a source-scanning algorithm, we investigated the locations of passing points affecting the propagation of HFLPs. The passing points were estimated to be in the forearc region in the entire NE Japan, indicating that HFLPs are scattered waves that pass through the forearc region. The propagating HFLPs seem to bypass the backarc mantle wedge, as a consequence of the distinct attenuation contrast in the mantle wedge across the volcanic front in NE Japan. These HFLP observations suggest that the high-attenuation zone in the backarc mantle wedge controls propagations of the high-frequency waves of intraslab earthquakes, in addition to the scatterers possibly locate in the forearc region.
Highlights
In northern (N) and northeastern (NE) Japan, the Pacific plate is subducting beneath the Kuril and NE Japan arcs
We analyzed the amplitude records of intraslab earthquakes with High-frequency later phase (HFLP) and revealed that the HFLPs were scattered waves generated by scatterers at depths corresponding to the lower crust and the mantle wedge in the forearc region; their propagation paths are similar to those of the forearc intraslab earthquakes retrieved by Hasemi and Horiuchi (2010)
When the attenuation effects were ignored, the passing points appeared in the backarc mantle wedge and the forearc region
Summary
In northern (N) and northeastern (NE) Japan, the Pacific plate is subducting beneath the Kuril and NE Japan arcs. The oblique subduction of the Pacific plate is driving the southwestward migration of the Kuril arc, which collides with the NE Japan arc at the central part of Hokkaido, Japan (e.g., Kimura 1996). Seismograms observed for earthquakes occurred at both N and NE Japan record direct P and S waves, and a variety of other signals reflecting the highly heterogeneous structures related to the subduction of the Pacific plate. We investigate the high-frequency later phases (HFLPs) of backarc intraslab earthquakes in NE. Wu and Irving (2018) have already reported arrival delays of high-frequency waves in an earthquake cluster at a depth of ~ 150 km, but here we further identified the HFLPs of earthquakes over the entire Pacific slab in the study area. We analyzed the amplitude records of intraslab earthquakes with HFLPs and revealed that the HFLPs were scattered waves generated by scatterers at depths corresponding to the lower crust and the mantle wedge in the forearc region; their propagation paths are similar to those of the forearc intraslab earthquakes retrieved by Hasemi and Horiuchi (2010)
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