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Variations in earthquake-size distribution across different stress regimes

Nature volume 437, pages 539542 (22 September 2005) | Download Citation

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Abstract

The earthquake size distribution follows, in most instances, a power law1,2, with the slope of this power law, the ‘b value’, commonly used to describe the relative occurrence of large and small events (a high b value indicates a larger proportion of small earthquakes, and vice versa). Statistically significant variations of b values have been measured in laboratory experiments, mines and various tectonic regimes such as subducting slabs, near magma chambers, along fault zones and in aftershock zones3. However, it has remained uncertain whether these differences are due to differing stress regimes, as it was questionable that samples in small volumes (such as in laboratory specimens, mines and the shallow Earth's crust) are representative of earthquakes in general. Given the lack of physical understanding of these differences, the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b = 1 is a universal constant for earthquakes in general5. Here we show that the b value varies systematically for different styles of faulting. We find that normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress.

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Acknowledgements

We thank E. Hauksson, D. Giardini, M. Mai, M. Gerstenberger, D. Jackson, J. Woessner and G. Hillers for discussions. We thank the Northern California Seismic Network, US Geological Survey, Menlo Park, and the Berkeley Seismological Laboratory, University of California, Berkeley, for the catalogue including phase data, and the National Research Institute for Earth Science and Disaster Prevention for mechanism solutions of the Kanto-Tokai area.

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Affiliations

  1. Swiss Seismological Service, ETH Zürich, ETH Hönggerberg, Schafmattstr. 30, 8093 Zürich, Switzerland

    • Danijel Schorlemmer
    •  & Stefan Wiemer
  2. World Agency of Planetary Monitoring and Earthquake Risk Reduction, Route de Jargonnant 2, 1207 Genève, Switzerland

    • Max Wyss

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Correspondence to Danijel Schorlemmer.

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https://doi.org/10.1038/nature04094

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