Schaff, D., Bokelmann, G.H.R., Beroza, G.C., Waldhauser, F., Ellsworth, W.L., 2002,
High resolution image of Calaveras fault seismicity,
Journal of Geophysical Research, 107, 633-668
By measuring differential arrival times between different earthquakes using waveform cross
correlation and locating earthquakes using the double difference technique, we are able to
reduce hypocentral errors by 1 to 2 orders or magnitude over routine locations for nearly 8000 events
along a 35 km section of the Calaveras Fault. This represents
approximately 92 percent of all seismicity since 1984 and includes the rupture zone of the
M 6.2 1984 Morgan Hill, California earthquake.
The relocated seismicity forms highly organized structures that were previously obscured
by location errors. There are abundant repeating earthquake sequences as well as linear clusters of
earthquakes. Large voids in seismicity appear with
dimensions of kilometers that have been aseismic over the 15-year time interval, suggesting
that these portions of the fault are either locked or creeping.
The area of greatest slip in the Morgan Hill mainshock coincides with the most prominent of these voids,
suggesting that this part of the fault may be locked between earthquakes. We find that the Calaveras
fault at depth is extremely thin, with an average upper bound on fault-zone width of 75 meters.
Given the location error, however, this width is not resolvably different
from zero. The relocations reveal active secondary faults, which we use to solve for the
stress field in the immediate vicinity of the Calaveras Fault.
We find that the maximum compressive stress is at a high angle, only 13 degrees from the fault normal
vector, supporting previous interpretations that this fault is weak.
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