Earthquakes along the Ryukyu‐Kyushu Arc: Strain segmentation, lateral compression, and the thermomechanical state of the plate interface

We systematically determined the focal depths and mechanisms of 49 large to moderate-sized earthquakes (mb≥5.4) that occurred along the Ryukyu-Kyushu arc since 1963 by inverting the waveforms and amplitudes of P and SH wave trains at teleseismic distances. The results are sufficiently precise to delineate seismogenic structures near the plate interface in detail, revealing features not predicted by plate kinematics. In contrast to previous studies of this arc and those along other subduction zones, shallow seismicity along the plate interface is systematic, showing two distinct layers of activities. The second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km, some 10–20 km directly beneath the seismogenic portion of the interplate thrust zone. These earthquakes indicate lateral compressional strain within the subducted slab as their P axes are subparallel to the local strike of the arc, not perpendicular to the arc as one would expect from the direction of plate convergence. The occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab. To our knowledge, similar earthquakes occurred only beneath the northern Indoburman ranges and along the Banda arc where subducted slabs, as part of the Indian plate, are being dragged northward with their northern edges bumping into east-west trending collision zones nearby. By drawing an analogy between the tectonic settings of these three regions, we interpret events beneath the plate interface along the Ryukyu-Kyushu arc as a consequence of ongoing collision between the Philippine Sea plate and Eurasia near Taiwan. The interplate thrust zone is largely aseismic down to a depth of approximately 30 km. A large number of earthquakes showing low-angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20–25 km. These observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25±5 km. Based upon available heat flow measurements between the trench axis and the volcanic arc, we estimated the temperature field and magnitude of shear traction along the interplate thrust zone, the deepest interplate earthquakes (∼40–50 km) correspond to temperatures of approximately 730–980°C, comparable to the limiting temperature of intraplate mantle earthquakes (∼800°C) and that of intermediate- and deep-focus earthquakes at Wadati-Benioff zones (potential temperature ∼900°K). Therefore, the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions. The magnitude of shear traction along the seismogenic portion (∼40–50 km depth) of the interplate thrust zone seems to be of the order of 100 MPa which, in turn, implies a low average coefficient of friction of only 0.10±0.05, considerably less than those observed for laboratory specimens. The inception of intermediate-focus earthquakes within the subducted slab is at a depth of about 100 km. Whereas shallow earthquakes show no apparent variations along the entire arc, intermediate-depth earthquakes indicate downdip extension along the northern end of the arc near Kyushu but abruptly change to downdip compression along the rest of the arc. All available evidence indicates that this is a spatial pattern unrelated to earthquake cycles. At the moment, tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic. Without the presence of clear discontinuities in the age of the subducted slab, the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc. In any case, this pattern of strain segmentation requires the presence of a major accommodation structure, such as a tear fault, in the subducted slab near the Tokara channel. Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Ave., N.W. , Washington, DC 20009. Document B91-004; $2.50. Payment must accompany order.