Annual Meeting - SCEC.org

Poster Abstracts
bearing faults as the dominant fracture type. Fracture density drops off sharply within the northern damage zone, resulting in
a highly asymmetric damage structure on a km-scale. Although fracture density is higher in the central zone, syn-tectonic
sealing of both micro- and macro-fractures by epidote, K-feldspar, and chlorite minerals was pervasive, resulting in low
permeabilities (~10-21m2). Fracture density is lower in the damage zones, and partial healing results in higher sample
permeabilities (~10-18m2). Laboratory P-wave velocities correlate well with both the architecture and sealing characteristics of
the fault zone. P-wave velocities are uniformly high (up to 6km/s) both within and immediately surrounding the central zone,
consistent with pervasive sealing of fractures and low sample permeability. In the damage zones P-wave velocities are much
lower (3-4km/s) due to the presence of open fractures. Such relationships highlight the close interplay between fracturing,
fluid flow, mineralization, and the strength of large fault zones. Importantly, they demonstrate that seismic wave velocities
and permeability depend on both fracture density and the degree of fracture sealing, which has implications for the
interpretation of active fault zone structure based on geophysical data.
TEMPERATURE DEPENDENCE OF FRICTIONAL HEALING: EXPERIMENTAL OBSERVATIONS AND
NUMERICAL SIMULATIONS (A-090)
E.K. Mitchell, Y. Fialko, and K.M. Brown
We performed a series of slide-hold-slide experiments on Westerly granite using a direct shear apparatus at ambient
temperatures between 20-550 ºC. We measured changes in static coefficient of friction after a period of time over which the
sample was held in nominally stationary contact. Friction increases in proportion to the logarithm of hold time at a rate of
about .02 per decade, similar to findings of previous studies conducted at room temperature. Friction also linearly increases
with temperature at about .02 per 140 ºC. We found that temperature has little effect on the rate of change in static friction
with hold time. We interpret these results using a numerical model that incorporates viscoelastoplastic rheology and a fractal
geometry of contact surfaces. We explore to what extent the observed time and temperature dependence of static friction can
be explained in terms of increases in the true contact area due to creep at highly stressed microasperities. We performed finite
element simulations of a contact consisting of a fractional Brownian surface pressed against a rigid flat surface. Changes in
contact area between the surfaces with respect to an initial contact area are compared to changes in static friction coefficient
with respect to its initial (reference) value. We find that the power-law rheology with stress exponent n = 3 (e.g., expected of
dislocation creep) does not fit the experimental data. It fails to generate an increase in the contact area over short (< 1000 s)
timescales; once the highly stressed contacts begin to relax, they do so too quickly. For the power-law rheology to provide a
reasonable fit to the data, the stress exponent needs to be increased to n = 45. In this case, significant (up to 500 degrees)
changes in contact temperature have little effect on the rate of increase in contact area.
REMOTE MAPPING OF A LARGE STEPOVER IN THE 4 APRIL 2010 EL MAYOR CUCAPAH EQ (A-117)
A.E. Morelan and M.E. Oskin
The 4 April 2010 MW 7.2 El Mayor Cucapah earthquake has been valuable to study because of its rich rupture exposure and
preservation. This dextral-oblique normal event ruptured on the NW-striking Pescadores, Borrego, Paso Inferior and Paso
Superior faults in the Sierra El Mayor and Sierra Cucapah mountain ranges, part of the active Pacific-North America plate
boundary between the northern tip of the Gulf of California and US- Mexico border. In total, the rupture consists of six major
faults, ~120 km in total length. Initial field work mapped the rupture across an 11 km left stepover in rugged terrain between
the Pescadores and Borrego faults. If accurate, this rupture would violate many accepted theories concerning the mechanics of
how ruptures terminate through stepovers; 11km is thought too great a distance for rupture to jump in a single event.
Although most models for stepovers use pure strike- slip motion on faults, this fault ruptured in a dextral- oblique normal
fashion with the ratio between normal and dextral motion between ~2- 3:1. This rupture characteristic may explain the
somewhat odd nature of this large stepover. Our remote mapping using high-resolution airborne LiDAR topographic data,
collected soon after the event, has yielded 2010 rupture on previously unidentified faults within the stepover. The distance
between rupture in the stepover, based on this new mapping, is 4km significantly less than the previously mapped 11km.
Abundant bedrock scarps were also remotely mapped in this rugged accommodation zone and are interpreted as rupture on
faults that pre-date the 2010 earthquake. These features turn out to be quite important because they beg the question of
whether or not this event was typical of this fault system. An 1892 event on the Laguna Salada fault ruptured along the
western range front of the Sierra Cucapah in a west- down motion, in an opposite sense of the 2010 event. Observation of and
comparison with such previous ruptures in the same region can increase our knowledge about the evolution of rupture style
along this fault system. Our remote mapping documents patterns of distributed faulting and displacement that allows us to
better understand strain accommodation and fault continuity during large oblique earthquakes. The airborne LiDAR dataset
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