Junichi Nakajima

We estimated both P ( V p ) and S wave velocity ( V s ) structures beneath northeastern Japan by applying a tomographic method to 169,712 P and 103,993 S wave arrival time data from 4338 local events. The average value of V p / V s ratio is ∼1.69 in the upper crust, ∼1.75 in the lower crust, and ∼1.77 in the uppermost mantle. These differences in V p / V s ratio may mainly reflect the lithological variations with depth. Low‐ V p , low‐ V s , and high V p / V s zones are extensively distributed a

We carry out high‐resolution three‐dimensional seismic tomography of the crust and upper mantle beneath southwestern (SW) Japan using arrival‐time data obtained from the nationwide seismograph network. The tomographic images provide new insights into the configuration of the Philippine Sea slab and arc magmatism. The results confirm the existence of an aseismic portion of the Philippine Sea slab at greater depths beyond the seismic portion. The Philippine Sea slab is subducting aseismically down

We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1‐D velocity model. Then, traveltime tomography is carried out to estimate three‐dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130–140 km

A detailed investigation of the hypocenter distribution beneath Kanto, Japan, reveals a NW‐SE‐trending linear alignment of seismicity within the subducted Pacific slab. We estimate the 3D seismic velocity structure in the Pacific slab to understand the factors controlling the genesis of such intraslab earthquakes. A narrow low‐velocity zone is imaged within the subducted slab over a length of ∼150 km, which partly penetrates into the mantle portion of the slab. The low‐velocity zone correlates i

Abstract We apply a three‐step approach to estimate three‐dimensional (3‐D) P wave attenuation ( Q p −1 ) structure beneath northeastern Japan. First, corner frequencies of earthquakes are determined using the spectral‐ratio method for S‐coda waves. Then, whole‐path attenuation terms, t *, and site‐amplification factors are simultaneously estimated by a joint inversion. The set of t * is finally inverted for 3‐D attenuation structure. The results show that the mantle wedge has low attenuation in

Shear‐wave splitting in the southwestern part of the Kurile arc and the northeastern (NE) Japan arc is investigated using the waveforms from local earthquakes. For both arcs observed shear‐wave splitting shows clear evidence for a striking rotation of fast direction across the arc, suggesting the different feature of anisotropy between the fore‐arc and back‐arc sides. Trench‐parallel fast directions are observed in the fore‐arc side, which is consistent with the anisotropy expected from the defo

We perform travel‐time tomography to estimate detailed seismic velocity structures in the crust of the Pacific slab from northeastern (NE) Japan to the Kanto district, Japan, and reveal that the depth extent of the low‐velocity (hydrated) oceanic crust varies along the arc. The low‐velocity oceanic crust is subducting to depths of 120–150 km beneath Kanto, which is 40–70 km deeper compared to NE Japan. Such deeper preservation of the low‐velocity oceanic crust beneath Kanto can be explained by l

Abstract We carried out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional