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Fault damage zones and friction: insights from shallow Japan Trench drill core




Keren, Tucker T., author
Kirkpatrick, James D., advisor
Caine, Jonathan S., committee member
Bareither, Christopher A., committee member

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The integrated deformation in a fault damage zone provides a record of repeated slip on faults and the properties that control slip behavior. Shear displacement on faults is a frictional phenomenon in the sense that slip begins when the ratio of shear to normal stress surpasses the static friction coefficient (μs). Thus, earthquakes and the associated patterns of slip are controlled by the frictional properties of faults. The March 11, 2011 Mw 9.0 Tohoku-oki earthquake resulted in an unprecedented coseismic slip of >50 m in the shallow portion of the Japan Trench subduction zone. This thesis presents analysis of drill core recovered during Integrated Ocean Discovery Program (IODP) Expedition 343/343T (JFAST) that characterizes structures surrounding the inferred Japan Trench décollement, and uses the results to constrain the relative magnitude of the long-term, or static friction coefficient (μs) on the fault. Datasets include distinction of tectonic and induced structures in drill core, measurements of structure intensity at the core and microscopic scales, and core-scale structure orientations. The results show a sharp decrease in tectonic core-scale structure and microstructure intensity with distance from the décollement, with small spikes corresponding to locations of some known secondary faults. Orientation data reveals a nearly full range of structure dip angles <70° at all depths. The fall-off in structure intensity is fit by power law functions, with decay exponent (n) values consistently >1 in the footwall (underthrust sedimentary units) and <1 in the hanging wall (frontal prism sedimentary units). Estimations of the damage zone thickness from structure intensity data results in an order of magnitude approximation of a few to several tens of meters. Faults of similar displacement report smaller damage decay exponents and larger damage zone thickness estimations. A quasi-static analytical model of a wavy, frictional fault is used to test the sensitivity of damage zone characteristics to the static friction coefficient (μs), and if friction may be responsible for distinct damage zone characteristics in the maximum coseismic slip region of the Japan Trench during the Tohoku-oki earthquake. Modeling suggests the comparatively narrow damage zone, steep damage decay exponents, and spread of structure orientations can all be a consequence of a frictionally weak fault with a low static friction coefficient (μs ≈ 0.1), consistent with existing constraints for the coseismic frictional strength of the Japan Trench décollement. Results and modeling suggest the shallow Japan Trench décollement is thus too weak to store significant elastic strain energy. Implications include constraints on models for the large shallow coseismic slip during the Tohoku-oki earthquake, and the potential for propagation of future large earthquakes through the shallow décollement without the presence of a shallow strong patch of the fault.


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