P and S wave tomography and anisotropy in Northwest Pacific and East Asia: Constraints on stagnant slab and intraplate volcanism

Abstract

We determined three-dimensional P and S wave velocities and P wave azimuthal anisotropic tomography of the Northwest Pacific subduction zones by inverting 1,225,086 P wave and 335,117 S wave arrival times from 13,413 earthquakes. Our results show some differences between P and S wave images for the stagnant Pacific slab in the mantle transition zone (MTZ) beneath Northeast China. The stagnant slab looks thicker in the P wave image than that in the S wave image, which may reflects the effects of both hydration and lower temperature in the MTZ, though differences in the resolution of P and S wave tomography may also have some effects. The Changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant Pacific slab. Our P wave anisotropy tomography shows that the fast velocity direction (FVD) in the subducting Philippine Sea plate beneath the Ryukyu arc is NE-SW (trench parallel), which is consistent with the spreading direction of the West Philippine Basin during its initial opening stage, suggesting that it may reflect the fossil anisotropy. A striking variation of the FVD with depth is revealed in the subducting Pacific slab beneath the Northeast Japan arc, which may be caused by slab dehydration that changed elastic properties of the slab with depth. The FVD in the mantle wedge beneath the Northeast Japan and Ryukyu arcs is trench normal, which reflects subduction-induced convection. Beneath the Kuril and Izu-Bonin arcs where oblique subduction occurs, the FVD in the mantle wedge is nearly normal to the moving direction of the downgoing Pacific plate, suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow.

Philippine Sea Plate inception, evolution, and consumption with special emphasis on the early stages of Izu-Bonin-Mariana subduction

“…A continuous slab-like high-velocity zone (HVZ), dipping roughly 45°, extends beneath the 1200 km-long Ryukyu Arc down to ∼300 km depth. This is consistent with the edge of the seismicity around 250 km according to Wei et al (2015), whereas Bijwaard et al (1998) imaged the high-velocity anomaly deeper, down to at least a∼500-600 km depth where it merges with the flat-lying Pacific slab. Lallemand et al (2001) suspected that the deepest portion of the PSP slab detached during the Upper Miocene -Lower Pliocene east of the Gagua Ridge (Fig.…”

“…Constraints and speculations on the subducted portions of the PSP The slab beneath the Ryukyu and SW Japan arcs A significant part of the PSP has been subducted beneath the Ryukyu Arc as attested by Wadati-Benioff zone seismicity and regional scale tomographic studies (e.g., Bijwaard et al 1998;Widiyantoro et al 1999;Wang et al 2008;Li and van der Hilst 2010;Wei et al 2012Wei et al , 2015. A continuous slab-like high-velocity zone (HVZ), dipping roughly 45°, extends beneath the 1200 km-long Ryukyu Arc down to ∼300 km depth.…”

“…3). Wei et al (2015) mentioned that the HVZ extends into the uppermost upper mantle near Taiwan, but only in V S tomography, which could suggest that an old and more rigid part of the PSP has penetrated into the lower mantle there. That deep slab could be connected with the detached slab east of the Gagua Ridge (see Fig.…”

Sequence of deep-focus earthquakes beneath the Bonin Islands identified by the NIED nationwide dense seismic networks Hi-net and F-net

“…Recent seismic surveys have revealed the complex configuration and subduction system of the Pacific slab beneath the Izu-Bonin arc (e.g., Miller et al 2004;Fukao and Obayahashi 2013;Wei et al 2015;Porritt and Yoshioka 2016). Such lateral tension events are consistent with the proposed slab geometries (e.g., Fukao and Obayahashi 2013; Wei et al 2015;Obayashi et al 2017). The hypocenters of the earthquake sequence are located in the transition region from slab stagnation to penetration.…”

The isolated ∼680 km deep 30 May 2015 MW 7.9 Ogasawara (Bonin) Islands earthquake

“…The slab has to tear, fold or buckle to extend continuously to the position of this event (Fig. 9b, c) as also suggested by tomographic models (e.g., Wei et al, 2015), and such deformation could broaden the horizontal width of the cold region. Alternatively, there could be a chunk of detached slab (e.g., Chen and Brudzinski, 2001), but we believe this is unlikely given the progressive steepening of the Benioff-zone seismicity band toward the south of the Bonin slab.…”