A Comprehensive Analysis of Deformation Data at Parkfield, California: Detection of a Long-term Strain Transient


More than a decade of high quality data from geodetic and borehole strain instrumentation in the Parkfield, California area (1985-1997) reveal a significant transient in apparent slip-rate along the San Andreas fault. This transient consists of two components: decreased slip-rate (by 16 ± 5%) during 1991.0 - 1993.0 followed by an increased rate (by 34 ± 6%) from 1993.0 for at least 3 years. The second component was observed by Gwyther et al. (1996) from tensor strain data. This transient is clear in all the deformation data used in this study, including those from 2-color laser geodimeters, borehole strain dilatometers, and creepmeters. Compared with pre-1991 levels, seismicity is greater during this episode, especially for the period of increased slip-rate, during which the four largest earthquakes in the observing period occurred. Since there was also a significant increase in precipitation during the period 1991-1996, we tested for the relative contributions of tectonic and hydrologic subluences to the observed signals. For this purpose we analyzed the geodimeter data, using for each baseline, the decade-long secular change in baseline length as a measure of tectonic sensitivity, and the amplitude of the annual cycle as a measure of hydrologic sensitivity. We found that the anomaly is significantly correlated with tectonic sensitivity at the 95% confidence level, but not with hydrologic sensitivity. From this we conclude that the observed transient is primarily tectonic, and most easily explained by local variations in slip-rate along the San Andreas fault. However, there may still be a hydrologic role in the modulating tectonic activity, through, for example, precipitation-induced variations in pore-pressure within the fault zone or in surface loading. We are presently examining this possibility.

Meeting Name

Fall Meeting (1997: Dec. 1, San Francisco, CA)


Geosciences and Geological and Petroleum Engineering

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Article - Conference proceedings

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© 1997 American Geophysical Union (AGU), All rights reserved.

Publication Date

01 Dec 1997

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