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Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site

Nature volume 454, pages 204–208 (2008)Cite this article

Abstract

Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies1,2.

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Figure 1: Map of the experiment site.
Figure 2: An example of the raw seismograms obtained from a horizontal component in the two periods.
Figure 3: Estimated delay times for the two periods.
Figure 4: A comparison of delay time variations with local seismicity and other deformation measurements.

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Acknowledgements

We thank the NSF funded SAFOD programme and those involved in providing the experiment site, R. Trautz for supplying the barometric pressure logger, M. Zumberge for providing the SAFOD strainmeter data, and D. Lippert and R. Haught for helping with field work. This work was supported by the NSF, Rice University, the Carnegie Institution of Washington, and Lawrence Berkeley National Laboratory of the US Department of Energy under contract DE-AC02-05CH11231.

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Authors and Affiliations

  1. Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, Texas 77005, USA,

    Fenglin Niu & Xin Cheng

  2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington DC 20015, USA,

    Paul G. Silver

  3. Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA,

    Thomas M. Daley & Ernest L. Majer

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  1. Fenglin Niu
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  5. Ernest L. Majer
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Correspondence to Fenglin Niu.

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Niu, F., Silver, P., Daley, T. et al. Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site. Nature 454, 204–208 (2008). https://doi.org/10.1038/nature07111

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