| | AbstractFor a decade, the
U.S. Geological Survey has used the Parkfield Earthquake Prediction
Experiment scenario document to estimate the probability that
earthquakes observed on the San Andreas fault near Parkfield will turn
out to be foreshocks followed by the expected magnitude 6 mainshock.
During this time, we have learned much about the seismogenic process at
Parkfield, about the long-term probability of the Parkfield mainshock,
and about the estimation of these types of probabilities. The
probabilities for potential foreshocks at Parkfield are reexamined and
revised in light of these advances. As part of this process, we have
confirmed both the rate of foreshocks before strike-slip earthquakes in
the San Andreas physiographic province and the uniform distribution of
foreshocks with magnitude proposed by earlier studies. Compared to the
earlier assessment, these new estimates of the long-term probability of
the Parkfield mainshock are lower, our estimate of the rate of
background seismicity is higher, and we find that the assumption that
foreshocks at Parkfield occur in a unique way is not statistically
significant at the 95% confidence level. While the exact numbers vary
depending on the assumptions that are made, the new alert probabilities
are lower than previously estimated. Considering the various assumptions
and the statistical uncertainties in the input parameters, we also
compute a plausible range for the probabilities. The range is large,
partly due to the extra knowledge that exists for the Parkfield segment,
making us question the usefulness of these numbers.Return to Table of
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