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Low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress

Nature Geoscience volume 8, pages 409–415 (2015)Cite this article

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Abstract

At subduction zones, the level of friction on the deep part of the plate boundary fault controls stress accumulation and release, which govern the transfer of stress to the shallower, locked part of the fault that can slip in a megathrust earthquake. In some subduction zones, the deep fault slips slowly, at speeds far less than shallower, regular earthquakes, and is often accompanied by weak seismic waves called tremor. Tremor and slow slip can be triggered by small stress changes induced by ocean or solid Earth tides. Here I use seismic data combined with calculations of tidal stress to determine the influence of tides on 31,000 tremors generated by six large slow-slip events in Cascadia between 2007 and 2012. I find that the sensitivity of tremor to tidal stresses rises during each slip event, as slip at each spot on the fault accumulates. Specifically, tremor rate is an exponential function of tidal stress, and this exponential sensitivity grows for several days, implying that the fault weakens during slip. I use the relationship between tidal stress and tremor to calculate a coefficient of intrinsic friction for the fault and find values of 0 to 0.1, which indicate that the deep fault is inherently weak.

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Figure 1: Consistency of tremor with sense of tidal stress.
Figure 2: Evolution of tremor response to tides.
Figure 3: Influence on tremor of shear versus normal stress changes.
Figure 4: Pore-pressure models and friction coefficient.
Figure 5: Model of stress and strength evolution during ETS.

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Acknowledgements

I thank J. Hawthorne for assistance with calculations of tidal stresses, S. Ide for suggesting the use of a Weibull distribution, and J. Vidale and K. Creager for helpful suggestions. Reviews from R. Burgmann and the Editor helped improve the paper. This work was supported by NSF grant EAR-1447005.

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  1. Department of Earth and Space Sciences, University of Washington, Johnson Hall 070, 4000 15th Avenue NE Seattle, Washington 98195-1310, USA,

    Heidi Houston

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Correspondence to Heidi Houston.

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Houston, H. Low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress. Nature Geosci 8, 409–415 (2015). https://doi.org/10.1038/ngeo2419

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