Seismogenic Mega-splay Fault in Subduction Zone-Modern and Ancient Examples from Southwest Japan

In the Nankai Trough, SW Japan, 1944 Tonankai earthquake took place along a mega splay fault branching from the plate boundary between the upper Eurasian and lower Phillipine Sea Plate (Park et al., 2002). The splay fault is recognized as a strong reflector showing partly negative polarity with a wavelength of ca. 200 m (Park et al., 2002). The fault appears to make a boundary between the underthrusted sediments in the deep portion (> 6km), cut the Nankai accretionary prism in a shallow part, and appears a cold seep spot on the sea bottom surface (Ashi, person. Com.). A major out-of-sequence thrust (the Nobeoka Thrust) cutting the ancient accretionary prism of the Shimanto Belt into two subbelts is observed in the Kyushu Island, SW Japan (Murata, 1986; Kondo et al., in press). The fault was located under the thermal condition ca. 250¡OC in terms of vitrinite reflectance and fluid inclusion geo-thermo-barometry, that is enough within the seismogenic zone by thermal model (Hyndman et al., 1997) and is a good analog for the modern splay fault in the Nankai Trough mentioned above. Structural analysis of the thrust shows that the damage zone around the thrust is a few hundred meters thick (Kondo, et al., in press) and is characterized by strong brittle deformation with ubiquitous development of crack-seal and shear-parallel veins. Hanging-wall side of the Nobeoka Thrust is composed of shale/sandstone dominated phyllites, of which deformation is characterized primarily by plastic flow and pressure solution overprinted by brittle cataclastic shear and cracking. Several thin cataclasitc shear zones represent extremely fine fragmental components or re-crystalized clays. Their chemical aspect indicates preferential Fe and Mg concentration, which is suggestive of preferential melting of micaceous and chrolite of host rocks due to frictional heating. Asymmetric development of cracks surrounding the thin shear zones is also characteristic. The asymmetric cracks are quite resemble to that developed in the process zone of fault (Vermilye and Sholz, 1998), that indicates upward propagation of rupture. The footwall side of the Nobeoka Thrust is completely brittle, and crack seal or shear zone filling quartz or calcite veins are developed. Geometric relationship of these veins with the fault is quite systematic, therefore indicates quite active fluid deposite and flow in the damage zone. Fluid inclusion analysis indicates that fluid passing through strongly deformed part of damage zone was hotter than that in less deformed part. Such a passage may be possible only during dynamic dilatant rupture associated with seismic event.