Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (M_w9.0) and subduction dynamics

Abstract

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate-to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (M w 9.0) occurred. The 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-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (M w > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Methodology of Seismic Tomography

“…The fourth reason is that very small blocks, or grid intervals, are required to express the lateral undulations of the three discontinuities, which are impossible to realize in the actual tomographic inversion, because the available seismic stations and earthquakes are not distributed densely and uniformly enough to allow the use of small blocks, or a grid, for the tomographic inversion. Therefore, it was, and still is, the best choice to take into account the three discontinuities and the Pacific slab in the starting velocity model for the tomographic inversion (e.g., Zhao et al 1992Zhao et al , 2012Nakajima et al 2005;Wagner et al 2005;Huang et al 2011a;Tian and Zhao 2012;Liu et al 2013Liu et al , 2014.…”

“…Here, we introduce a currently well-used Q tomography method utilizing seismic waveform data, following Liu et al (2014) who determined high-resolution 3-D P-and S-wave attenuation (Qp and Qs) models of the Tohoku subduction zone. They measured a large number of high-quality t* data precisely from P-and Swave velocity amplitude spectra of local earthquakes over a frequency range of 0.5-25.0 Hz.…”

“…As shown in Eq. (2.39), the corner frequency f ci is coupled with the wholepath attenuation operator t * , and so trade-off may occur between them during the spectra-fitting procedure estimating them simultaneously (e.g., Scherbaum 1990; Ko et al 2012;Liu et al 2014). To tackle this trade-off problem and obtain highquality t * data, the multi-window spectral ratio method (Imanishi and Ellsworth 2006) is used to first estimate the corner frequencies of local earthquakes independently and then to determine the t * values with the corner frequencies obtained.…”

“…Equation (2.41) shows that the spectral ratio method can estimate precisely the corner frequencies, largely eliminating the whole-path attenuation ( t * ) effect (e.g., Frankel and Wennerberg 1989;Imanishi and Ellsworth 2006;Liu et al 2014). Liu et al (2014) estimated the velocity spectral ratio for a pair of nearby earthquakes recorded at the same station, and did this for all the 316 seismic stations in their study region. All of the spectral ratios for an event pair are stacked if they have a sufficient signal-to-noise (S/N) ratio (> 2.0) in the frequency band 0.5-25.0 Hz.…”

The crust and uppermost mantle s-wave velocity structure beneath japan islands revealed by joint analysis of p-and s-wave receiver functions

“…Many seismic researches have been conducted in order to detect the crust and upper mantle structure beneath Japan Islands. Using seismic body wave travel time tomography method and the seismic data recorded by more and more densely distribution of seismic stations in Japan, some scientists have obtained detailed P wave and S wave velocity perturbation images within upper mantle depth which has greatly improved the understanding about the deep structure and the tectonics beneath Japan and the relationship between large earthquakes and the deep velocity structure [3][4][5][6][7][8]. To the purpose of exploring the deep tectonic causes of 2011 Tohoku earthquake, some scientists studied the seismic attenuation of Northeast of Japan arc [9].…”