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Earthquake slip weakening and asperities explained by thermal pressurization


An earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1,2. This slip weakening occurs over a critical distance, Dc. Understanding the controls on Dc in nature is severely limited, however, because the physical mechanism of weakening is unconstrained. Conventional friction experiments, typically conducted at slow slip rates and small displacements, have obtained Dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3,4,5,6. Here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the Median Tectonic Line fault zone, Japan. We show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7,8,9,10 is the slip-weakening mechanism. Our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity.

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Figure 1: The nature of a localized slip zone in a large geological fault.
Figure 2: Permeability and porosity data for slip zone rocks.
Figure 3: Hydraulic diffusion lengths during thermal pressurization as a function of slip zone width.
Figure 4: Shear stress modelling for the thermal pressurization process.

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C.A.J.W. acknowledges financial support from the Japan Society for the Promotion of Science (JSPS), and assistance in the field from S. Uehara. This work was partially supported by a Grant-in-Aid for Scientific Research, JSPS, and by the Kyoto University ‘Active Geosphere Project for the 21st Century’.

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Correspondence to Christopher A. J. Wibberley.

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Wibberley, C., Shimamoto, T. Earthquake slip weakening and asperities explained by thermal pressurization. Nature 436, 689–692 (2005).

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