Seismic Velocity and Attenuation Tomography of the Source Zone of the 2011 Tohoku-oki Earthquake (Mw 9.0)

Wednesday, 17 December 2014
Dapeng Zhao, Xin Liu and Zhouchuan Huang, Tohoku University, Sendai, Japan
We investigated 3-D P and S wave velocity (Vp, Vs) and attenuation (Qp and Qs) structures of the crust and upper mantle under the entire Northeast Japan arc from the Japan Trench to the Japan Sea coast. We used data from local shallow and intermediate-depth earthquakes recorded by a dense seismic network in Northeast Japan. Many suboceanic earthquakes under the Pacific Ocean and the Japan Sea are used and they are relocated precisely using sP depth phases. Vp and Vs tomography is determined using a large number of high-quality arrival-time data (Zhao et al., 2011; Huang and Zhao, 2013), whereas Qp and Qs tomography is obtained using a large number of t* data measured precisely from P and S wave spectra of local earthquakes (Liu et al., 2014). Our results reveal the high-V and high-Q subducting Pacific slab, and significant low-V and low-Q anomalies in the crust and mantle wedge under the volcanic front and the back-arc area. Large megathrust earthquakes (M > 6.0) during 1900-2013 including the great 2011 Tohoku-oki earthquake (Mw 9.0) sequence are generally located in high-V and high-Q patches in the megathrust zone which are surrounded by low-V and low-Q anomalies. The high-V/high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-V/high-Q patches represent asperities in the megathrust zone resulting from subducted oceanic ridges, seamounts and other topographic highs, whereas the low-V/low-Q anomalies reflect weakly coupled or even decoupled areas which contain more subducted sediments and fluids associated with slab dehydration. Our tomographic images are in good agreement with results of active-source seismic surveys conducted in the Tohoku fore-arc region and the Japan Trench (e.g., Tsuru et al., 2002; Fujie et al., 2013). These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.