Masaaki Shirai

Submarine Slope Response to Earthquake Shaking Within Western Sagami Bay, Central Japan

Earthquakes are a major trigger of submarine landslides. Strong ground shaking of the sea floor by an earthquake increases the sediment pore pressure and induces slope failure. As submarine landslides may generate tsunamis, it is important to understand the submarine slope response to earthquake ground shaking. Slope response may change spatially according to the strength of ground shaking and slope characteristics, such as submarine topography, gradient, sea bed materials, and sedimentation rate. For a better understanding of slope response, it is necessary to obtain data on changes to the sea floor following an earthquake. In 1997, 1998, and 2006, the JAMSTEC Hatsushima Deep-Sea Observatory recorded turbidity currents triggered by the Izu-toho-oki earthquake swarm (magnitude ∼6) off Hatsushima Island in western Sagami Bay, Central Japan. In 2004, cores of undisturbed surface sediment were collected using a multiple corer at the foot of the submarine slope near the Observatory. No clear gravity-flow deposits (e.g., turbidites and debrites) were observed near the tops of the cores, indicating that earthquakes of magnitude ∼6 have insufficient energy to generate clearly defined sandy turbidites on this slope. In contrast, several sandy turbidite layers are present in a piston core collected from a nearby site, suggesting that this site is affected by a larger magnitude of ground shaking than that produced by the Izu-toho-oki earthquakes. The most likely origin of this greater ground shaking is the Kanto earthquakes (magnitude ∼8), the epicenter of which is located in the northwestern Sagami Trough.