Seismic Velocity Structure in and around the Japanese Island Arc Derived from Seismic Tomography Including NIED MOWLAS Hi-net and S-net Data

Abstract

Japanese Islands are composed of four plates, with two oceanic plates subducting beneath the two continental plates. In 2016 the National Research Institute for Earth Science and Disaster Resilience (NIED) Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) started seismic observation of the offshore Hokkaido to Boso region in the Pacific Ocean, and Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) was transferred to NIED. We add the NIED S-net and DONET datasets to NIED highsensitivity seismograph network (Hi-net) and full range seismograph network (F-net) datasets used in the previous study and obtain the three-dimensional seismic velocity structure beneath the Pacific Ocean as well as Japanese Islands. NIED S-net data dramatically improve the resolution beneath the Pacific Ocean at depths of 10-20 km because the seismic stations are located above the earthquakes and on the east side of the Japan Trench. We find a NS high-Vp zone at depths of 20-30 km. The 2018 Eastern Iburi earthquake occurred below the northern part of this high-V zone. The coseismic slip plane of the 2011 Tohokuoki earthquake has low Vp/Vs, but its large slip region has high Vp. The broad low-Vp/Vs region may play a role in large earthquake occurrence.

Cascading rupture of patches of high seismic energy release controls the growth process of episodic tremor and slip events

“…This difference among hypocenters is small relative to the size of the high-energy patches. The effect of a heterogeneous velocity structure on the tremor locations in this study would be comparable to that of Matsubara et al (2019). It is therefore unlikely that a heterogeneous velocity structure accounts for the observed high-energy patches.…”

“…Velocity structure heterogeneities may cause tremor to concentrate in certain locations. Among hypocenters determined by seismic tomography by Matsubara et al (2019), those determined within a one-dimensional velocity structure differ from those determined within a three-dimensional structure by 1 km on average, and by as much as 8 km for hypocenters at depths of 0-50 km in this region (M. Matsubara, personal communication). This difference among hypocenters is small relative to the size of the high-energy patches.…”

Seismic Evidence for Water Transportation in the Forearc off Northern Japan

“…These high‐V anomalies are not evident in the previous tomographic results of Northern Japan obtained using the land‐based Kiban‐net data (e.g., Huang et al, ; Liu et al, ). A recent tomographic study using the S‐net data (Matsubara et al, 2019) revealed similar high‐V anomalies in the forearc mantle, but the resolution of their model in the forearc region is not very high (Figures S8 and S9). We also compare our new model with three seismic refraction profiles obtained with active‐source OBS data in Northern Japan (Ito et al, ; Nakanishi et al, ; Takahashi et al, ) that have higher resolutions in the crust and uppermost mantle (Figure S10).…”

Passive seismic reflection imaging based on acoustic and elastic reverse time migration without source information: theory and numerical simulations

“…Some techniques that utilise such records are seismic tomography using earthquakes (e.g. Aki and Lee 1976;Dziewonski, Hager, and O'Connell 1977;Nakajima and Hasegawa 2007;Matsubara et al 2019), a major method for investigating the velocity structure of the deep Earth, and receiver function analysis (e.g. Langston 1979;Yamauchi, Hirahara, and Shibutani 2003), which images subsurface conversion interfaces using multicomponent earthquake waveforms.…”