P and S wave attenuation tomography of the Japan subduction zone

Abstract

We determine the first high‐resolution P and S wave attenuation (Q) tomography beneath the entire Japan Islands using a large number of high‐quality t∗ data collected from P and S wave velocity spectra of 4222 local shallow and intermediate‐depth earthquakes. The suboceanic earthquakes used in this study are relocated precisely using sP depth phases. Significant landward dipping high‐Q zones are revealed clearly, which reflect the subducting Pacific slab beneath Hokkaido and Tohoku, and the subducting Philippine Sea (PHS) slab beneath SW Japan. Prominent low‐Q zones are visible in the crust and mantle wedge beneath the active arc volcanoes in Hokkaido, Tohoku, and Kyushu, which reflect source zones of arc magmatism caused by fluids from the slab dehydration and corner flow in the mantle wedge. Our results also show that nonvolcanic low‐frequency earthquakes (LFEs) in SW Japan mainly occur in the transition zone between a narrow low‐Q belt and its adjacent high‐Q zones right above the flat segment of the PHS slab. This feature suggests that the nonvolcanic LFEs are caused by not only fluid‐affected slab interface but also specific conditions such as high pore pressure which is influenced by the overriding plate.

Age of the Subducting Philippine Sea Slab and Mechanism of Low‐Frequency Earthquakes

“…The youngest part (~15 Myr) of the PHS slab in SW Japan is located at the fossil ridge of the Shikoku basin where no LFE occurs, and the slab should be very thin and hot (Figure 4). Tomographic studies show a low Poisson's ratio and a high Q (low attenuation) at the Kii gap, which are generally considered to be a consequence of low fluid content (Liu & Zhao, 2015;Wang et al, 2006;Z. W. Wang et al, 2017), high permeability of the overlying plate (Nakajima & Hasegawa, 2016), or the lack of hydrous minerals (Seno & Yamasaki, 2003).…”

“…The degree of prograde metamorphism above the megathrust controlled by temperature distribution and lithosphere age of the subducting slab (Wang et al, 1995) may be proportional to the rate of fluid leakage from the subducting slab to the overlying plate (Nakajima & Hasegawa, 2016). A high fluid content has been suggested by studies of seismic tomography, which show that the LFEs occur in areas with a high Poisson's ratio, low Q, and low seismic velocity (Liu & Zhao, 2014, 2015Matsubara et al, 2009;Wang et al, 2006;Z. W. Wang et al, 2017).…”

Three‐Dimensional Seismic Attenuation Structure of Central Japan and Deep Sources of Arc Magmatism

“…In Figure 3a, the imaged low-to moderateattenuation PHS slab is clearly subducting to~150-km depth. The volcanic gap is located immediately above the shallow subduction of the PHS slab at 30-to 80-km depth, and the continental crust beneath the volcanic gap possesses distinct low-attenuation bodies compared with those beneath the active volcanoes (e.g., Wang et al, 2017). The volcanic gap is located immediately above the shallow subduction of the PHS slab at 30-to 80-km depth, and the continental crust beneath the volcanic gap possesses distinct low-attenuation bodies compared with those beneath the active volcanoes (e.g., Wang et al, 2017).…”

“…American Geophysical Union. Nakamura et al, 2006;Sekiguchi, 1991;Wang et al, 2017), limited attention has been given to the heterogeneous structures beneath the active volcanoes. the mantle wedge beneath volcanoes contains high-attenuation zones that are interpreted as areas of partial melting (e.g., Eberhart-Phillips et al, 2008;Haberland & Rietbrock, 2001;Kita et al, 2014;Nakajima et al, 2013;Saita et al, 2015).…”

Experimental Study of Seismic Wave Velocity Tomography

“…Assume the path from the source point to the detection point is a straight line, thus to prepare for the inversion, units passed by every ray can be recorded and the number of rays passing through every grid unit can be counted. The other is curved ray simulation with several kinds, such as a shortest path algorithm based on Fermat Theory and Graph Theory which is proposed by Moser [6] , and the improved Dijkstra algorithm in paper [7] . The forward modeling provides theoretical arrival time of the direct wave, and prepares for the inversion.…”