SEISMOTECTONICS OF THE MEDIAN TECTONIC LINE IN SOUTHWEST JAPAN - IMPLICATIONS FOR COUPLING AMONG MAJOR FAULT SYSTEMS

The relationship between the slip activity and occurrence of historica l earthquakes along, the Median Tectonic Line (MTL), together with tha t of the fault systems extending eastward has been examined. The MTL i s divided into three segments, each containing diagnostic active fault s. No historical earthquakes have been recorded along the central segm ent, although the segment has faster Quaternary slip rates compared wi th the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explai ned by a microplate model of southwest Japan. The microplate model als o provides spatial and temporal coupling of slip on adjacent fault sys tems- In the context of this model, slip on adjacent faults reduces th e normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong g round motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where his torical earthquakes were recorded. Alternatively, the earthquakes at t he eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that t he MTL characteristically does not produce great earthquakes. The micr oplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough.