Abstract
The three-dimensional (3-D) P-wave attenuation (Q p -1) structure beneath Kanto, Japan, is estimated by using a large number of waveform data. Corner frequencies of earthquakes are initially calculated from spectral ratios of S-coda waves, followed by an inversion to simultaneously determine attenuation terms and frequency-dependent site amplification factors. The attenuation terms are then inverted for estimation of the 3-D Q p -1 structure. The obtained results show that seismic attenuation is highly heterogeneous, and pronounced high-attenuation areas are located in the continental lower crust and mantle of the Philippine Sea slab. Seismic activity is very low in the high-attenuation lower crust, which is most likely attributable to ductile deformation facilitated by fluids supplied from the underlying Philippine Sea slab. The high-attenuation area in the Philippine Sea slab represents the serpentinized mantle, and two M ~ 7 earthquakes are documented to have occurred along the western boundary of this area. Interplate earthquakes on the Pacific slab are absent in areas overlain by the serpentinized Philippine Sea slab, which is likely due to the low viscosity of serpentine promoting continuous ductile deformation rather than brittle failures along the plate boundary.
Highlights
Beneath the Kanto district of Japan, which contains the Tokyo metropolitan area, the Philippine Sea plate is subducting under the continental plate along the Sagami trough, and the Pacific plate is subducting below the Philippine Sea and continental plates along the Japan trench (Figure 1a)
20 50 km (d) Depth = 50 km (e) Depth = 60 km (f) Depth = 70 km Results High-attenuation (Qp−1 > 0.005) areas are distributed over a wide region of Kanto at a depth of 20 km, and a significant E-W trending high-attenuation area is observed along latitudes of 35.5 to 36.0 °N at depths of 30 and 40 km
When compared with iso-depth contours for the Philippine Sea slab, it is evident that the E-W trending highattenuation area occurs within the continental plate at a depth of 30 km, whereas it corresponds to the subducting crust at a depth of 40 km
Summary
Beneath the Kanto district of Japan, which contains the Tokyo metropolitan area, the Philippine Sea plate is subducting under the continental plate along the Sagami trough, and the Pacific plate is subducting below the Philippine Sea and continental plates along the Japan trench (Figure 1a). One of the most important findings was a low-velocity and high-Vp/Vs mantle of the Philippine Sea slab beneath eastern Kanto, which is interpreted as being the serpentinized mantle (Figure 1b) (Nakajima et al 2009). Nakajima and Hasegawa (2010) emphasized the important role of the western boundary of the serpentinized mantle on seismogenesis beneath Kanto and suggested that the western unmetamorphosed mantle moves 0.5 to 1 cm/year relative to the eastern serpentinized mantle. Seismic attenuation provides additional insights into seismogenesis and plays a crucial role in the estimation of strong surface motions, but there are few studies on three-dimensional (3-D) seismic attenuation beneath Kanto. Precisely estimating seismic attenuation is of considerable importance to understand
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