Tomography and Dynamics of Western-Pacific Subduction Zones

Abstract

We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V ) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V /low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off b...

Methodology of Seismic Tomography

“…The first is that detailed studies of reflected and converted waves, as well as the first P-and Swaves, revealed the existence of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab beneath NE Japan (Fig. 2.1c), and the three discontinuities exhibit significant lateral depth variations (see the review by Zhao 2012). All the previous tomographic studies prior to 1990 (e.g., Aki and Lee 1976;Thurber 1983) could not handle the complex-shaped velocity discontinuities in the study area.…”

“…All the previous tomographic studies prior to 1990 (e.g., Aki and Lee 1976;Thurber 1983) could not handle the complex-shaped velocity discontinuities in the study area. The second reason is that clear later phases (i.e., reflected and converted waves) associated with the three discontinuities were detected in the seismograms of local crustal earthquakes and intermediate-depth earthquakes in the subducting Pacific slab (see Zhao 2012 for the detailed review). These later-phase data contain important information on the Earth's structure, in particular, in and around velocity discontinuities, hence they should be used in tomographic inversion so that local earthquakes can be better located and a better 3-D velocity model can be obtained.…”

“…2.1b);and Dziewonski (1984) adopted spherical harmonic expansions. Each of these approaches has pros and cons (for detailed reviews, see Thurber and Aki 1987;Nolet 1987Nolet , 2008Hirahara 1990;Iyer and Hirahara 1993;Zhao 2001aZhao , 2012. Zhao (1991) and Zhao et al (1992) adopted a boundary-grid approach to express the 3-D Earth's structure in their studies of the Japan subduction zone (Fig.…”

Wave-equation-based travel-time seismic tomography – Part 1: Method

“…Ever since the pioneering works of Aki and Lee (1976) and Dziewonski et al (1977), tomographic images have provided crucial information for the understanding of plate tectonics, volcanism, and geodynamics (e.g., Romanowicz, 1991;Liu and Gu, 2012;Zhao, 2012). Seismic tomography itself has also gone through significant development over the last 3 decades, including advances in both methodology and data usage.…”

Subduction Zone Tomography

“…Subduction zones are convergent plate boundaries which are characterized geographically by deep ocean trenches and island arcs or continental margins; seismically by the most abundant and largest earthquakes and the dipping Wadati-Benioff zone of intermediate-depth and deep earthquakes; tectonically by extensive crustal faulting and mountain building; and magmatically by a trench-parallel linear belt of active volcanoes, i.e., the volcanic front (e.g., Zhao 2001aZhao , 2012. Plate subduction is a fundamental and unique process in terrestrial tectonics and geodynamics.…”