Toward understanding slip-to-the-toe earthquakes along the Japan Trench: results and a future plan of a geophysical-geological project
Abstract
Earthquake, tsunami and geodetic data of the 2011 Tohoku-oki earthquake show that the fault rupture propagated to a shallow part of the subduction zone. However, these observations alone cannot clarify whether the coseismic slip fault extended up the plate boundary interface all the way to the trench axis, mainly because of low near-trench resolution. In addition, several seismological studies suggested a structural anomaly, such as a subducted seamount, in an area of the rupture initiation, but a previous wide-angle seismic data show smoothly landward dipping oceanic crust. In order to examine a coseismic mega-thrust fault extending from the hypocenter to the trench axis, we processed seismic data acquired during a rapid response geophysical cruise soon after the earthquake, from 14th to 30th March in 2011. Comparing those data with one obtained before the earthquake in 1999, we found a remarkable coseismic structural difference in trench-filled sediment. As conventional multi-channel seismic data could not resolve a detailed fault image in the trench-filled sediment, additional high resolution seismic data were acquired from 22nd October to 11th November in 2011. We found, from the high resolution seismic data, a 3-km-long, ~350-m-thick deformed/upheaval structure showing a compressional structure (i.e., thrust-up structure) with reverse faults branching from an interface within the subducting sedimentary layer, which is interpreted as the coseismic master fault reaching to theseafloor. In the area around the hypocentre, seismic reflection data acquired after the earthquake show a strong landward dipping reflection which was not obvious before the earthquake. An important observation is that a depth-converted section by using a velocity structure obtained by wide-angle seismic data shows that this reflector is imaged within the subducted oceanic crust. This suggests that the fault rupture of the 2011 earthquake was likely initiated in the subducted oceanic crust, not along the plate interface. We believe that deformation structures we observed in the Japan trench axis preserve record of a slip-to-the-toe earthquake and can be used to identify where a large slip to the toe has occurred along the trench, not only during the 2011 earthquake but also during past tsunamigenic earthquakes. In order to understand along-trench extension and records of this type of earthquakes, we started a new five-years geophysical-geological project from 2011, including high resolution seismic, micro-bathymetry and piston-coring to cover the entire Japan trench. In our presentation, we will also introduce an outline and preliminary results from this project.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21F..02K
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 7240 SEISMOLOGY / Subduction zones