An alternative interpretation for slip vector residuals of subduction interface earthquakes: a case study in the westernmost Ryukyu slab

Abstract

Slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes. It is measured through contrasting the slip vector with respect to the local plate convergence direction defined on the surface of the Earth by the Euler vector. Interpretation of regional plate kinematics based on slip vector residuals thus defined may be misleading. Besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface, a more fundamental problem is the construction of the reference subduction flow field for the subducted slab based upon the known plate convergence. It can be shown that for complicated slab geometry, the conventional practice of intuitively rotating the Euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere. Alternatively, we propose to calculate the velocity field for the specific slab geometry based on the rationale that the subduction flow field should be the one that endures the least amount of intraplate deformation. The calculated flow field for the westernmost Ryukyu slab, that is in transition to the Philippine Sea plate (PSP)–Eurasian Plate (EP) collision near Taiwan, reveals quite distinct subduction flow field. The particle paths associated with the calculated flow field are in better accordance with the observed slip vectors while the corresponding strain-rate field is consistent with the previously reported lateral compression seismogenic zone. Furthermore, it is speculated that the plate kinematics of the northwestern corner of PSP that is impinging the vicinity of the collision might have also undergone significant readjustment to accommodate the potential intraplate deformation. It is noted that the westernmost subducted Ryukyu slab and the northwestern corner of PSP belong to a mechanically coherent tectonic unit. The 3D geometric constraints set up by the subducted slab and the “unfriendly” kinematic constraints imposed by the collision boundary both play important roles in configuring the intraplate deformation within PSP. Manifestation in the local kinematics might indicate deviation from the general PSP–EP convergence, and is thus of crucial importance to tectonic interpretations of geophysical observations in this area.