Letter abstract


Nature Geoscience 1, 59 - 63 (2008)
Published online: 9 December 2007 | doi:10.1038/ngeo.2007.36

Subject Categories: Seismology | Structural geology, tectonics and geodynamics

Persistent earthquake clusters and gaps from slip on irregular faults

Tom Parsons

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Earthquake-producing fault systems like the San Andreas fault in California show self-similar structural variation1; earthquakes cluster in space, leaving aseismic gaps between clusters. Whether gaps represent overdue earthquakes or signify diminished risk is a question with which seismic-hazard forecasters wrestle1, 2, 3, 4, 5. Here I use spectral analysis of the spatial distribution of seismicity along the San Andreas fault (for earthquakes that are at least 2 in magnitude), which reveals that it obeys a power-law relationship, indicative of self-similarity in clusters across a range of spatial scales. To determine whether the observed clustering of earthquakes is the result of a heterogeneous stress distribution, I use a finite-element method to simulate the motion of two rigid blocks past each other along a model fault surface that shows three-dimensional complexity on the basis of mapped traces of the San Andreas fault. The results indicate that long-term slip on the model fault generates a temporally stable, spatially variable distribution of stress that shows the same power-law relationship as the earthquake distribution. At the highest rates of San Andreas fault slip (40 mm yr-1), stress patterns produced are stable over a minimum of 25,000 years before the model fault system evolves into a new configuration. These results suggest that although gaps are not immune to rupture propagation they are less likely to be nucleation sites for earthquakes.

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  1. US Geological Survey, MS-999, 345 Middlefield Rd., Menlo Park, California 94025, USA

Correspondence to: Tom Parsons e-mail: tparsons@usgs.gov



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