| To understand the role of the deformation in the oceanic lithosphere, I have analyzed focal mechanisms from moderate (4.7 ;Re-analysis of the aftershocks of the November 14, 1986 earthquake (Wu et al., 1989) shows that the event ruptures a northwest-dipping plane, which implies that the oceanic crust of the Philippine Sea Plate is underthrusting the continental crust. Examination of the spatial distribution of the newly obtained focal mechanisms and the background seismicity indicates that, south of the November 14 hypocenter, there is shallow (depths ;Since the orogeny is younger in the south than in the north, these sections are also time lines of the orogeny (Teng, 1990). Thus, the oceanic lithosphere appears to thicken through conjugate faulting. As the oceanic lithosphere thickens, it comes into contact with deep weaker material on the continental margin. This results in a fault where the oceanic crust underthrusts the continental margin. Analysis of aftershocks shows an n aftershock alignment from the November 14, 1986 ;While the relationship between the underthrust and the Ryukyu subduction remains unclear, the presence of the latter clearly influences the deformation of the Philippines Sea Plate. In fact, the focal mechanisms from events east of Taiwan are mostly consistent with those expected from the subduction. Moreover, there are strike-slip focal mechanisms southeast of Taiwan that show east-west oriented P-axes and north-south oriented T-axes. These mechanisms, coupled with the presence of the Ryukyu subduction 100 km to the north, support Wu's (1978) hypothesis of northward escape of the oceanic lithosphere into the Ryukyu Subduction Zone. (Abstract shortened by UMI.);By applying this method to 40 moderate earthquakes, I am able to image some aspects of the internal deformation, both at the suture and in the oceanic lithosphere. Comparisons of the focal mechanisms with aftershock seismicity indicate that, in the few cases where causative faults could be determined, there is probably conjugate reverse faulting in the oceanic lithosphere. In fact, all of the reverse mechanisms in the oceanic lithosphere are consistent with conjugate faulting. |
