Abstract
It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15–18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.
Highlights
It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis
The weak coupling is supported by recent geophysical/geological studies: First, seismic studies reveal that slow slip events (SSEs), very-low-frequency earthquakes (VLFEs) and small repeating earthquakes are ubiquitously distributed, suggesting that the plate boundary is enriched in fluids and mostly creeping[5,6,7,8,9]
It has been recognized that slow earthquakes including LFE, VLFEs and SSEs are a key to understand the frictional properties along the plate interface[19]
Summary
It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. In the southern part of the Ryukyu Trench, there is a gap between the shallow tsunamigenic zone
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