Abstract
We systematically investigate spatial variations of temporal changes
and depth extent of damage zones along the Calaveras fault that ruptured
during the 1984 Morgan Hill earthquake by the waveform analysis of
333 sets of repeating earthquakes. We use a sliding window waveform
cross-correlation technique to measure traveltime changes in waveforms
generated by each repeating cluster. We find clear traveltime delays
in the S- and early S-coda waves for events immediately after the
Morgan Hill main shock. The amplitudes of the time delays decrease
logarithmically with time since the main shock, indicating a time-dependent
recovery (healing) process following the abrupt coseismic temporal
changes. The largest temporal changes are observed at station CCO
that is the closest to the rupture zone of the Morgan Hill main shock.
The time delays at this station are larger for clusters in the top
6 km, and decrease systematically at larger depth. In comparison,
the time delays observed at other five stations are much smaller,
and do not show clear relationship with hypocentral depth. We suggest
that the temporal changes at these five stations mostly occur in
the top few hundred metres of the near-surface layers, while the
temporal changes at station CCO are likely associated with the damage
zone around the Calaveras fault that is well developed in the top
few kilometres of the upper crust. Our results are consistent with
the inference of a widespread damage and non-linearity in the near-surface
layers associated with strong ground motions of nearby large earthquakes,
and localized damages and flower-type structures around active faults
based on previous studies of fault zone structures and recent 3-D
numerical simulations.
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