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Group 2 – FARM | Poster Abstracts
for dry diabase and dry novaculite. Another trend in the onset of stick slip occurs due to normal
stress. Seen best at higher temperatures, a transition from stable sliding to stick slip occurs with
increasing normal stress. Our observations are broadly consistent with predictions of the rate and
state friction theory, indicating that lower slip rates and higher normal stresses result in enhanced
interlocking of contacts on a frictional interface. Our data show that elevated temperatures give rise
to the same effect, suggesting a thermally-activated nature of the asperity contacts. Surprisingly,
we did not observe a high-temperature transition from stick-slip back to stable sliding in novaculite
(purely silicic lithology), even at temperatures as high as 500°C. Such a transition is widely
believed to be responsible for the brittle-ductile boundary defining the bottom of the seismogenic
layer. Our observations may highlight the role of water on the brittle-ductile transition and suggest
that the velocity weakening behavior can extend considerably deeper than typically thought, at
least in the dry middle-to-lower crust. We also point out potential similarities between the periodic
accelerated creep observed in our experiments at the boundary between the stick-sip and stable
sliding regimes, and episodic slow-slip events reported near the velocity-neutral transition in a
number of subduction zones. The slip rates observed during slow-slip events in our experiments
have the same order of magnitude (10-8 m/s) as the slow-slip events in the Cascadia subduction
zone.
2-093
HETEROGENEITY OF FOCAL MECHANISM ORIENTATIONS IN DIFFERENT
PARTS OF THE SAN JACINTO FAULT ZONE Bailey IW, Ben-Zion Y, Becker TW, and
Holschneider M
We investigate earthquake heterogeneity associated with different parts of the San Jacinto fault
zone in terms of the statistical variation of focal mechanism orientations. Our analysis is based on a
catalog of ~12,000 focal mechanisms for earthquakes with magnitude between zero and five
recorded between January, 1984 and July, 2003. The focal mechanisms are double-couple solutions
computed from first-motion data using the program HASH (Hardebeck & Shearer, 2002).
Individual focal mechanisms are associated with one of seven sections of the San Jacinto fault,
based on their horizontal distance from the fault trace according the USGS quaternary fault map.
We assess the deformation properties of fault sections by summation of their potency tensors. We
investigate the earthquake heterogeneity for each fault section based on the orientation statistics of
the double-couples, which are described by distributions of rotation angles and rotation axes for
the minimum rotation between all pairs within each population. We relate the heterogeneity of
earthquakes to fault heterogeneity by numerical simulations that consider slip along a set of fault
planes with varied orientations. The slip direction of each fault is computed from the maximum
shear stress produced by a specified regional stress tensor. Our inferences from the simulations are
compared to measures of fault complexity inferred from the patterns of fault traces.
2-094
TRANSITIONS TO CHAOS IN DIETERICH-RUINA FRICTION Erickson BA, Birnir B,
and Lavallee D
We began investigations into the Dieterich-Ruina (D-R) friction law in previous work by studying
the behavior of a single slider-block under this law. We found transitions to chaos in the numerical
solution to this system when a specific parameter was increased. This parameter, ? = (B-A)/A is the
ratio of the stress parameters (B-A) and A in D-R friction. The parameter A = d(?)/d(log(v)), where
? is the frictional stress and v is the velocity of the slider, is a measure of the direct velocity
dependence (sometimes called the "direct effect") while (A-B) = d(?_{ss})/d(log(v_{ss})), is a
measure of the steady-state velocity (v_{ss}) dependence. When compared to the slip weakening
friction law, the parameter (B-A) plays a role of a stress drop while A corresponds to the strength
2008 SCEC Annual Meeting | 191