Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

Abstract

An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.