Can Great Earthquakes Occur in the Southernmost Ryukyu Arc-Taiwan Region?


Seismogenic conditions associated with the plate interface in the southernmost Ryukyu arc-Taiwan region is studied in four different aspects. From the history of large and great earthquakes, it is inconclusive whether Mw >= 8 events have ever occurred along the plate interface, but there seems to be no doubt that at least a few earthquakes with Mw >= 7.0-7.5 have occurred in the region since the turn of this century. Earthquake focal mechanisms show the co-existence of extensional earthquakes in the vicinity of trench and within the subducted lithosphere, suggesting no patterns of a seismic cycle due to strong interface coupling. The lack of ¡§transitional thrust¡¨ earthquakes near the lower portion of the interface further supports that the interface is not strongly coupled. The maximum Mw of an interface earthauake is estimated to be 7.6-7.7, as inferred form the geometric configuration of the interface. The average slip obliquity of interface earthquakes is 35 while the convergence obliquity is 70. Such a pattern of slip partitioning indicates that the rheological behavior of the Ryukyu fore-arc in this region is not completely elastic, thus is unlikely to generate Mw > 8 subduction earthquakes. However, there is strong evidence in focal mechanisms to indicate interactions among different tectonic stress regimes. Although such interaction may not necessarily mean a bigger earthquake, they are possible to increase the frequency of earthquake occurrence because the combined stress regime would reach the failure criteria of geological materials more effectively. Given that NE Taiwan is the most likely place that interactions among various stress regimes could take place, it is suggested the seismicity there to be closely monitored. From the seismic hazard point of view, the potential threat form frequent occurrences of Mw > 7 events in the region cannot be ignored. Due to the short distances from NE Taiwan to the three largest metropolitan areas on the island, improving and strictly enforcing building code and careful planning of major industrial facilities are probably the most effective measures to prevent the potential devastation due to earthquakes.

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