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SCEC Research Accomplishments | Report
Figure 2. (Left) Map of El Mayor sequence and the background seismicity from 1 Jan. to 21 Nov. 2010. (Right) Time-space
plot of latitude versus time of El Mayor aftershocks triggered earthquakes to the north along the San Jacinto and Elsinore
fault zones.
Observations of Tremor in California
Bürgmann et al. examine triggering of tremor and
repeating earthquake sequences (RES) at Parfield. They
explore whether tidal triggering of tremor occurs by
comparing the timing of 600,000 low-frequency
earthquakes (LFEs) reported in the Shelly and
Hardebeck [2010] catalog. They find that the shear stress
perturbations correlate with tremor occurrence better
than normal stress perturbations (Figure 3). They also
find that short-term triggering may occur between
neighboring events as shown by the joint occurrence of
RES and nearby background earthquakes. The
immediate triggering of few second to minutes can
happen when the separation distance is within a few km.
When earthquakes are farther apart, triggering is less
efficient.
Peng et al. explore and compare triggered tremor in
California. They find that along the San Andreas fault
triggering generally occurs after the S-wave arrival with
the most intense triggering during large-amplitude
Figure 3. Along-fault section near Parkfield showing LFE locations colored
by the percent excess value (Nex), the difference between the actual and
expected number of events that occur under a particular tidal loading
condition divided by the expected number of events. Positive Nex values
indicate a surplus of events and negative values, a deficit. Values
representing correlation of LFE occurrence with right-lateral shear stress
(RLSS, top) and fault normal stress (FNS, bottom) are shown.
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