| | AbstractA
high-resolution tomographic study, using cubic B-splines
parameterization and employing a systematic approach to the choosing of
appropriate damping and smoothing parameters, provided a
three-dimensional P-wave velocity map of the Loma Prieta area.
Used in the inversion were 11,977 high-quality raypaths from 844
aftershocks of the 1989 Loma Prieta earthquake. The velocity model
exhibits a low-velocity feature between the San Andreas and
Zayante-Vergeles faults in the top 10 km of the crust. This low-velocity
feature is interpreted as a sedimentary unit exposed to the northwest
and separated from the Salinian block by the Zayante-Vergeles fault.
Below 10 km, no consistent change is observed between the Salinian and
the Franciscan blocks. There appears to be a high correlation of
aftershock activity and localized high-velocity anomalies southeast of
the Loma Prieta mainshock. Whereas this anomaly may represent brittle
rocks associated with a fault-zone asperity that failed after the
mainshock, there is evidence to suggest it may be a body of
serpentinite. The serpentinite exhibits high velocities and is
potentially less competent than surrounding country rock, thus providing
a sector along the fault more likely to be associated with many smaller
earthquakes or creep behavior.Return to Table of
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