The Taiwan‐Ryukyu subduction‐collision complex: Folding of a viscoelastic slab and the double seismic zone

Abstract

[1] The termination of the Ryukyu trench against Eurasia and the oblique subduction of the Philippine Sea plate create a subduction-collision complex offshore Taiwan, which has not previously been elucidated in detail. We combine traveltime data from the seismic networks in Taiwan and Japan to better illuminate how the subducting Ryukyu slab deforms in this subduction-collision zone. More than 5000 events recorded by both networks were relocated with the double-difference method using an optimal regional one-dimensional velocity model. The offshore seismicity indicates that the double seismic zone, with a gap of 15-20 km, exists in the subducting slab in the depth range of 40-80 km. Focal mechanisms suggest that the double seismic zone is caused by east-west compression resulting from oblique convergence. The improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the Ryukyu trench in the depth range 50-100 km. The anomalous curvature, together with the focal mechanisms, suggests that the slab folds against the Eurasian lithosphere and that this deformation cannot be fully elastic. We model this deformation mode as the developing instability of a viscoelastic Maxwell layer embedded in a viscous medium. The characteristic wavelength of the instability, i.e., $250 km, is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along-strike elastic membrane strain as small as 0.01, or more than 3 orders of magnitude higher if 5% elastic strain is allowed.

A model for the termination of the Ryukyu subduction zone against Taiwan: A junction of collision, subduction/separation, and subduction boundaries

“…Another alternative model of the “kink” was proposed by Chou et al [2006]. They used a similar combination of CWB and JMA data set as we have and found that the slab to the west of about 122.2°E defined by seismicity deeper than 50 km assumes a noticeable different trend from that to the east of 122.2°E.…”

“…We use the results to construct a consistent plate model and then extrapolate to the surface to define the boundaries. Furthermore, to get a sense of the overall PSP subduction zone between the end of the well‐defined Ryukyu Trench and eastern Taiwan, we combine data from the Ryukyu Islands and northern Taiwan to locate events in this area, again using the double‐difference method [see also Font et al , 2004; Chou et al , 2006]. …”

Focal-Mechanism Determination in Taiwan by Genetic Algorithm

“…Many studies based on limited focal mechanisms have been conducted in the past in Taiwan tectonics (Kao, Jian, et al, 1998;Kao, Shen, and Ma, 1998;Kao and Rau, 1999;Chou et al, 2006). Focal mechanisms determined in this study provide an alternative database for the study of subduction systems.…”

Origin of the southern Okinawa Trough volcanism from detailed seismic tomography

“…However, the westernmost part of the Ryukyu slab was not clearly imaged. Recently, Chou et al [2006] have combined data from two networks and relocated more than 5000 earthquakes. The shape of the Ryukyu slab obtained from this relocated hypocenter distribution shows a continuous slab configuration from 121.5°E to 124°E longitude and a folding of the slab around 123°E rather than a slab tear as proposed by Deschamps et al [2000].…”