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

Seismogenic conditions associated with the plate interface in the south ernmost Ryukyu arc-Taiwan region is studied in four different aspects.From the history of large and great earthquakes, it is inconclusive whether M � events have ever occurred along the plate interface, but there seems w to be no doubt that at least a few earthquakes with M �.0-7.5 have ocw curred 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 thrustn earthquakes near the lower portion of the interface further sup ports that the interface is not strongly coupled.The maximum M of an w interface earthquake is estimated to be 7.6-7.7,as inferred from the geometric configuration of the interface.The average slip obliquity of inter face earthquakes is 35° while the convergence obliquity is 70°.Such a pat tern 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 M w >S subduction earthquakes.However, there is strong evidence in focal mechanisms to indicate interactions among different tectonic stress regimes.Although such interactions may not necessarily mean a bigger earthquake, they are possible to increase the frequency of earthquake occurrence be cause the combined stress regime would reach the failure criteria of geo logical 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 from 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 indus trial facilities are probably the most effective measures to prevent the po tential devastation due to earthquakes.