Structural features of the subducting slab beneath the Kii Peninsula, central Japan: Seismic evidence of slab segmentation, dehydration, and anisotropy

摘要

Seismic evidence of lithospheric slab segmentation, dehydration, and anisotropic rocks beneath the Kii Peninsula, central Japan, is detected with receiver function (RF) analyses. By stacking radial and transverse RFs with moveout perturbations, we estimate the downdip azimuth, dip angle, and depth of the slab Moho beneath each seismic station. Comparing the downdip azimuth to the slab convergence azimuth, we can classify the slab into three segments. The segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity. The slab Moho lies just above the intraslab seismicity in the southwestern Kii Peninsula. Slab Moho lies deeper at the eastern part of the peninsula, at the bottom of seismicity. This suggests that earthquakes occur only within the slab mantle under the southern Kii Peninsula. The dip angle of slab Moho becomes steeper at 40- to 50-km depth, perhaps caused by dehydration transformation of basalt to eclogite. At some stations located in the western part of the Kii Peninsula, RF-estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity. Since the local seismicity distribution reflects well the larger-scale geometry of the slab, the RF-estimated dip angle may imply anisotropic wave speeds within strongly sheared and/or hydrated media around the slab Moho. These detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the Nankai region.