The Great 1933 Sanriku-oki Earthquake: Possible Compound Rupture of Outer Trench Slope and Triggered Interplate Seismicity

The 1933 Sanriku-oki earthquake offshore northern Honshu, Japan (Mw8.4) is the largest earthquake that occurred outer-rise/outer-trench-slope region. The spatial extent of the aftershocks and possibility of a triggered seismicity was estimated by using modern relocation method and velocity structure. Land-station based hypocenter determination by using 3D velocity structure was firstly applied to the off-Sanriku, near-trench region where systematic hypocenter shifts are recognized in the previous studies. The improvement of hypocenter locations near the trench were confirmed by examinations of recent earthquakes that are accurately located based on OBS data. The earthquakes after the 1933 Sanriku-oki earthquake are located about 200 km long region under the outer trench slope that is separated from the aftershock seismicity under the inner trench slope. The outer-trench-slope earthquakes are shallow (depth <=50km) and has V-shape distribution in the trench-normal cross-section. The aftershock distribution suggests shallow rupture area and possibly a compound rupture for the 1933 main shock. We found the V-shaped compound rupture model explains better the polarity of Tsunami waves at the Sanriku coast than a single west dipping fault. This indicates that the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks both in outer- and inner trench slope regions was confirmed by the investigation of dominant wave frequency which is seen in the recent precisely located earthquakes in the two regions (Gamage et al., 2009). The earthquakes under the inner trench slope were shallow (depth <=30km) and located where recent activity of interplate thrust earthquakes is high. This suggests the deformation of the 1933 outer-rise earthquake triggered the interplate earthquakes. Recent (2001-2012) seismicity around the source area by the same method show the seismicity at the outer trench-slope region of northern Honshu can be divided into several groups of earthquakes along the trench; one group roughly corresponds to the aftershock region of the 1933 earthquake. Comparison of the 1933 rupture dimension based on our relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited by the region where fault scarps are trench parallel and cross cutting seafloor spreading fabric. These suggest bending and structural segmentation largely controls the horizontal and vertical extent of the fault. The re-examined aftershock distribution in this study provides a constraint on the stress state of the subducting plate and water supply to deep earth. They also suggest triggered of interplate seismicity that imply the outer rise /outer trench slope earthquake is closely involved in the earthquake cycle of interplate earthquake.