Changes in Seismic Waveform Attributes after Loma Prieta: A Discriminant for the Role of Fluids in the Fault Zone ?
G. H. R. Bokelmann, Geophysics, Stanford University,
Stanford, CA 94305-2215, USA
Baisch, S., Universitaet Bochum, 44780 Bochum, Germany
During 1987 to 1995, several clusters of nearly identical events
occured near the Loma Prieta source region.
These multiplets allow us to investigate and demonstrate clear changes in seismic
wave character associated with the 1989 Loma Prieta main shock.
These are apparent in seismogram similarities, in spectral ratios and in
velocity changes. This investigation follows the first two approaches.
1) Monitoring seismogram similarities in time and frequency domain.
Here, the time-spatial pattern gives evidence for an anomalous region in the crust
which is most likely distorted by the main shock and shows a progressive recovery process
that is well fitted by a logarithmic dependence. Over the time span of 6 years after
the main shock, the anomalous region is nearly completely ''healed''.
This region is apparently located close to the main shock hypocenter.
2) Monitoring spectral ratios between pre-main-shock and post-main-shock
multiplet members. From relative changes in the spectral ratios,
changes of the generalized attenuation can be infered.
The observed changes of spectral ratios show a similar time signal,
where spectral ratios change with the main shock
and thereafter progressively approach the pre-main-shock
state.
This corroborates previous observations of velocity changes in the Loma Prieta
region.
A possible cause for the observed anomaly is
a main-shock induced hydraulic
opening of crack systems. Then the observed time signal
would reflect closure and/or crack healing following the main shock.
Back to My Homepage , Stanford University or Department of Geophysics