SSA '98: Abstracts Online

Please Note: Abstracts appear here exactly as they were submitted through our 
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PARKFIELD EARTHQUAKE: NOT LIKELY THIS YEAR 

JACKSON, D.D. and KAGAN, Y.Y., 
Southern California Earthquake Center, University of California, Los Angeles,
CA 90095-1567, djackson@ucla.edu 

 The probability of a moderate earthquake at Parkfield is officially taken to
be about 10 percent per year, according to a recent report by Roeloffs and
Langbein (Rev. Geophys. 32, 315, 1994). This figure is near the middle of
several published estimates, all based on three assumptions: (1) the
characteristic earthquake hypothesis, (2) quasiperiodic earthquake recurrence
on the "Parkfield segment," and (3) that moderate earthquakes in 1857, 1881, 
1901, 1922, 1934, and 1966 were all characteristic Parkfield earthquakes. Davis
et al. (BSSA 79, 1439, 1989) showed that even if these assumptions are
adopted, allowing for the open interval since 1966 decreased the estimated
probability to a value of about 7 percent per year in 1989. The equivalent
rate is now about 5 percent per year. However, the three basic assumptions
must be questioned. As applied to the Pacific Rim, the characteristic
earthquake hypothesis has failed important statistical tests, and has not been
statistically validated anywhere. Without the characteristic hypothesis,
quasiperiodic recurrence loses its meaning. Moreover, geodetic studies by
Segall and others show that the 1934 and 1966 earthquakes had quite different
rupture distributions; and evidence for the locations and magnitudes of the 
first three events is largely circumstantial. Without the three assumptions,
the Parkfield region can be interpreted as an area which by chance endured a
larger than average number of earthquakes in the last century and half. Its
selection for special study may be largely a result of that chance. Assuming
that the "Parkfield Study Region" has earthquake magnitudes distributed
according to a truncated Gutenberg-Richter distribution with a maximum 
magnitude of about 8 and a rate sufficient to release all of the long-term slip
on the San Andreas, the probability of a magnitude 6 or larger event there is
less than 1 percent per year.