Anomalously steep dips of earthquakes in the 2011 Tohoku-Oki source region and possible explanations
The 2011 M_w 9.1 Tohoku-Oki earthquake had unusually large slip (over 50 m) concentrated in a relatively small region, with local stress drop inferred to be 5–10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (M_w 5.5–7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the M_w 9.1 event had steeper dip angles (by 5–10°) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5–10°, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues.
Additional Information© 2012 Elsevier B.V. Received 19 May 2012. Received in revised form 26 July 2012. Accepted 27 July 2012. Editor: P.Shearer. Available online 7 September 2012. We thank Seiichi Miura, Narumi Takahashi, Aki Ito and Ryota Hino for providing their velocity models or earthquake catalog. We thank Robert Graves, another anonymous USGS internal reviewer and two anonymous reviewers for their comments that improved the manuscript. The Incorporated Research Institutions for Seismology (IRIS) provided the seismic data. All figures are made with GMT. This work is supported by the National Science Foundation through grant number EAR-1142020. Contribution #10080 of the Tectonic Observatory, California Institute of Technology.
Supplemental Material - mmc1.pdf