JDASH – Japan Trench Deep-sea Research for Assessing Shallow Seismic Slips and Their History

Abstract:

We started a multidisciplinary research project to elucidate the history of recurring tsunamigenic slips on the shallowest portion of the plate boundary fault along the Japan Trench before the 2011 earthquake and to understand post-seismic transient processes since the massive rupture of 2011. In the 5-year project, we will: 1) Conduct high-resolution seismic surveys and use the resultant data to both elucidate the spatial extent of past shallow seismic slip events and identify deformation structures characteristic of the events. 2) Determine the timing of past slip events by collecting sub-seafloor sedimentary core samples from the ultra-deep environment near the Japan Trench and identifying and dating earthquake-generated turbidite layers. 3) Identify postseismic deformation processes on the shallow fault by using dense arrays of broadband seismic sensors and monitoring seafloor deformation near the trench axis. The geodetic monitoring includes continuous measurement of baseline lengths across the trench axis, which reflects relative motion of the overriding North American (or Amur) plate and the subducting Pacific plate.

 Systematic studies on past shallow seismic slip events will improve our understanding of the space–time history of large earthquakes, which has mostly derived from geological studies on tsunami deposits along coasts. The space–time history of large earthquakes will provide clues to understanding why earthquakes of M <8 have been frequent and those of M~9 infrequent in the same subduction system, a fundamental question raised by the 2011 Tohoku earthquake. The post-2011 deformation near the trench will put strong constraint on the frictional behavior of the updip end of the megathrust. Since shallow slip events increase size of tsunamis associated with megathrust earthquakes, history of past events is crucial for evaluating tsunami hazard in regions facing to subduction zones. This project will establish a methodology to clarify reliable reccurence history of shallow seismic slip events in subduction zones in the world.