Seismicity Precursors of the M6.0 2004 Parkfield and M7.0 1989Loma Prieta Earthquakes Page: 4 of 35
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M7.9 Fort Tejon (Langbein et al., 2005) and M7.7 1906 San Francisco (Wald et al.,
1999) have an average slip of about 4 m (Wald et al, 1993), which translates to an
average recurrence time interval of every 100-200 years. This evaluation assumes
approximately 2-3 cm/yr of average tectonic plate displacement by SAF and also
accounts for some incomplete release of accumulated strain for those events. With
catastrophic events occurring so rarely, even moderate uncertainty in prediction makes it
unrealistic to use GRR-derived statistics for disaster-related warnings. Moreover,
predictions expressed in terms of probabilities are inappropriate for rare earthquake
occurrences, since definitions of probability are based on statistical limits of multiply
occurring events. Practically applicable prediction methods need to be based on causal
approaches.
In this paper, a selective seismicity analysis is used in which only events having a
direct relationship with strain-buildup processes are included. The idea of selectiveness
is partially based on the results of a Vibroseis monitoring experiment in which seismic
waves repeatedly illuminated the epicentral region of the expected M6 event at Parkfield
from June 1987 to November 1996. Data collected by the borehole network were
examined for evidence of changes associated with the nucleation process of the
anticipated M6 earthquake at Parkfield (Karageorgi et al., 1992, 1996; Korneev et al.,
2000, Korneev and Nadeau, 2004). These investigations reported significant travel-time
changes for paths crossing the fault zone in the locked southeast part of SAF, while in
the northwest (creeping) part of the SAF, no changes were observed. This result
suggests that little or no information about stress accumulation in the SAF can be
gathered from the seismicity of the SAF's creeping part, where a weak fault steadily
releases small stress changes and the seismicity mostly represents a stationary random
process. Indeed, the weak creeping faults can be modeled as large-scale fractures
having very low friction and the capability to immediately discharge any applied shear
stresses. The stress-strain conditions on both sides of such fractures generally stay
unchanged, with just small fluctuations and no dependence on the regional stress
buildup. At the same time, the seismicity associated with weak faults makes a dominant
contribution to regional event statistics, overshadowing the seismicity directly related to
regional stress accumulation. Therefore, all events with hypocenters within the active
fault zone are excluded from the results shown in this paper. The transition zone
between the locked and creeping parts of SAF is a northwesterly dipping structure,
oriented at approximately 450 and extending for about 5 km along the fault (Korneev et4
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Korneev, Valeri A. Seismicity Precursors of the M6.0 2004 Parkfield and M7.0 1989Loma Prieta Earthquakes, report, March 9, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc891334/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.