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Group 2 – Seismology | Poster Abstracts
2-110
PRECISE RELATIVE LOCATION OF SAN ANDREAS FAULT TREMORS NEAR
CHOLAME, CA USING SEISMOMETER CLUSTERS Shelly DR, Ellsworth WL, Nadeau
RM, Burgmann R, Ryberg T, Haberland C, Murphy JM, and Fuis GS
Non-volcanic tremor, similar in character to that generated at some subduction zones, was recently
identified beneath the strike-slip San Andreas Fault (SAF) in central California. Using a matched
filter method, we closely examine a 24-hour period of active SAF tremor and show that, like tremor
in the Nankai Trough subduction zone, this tremor is composed of repeated similar events. We
take advantage of this similarity to locate detected similar events relative to several chosen events.
While low signal-to-noise makes location challenging, we compensate for this by estimating eventpair
differential times at “clusters” of nearby temporary and permanent stations rather than at
single stations. We find that the relative locations consistently form a near-linear structure in map
view, striking parallel to the surface trace of the SAF. Therefore, we suggest that at least a portion
of the tremor occurs on the deep extension of the fault, similar to the situation for subduction zone
tremor. Also notable is the small depth range (a few hundred meters or less) of many of the located
tremors, a feature possibly analogous to earthquake streaks observed on the shallower portion of
the fault. The close alignment of the tremor with the SAF slip orientation suggests a shear slip
mechanism, as has been argued for subduction tremor. At times, we observe a clear migration of
the tremor source along the fault, at rates of ~20 km/hr.
2-111
ESTIMATING THE RUPTURE VELOCITY OF SMALL EARTHQUAKES McGuire JJ,
Ide S, Kim M, Iidaka T, and Hirata N
Studies of earthquake scaling are extremely common but have been inconclusive in determining
the existence of any fundamental differences in the mechanical processes controlling small and
large earthquakes. While many studies have found a deficit of radiated energy per unit moment
release in small earthquakes, work by Ide and Beroza (2001) called these interpretations into
question. Much of the uncertainty in whether there are major differences between small and large
earthquakes results directly from the difficulty in measuring the energy radiated by small
earthquakes. Kanamori and Rivera (2004) pointed out that if the observed deficiency in radiated
energy for small earthquakes is real, then either the rupture velocity and/or the static stress drop
must decrease for small earthquakes. A surprising result since some studies have suggested that
small earthquakes have higher stress drops. If this difference was reflected primarily in rupture
velocity, the velocity of magnitude 3 earthquakes would be at most half (for e=0.5) of that in
magnitude 7 earthquakes.
We estimated the rupture velocity of earthquakes between M3 and M5 for faults in California and
Japan using an Empirical Green's Function (EGF) deconvolution technique. The time functions
resulting from the EGF deconvolution show clear variation in their durations as a function of
azimuth for a given earthquake. This signal is inverted for the second moments of the earthquake's
space-time moment-release distribution which provide a lower bound on rupture velocity
(McGuire 2004). The EGF approach leads to relatively tight constraints on earthquake scaling
because the azimuthal variation in time function duration is more clearly resolved than the
azimuthal variation in corner frequency. We find no evidence for a decrease in rupture velocity
with earthquake size. Magnitude 3 events within the rupture zones of the 2000 M6.5 Tottori and
2004 M6 Parkfield earthquakes have rupture velocities that are indistinguishable from those of the
mainshocks on the same faults and error analysis demonstrated that rupture velocities faster than
2.0 km/s are required with best fit values closer to 3 km/s. These general patterns hold true for
crustal earthquakes in many regions of Japan recorded by the HINET array. We find that the value
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