The post-earthquake stress state on the Tohoku megathrust as constrained by reanalysis of the JFAST breakout data

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Author(s): Brodsky, Emily E.; Saffer, Demian M.; Fulton, Patrick M.; Chester, Frederick M.; Conin, Marianne; Huffman, Katelyn Allison; Moore, J. Casey; Wu, Hung-Yu
Author Affiliation(s): Primary:
University of California, Santa Cruz, Santa Cruz, CA, United States
Pennsylvania State University, United States
Texas A&M University, United States
Université de Lorraine, France
Chevron Corporation, United States
Japan Agency for Marine-Earth Science and Technology, Japan
Volume Title: AGU 2017 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2017; American Geophysical Union 2017 fall meeting, New Orleans, LA, Dec. 11-15, 2017. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The JFAST drilling project endeavored to establish the stress state on the shallow subduction megathrust that slipped during the M9 Tohoku earthquake. Borehole breakout data from the drillhole can constrain both the orientation and magnitude of the principal stresses. Here we reanalyze that data to refine our understanding of the stress state on the fault. In particular, we: (1) Improve the identification of breakouts, (2) Consider a fuller range of stress states consistent with the data, and (3) Incorporate new and more robust laboratory constraints on rock strength. Based on this reanalysis of the breakout data, we conclude that: (1) The orientation of the maximum horizontal stress is 28° from the convergence direction. Although a parallel orientation cannot be ruled out by the data, it is not the optimal solution. The lack of consistency suggests stress changes during the earthquake comparable to the ambient stress. (2) All possible solutions of the data that utilize the laboratory constraints on strength and a 3-D failure criterion (Modified Weibol-Cook) require that the system was in a normal faulting regime 14 months after the 2011 Tohoku earthquake. This is consistent with a broad range of independent observations in the region, including seafloor surveys that indicate post-seismic extensional failure on presumed normal faults and normal faulting aftershocks. Again, this observation implies near total stress drop. (3) Extrapolating the stress state from the breakout zone results in an effective friction on the fault that is lower than the friction averaged during all of slip. The combined analysis suggests the earthquake released sufficient elastic strain energy to reset the local stress field.
Year of Publication: 2017
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 19 Geophysics, Seismology; Earthquakes; Integrated Ocean Drilling Program; Japan Trench Fast Drilling Project; Seismotectonics; Tectonics; Tohoku-Oki earthquake 2011
Record ID: 2019016291
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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