HETEROGENEOUS CRUSTAL STRUCTURE ACROSS A SEISMIC BLOCK BOUNDARY ALONGTHE NANKAI TROUGH

The Nankai trough, which runs along the western Japanese islands, is w idely known for its systematic activity of large earthquakes with the Magnitudes greater than 7. According to the fault-plane studies of the se earthquakes, a major seismic block boundary is supposed to run off the Kii Peninsula almost perpendicular to the trough axis. A seismic e xperiment was conducted across this boundary region in order to determ ine seismic crustal structure. P-wave travel time analyses were applie d to the data from the OBSs deployed along the trough axis, including travel time nonlinear inversion. The result shows an abrupt change in thickness of the subducting Philippine Sea Plate's crust at the expect ed location of the seismic block boundary. The crust is thicker in the east by about two kilometers, and the interface between the overridin g and subducting plates runs shallower in the east. These structural f eatures may affect the mode of earthquake occurrence.