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Report | SCEC Research Accomplishments
surface wave arrivals. They find an apparent migration of
triggered LFEs along strike at 80 km/hr. The observation of
triggered tremor on northern and southern California faults is
less frequent and these tremor episodes have lower
amplitudes (Figure 4).
Cochran et al. are developing a neural network approach to
detect tremor within continuous data. The approach uses Self
Organizing Maps (SOMs) based on the work of Köhler et al
[2009] to classify the time-series data into prototype vectors
based on waveform characteristics including frequency
content, amplitude, and polarization. These prototype vectors
can then be associated with different signal types including
earthquakes, tremor, and noise. The method is being applied
to data collected during a temporary deployment of
broadband instruments near Cholame, California.
Fault Structure and Damage Zones
Li et al. completed a detailed study of the on- and near-fault
damage and healing at the San Andreas fault (SAF) Parkfield
and the Longmen-Shan Fault (LSF) in Sichuan, both of which
ruptured in the recent major earthquakes. Along the SAF,
they find a waveguide that is interpreted to be a damage zone
of reduced seismic velocities with a width of 100-200 m that
extend to at least 7 km depth (Figure 5). The Wenchuan fault
is found to have a 200-400 m wide damage zone with
velocities reduced by up to 50% at shallow depths.
Ma modeled the effect of heterogeneity of fault geometry on
rupture dynamics in three dimensions with off-fault
plasticity (Figure 6). He finds that fault roughness results in
irregular rupture fronts that could result in greater highfrequency
radiation. In addition, he finds that fault roughness
results in more pulse-like ruptures due to rapid healing.
Duan models the effect of off-fault damage and pore-pressure
variations on the dynamics of parallel strike-slip faults. He
finds that when effects of off-fault plastic yielding and timedependent
pore pressure changes are combined, the effect of
time-dependent pore-pressure dominates, while off-fault
plastic yielding only exerts a second-order effect.
References
Aiken, C., Z. Peng, and C. Wu (2011), Dynamic triggering of
microearthquakes at three geothermal regions in
California, Seismological Society of America Annual
Meeting, Memphis, TN.
Aguiar, A. C., J. R. Brown, and G. C. Beroza (2009), Nonvolcanic
tremor near the Calaveras Fault Triggered by
Mw~8 Teleseisms, Eos Trans. AGU, 90(54), Fall Meet.
Suppl., Abstract T23E-06.
Chao, K., Z. Peng, A. Fabian, and L. Ojha (2011),
Comparisons of Triggered Tremor in California, Seis. Res.
42 | Southern California Earthquake Center
Figure 4. Median amplitude of the high-frequency envelope
functions versus the peak surface wave amplitudes measured at
the broadband station PKD in central California for teleseismic
earthquakes. The values measured during the surface waves
and before the P-wave.
Figure 5. (a) Observed and synthetic vertical-component
seismograms for an on-fault aftershock recorded at an array near
Parkfield, showing fault-zone guided waves. Seismograms have
been