Tomography of the source zone of the great 2011 Tohoku earthquake

Yuanyuan Hua,Yixian Xu,Dapeng Zhao,Genti Toyokuni

doi:10.1038/s41467-020-14745-8

Abstract

The mechanism and rupture process of the giant 2011 Tohoku-oki earthquake (Mw 9.0) are still poorly understood due to lack of permanent near-field observations. Using seismic arrival times recorded by dense seismograph networks on land and at ocean floor, we determine a detailed seismic tomography model of the megathrust zone beneath the Tohoku forearc. Our results show that the coseismic slip of the 2011 Tohoku-oki earthquake initiated at a boundary between a down-dip high-velocity anomaly and an up-dip low-velocity anomaly. The slow anomaly at shallow depths near the Japan trench may reflect low-rigidity materials that are close to the free surface, resulting in large slip and weak high-frequency radiation. Our new tomographic model can account for not only large slip near the trench but also weak high-frequency radiation from the shallow rupture areas.

Highlights

The mechanism and rupture process of the giant 2011 Tohoku-oki earthquake (Mw 9.0) are still poorly understood due to lack of permanent near-field observations
It produced a huge tsunami and caused great damage to the local society and infrastructure in NE Japan. The occurrence of this great megathrust earthquake has updated our knowledge about megathrust ruptures, especially due to the large coseismic slip at shallow depth near the Japan trench[1,2,3,4,5,6,7] and high-frequency radiations from small-slip areas[8,9]
The Seafloor Observation Network for Earthquakes and Tsunamis (S-net) (Fig. 1) has been installed around the Japan trench and Kuril trench by the National Research Institute for Earth Science and Disaster Prevention (NIED), which consists of 150 ocean bottom seismometer (OBS) stations[26]

Summary

Results and discussion

Coseismic slip areas of large megathrust earthquakes (M ≥ 7.0) that occurred before 2011 are mostly located in the deeper high-V and high-Q (i.e., low seismic attenuation) zones with strong ground motions and positive residual gravity[34,35] (Fig. 2c), which are interpreted as strong asperities[23,25] or granite batholiths[21] For those megathrust earthquakes (M ≥ 7.0), the up-dip low-V anomaly may have been able to arrest rupture propagation to the shallow part due to their relatively low slip velocities. In the large-slip area, most of the GPS-A stations show landward movement and they are located in the low-V anomaly, except for two stations (G05, G03) that are located in a high-V anomaly and show small trench-ward movements (Fig. 2a) This difference may be caused by the variations of friction and material properties along the Tohoku megathrust zone[46] and the lithologic transition in the upper plate

30 Subducting

Methods

Code availability