Distribution of very low-frequency earthquakes in the Miyako Strait, central Ryukyu Trench
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A total of 1314 very low frequency earthquakes (VLFEs) were identified along the Ryukyu trench from seismograms recorded at broadband networks in Japan (F‐net) and Taiwan (BATS) in 2007. The spectra of typical VLFEs have peak frequencies between 0.02 to 0.1 Hz. Among those, waveforms from 120 VLFEs were inverted to obtain their centoroid moment tensor (CMT) solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11° × 14° in latitude and longitude and at depths of 0 to 60 km. Most of the VLFEs occur on shallow thrust faults that are distributed along the Ryukyu trench, which are similar to those earthquakes found in Honshu and Hokkaido, Japan. The locations and mechanisms of VLFEs may be indicative of coupled regions within the accretionary prism or at the plate interface; this study highlights the need for further investigation of the Ryukyu trench to identify coupled regions within it.
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Accretionary wedge
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Abstract Here we present the new model, the geometry of the subducted Philippine Sea Plate interface beneath the southern Ryukyu Trench subduction zone, estimated from seismic tomography and focal mechanism estimation by using passive and active data from a temporary amphibious seismic network and permanent land stations. Using relocated low‐angle thrust‐type earthquakes, repeating earthquakes, and structural information, we constrained the geometry of plate boundary from the trench axis to a 60 km depth with uncertainties of less than 5 km. The estimated plate geometry model exhibited large variation, including a pronounced convex structure that may be evidence of a subducted seamount in the eastern portion of study area, whereas the western part appeared smooth. We also found that the active earthquake region near the plate boundary, defined by the distance from our plate geometry model, was clearly separated from the area dominated by short‐term slow‐slip events (SSEs). The oceanic crust just beneath the SSE‐dominant region, the western part of the study area, showed high V p / V s ratios (>1.8), whereas the eastern side showed moderate or low V p / V s (<1.75). We interpreted this as an indication that high fluid pressures near the surface of the slab are contributing to the SSE activities. Within the toe of the mantle wedge, P and S wave velocities (<7.5 and <4.2 km/s, respectively) lower than those observed through normal mantle peridotite might suggest that some portions of the mantle may be at least 40% serpentinized.
Slab
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Convergent boundary
Seamount
Seismic Tomography
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Seismic zone
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Ocean bottom
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The minute investigation of the spatial distribution of intermediate-depth earthquakes reveals that the deep earthquake zone beneath the Kyushu-Ryukyu region is dislocated transversely and changes its strike under the Tokara Channel.Left-lateral dislocations of the isobaths of intermediate-depth earthquakes amount to 70-80km and 30-40km at depths of 80km and 150km, respectively. The isobath of the depth of 200km, however, is scarcely dislocated. The change in strike of the deep earthquake zone comes up to about 30° there.The discontinuity of the zone appears to be related closely to the surface geology. The Tokara Channel represents the left-lateral geological fault offsetting by 80-100km the tectonic belts in the pre-Miocene basement rocks running parallel to the island arc. In this region, there are a few left-lateral faults including the Tokara Channel, which traverse the Ryukyu Islands and divide the pre-Miocene tectonic belts into segments of 200 to 400km in strike length. But the other transverse discontinuities of the deep earthquake zone corresponding to the surface geological structure can not be found from the present analyses.
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Discontinuity (linguistics)
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