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Poster Abstracts
existing catalog of tremor events. Using the dense PASO array and various correlation methods, including autocorrelation
(Brown et. al. 2008), a scanning algorithm (Rowe, 2005), and cross correlation of template events (Shelly et al., 2007), we will
refine the locations of these known events and seek to identify undiscovered clusters of LFEs and tremor. After generating an
updated catalog initially for the month of September 2002, we will use S-wave arrivals from the 59 stations comprising the
PASO array to provide strong constraints on the locations of identified events.
KINEMATICS OF ROTATING PANELS OF E-W FAULTS IN THE SAN ANDREAS SYSTEM: WHAT CAN WE
TELL FROM GEODESY? (A-056)
J.P. Platt and J.P. Becker
Panels of E-W-trending sinistral and/or reverse faults occur in various locations within the San Andreas system, and are
commonly associated with paleomagnetic evidence for clockwise rotations over geologic time. These panels commonly cut
across the trend of active dextral faults, posing questions as to how displacement is transferred across them. Geodetic data
indicate that they lie within an overall dextral shear field, and the data are commonly interpreted to indicate little or no slip on
the E-W faults.
We model these faults and fault panels as rotating by bookshelf slip in a dextral shear field. This allows prediction of rates of
slip, rotation, fault-parallel extension, and fault-normal shortening. We decompose the geodetically determined velocity field
into these components, and compare them with our predictions,as illustrated by the following two examples. (1) The velocity
field around the central section of the Garlock fault is consistent with a combination of long-term sinistral slip rate of ~7.5
mm/yr on a locked fault, and a counterclockwise rotation rate of 5.2°/m.y. The resultant field comprises a dominant
component of dextral shear at a rate of 90 nanostrain/yr, and a residual component of fault-parallel motion that reflects the
elastic strain around the locked fault. (2) The E-W trending faults of the western Transverse Ranges currently lie at ~50° to the
Pacific – North America plate motion vector, having been rotated clockwise into this orientation during the since early
Miocene time. In this orientation the rate of sinistral slip is small, but they are still rotating at ~2.5°/m.y. Strike-parallel
extension at 4 mm/yr is also clearly detectable in the velocity field, as well as strike-normal shortening at up to 6mm/yr. The
rate of strike-normal shortening decreases westward, reflecting transfer of displacement from Inner Borderland dextral faults
south of the Transverse Ranges onto faults within the Salinia block.
These two examples demonstrate firstly, that significant sinistral slip on rotating E-W trending faults may only be detectable
through second-order elastic effects; and secondly, that previously active sinistral faults may become inactive because of
rotation, but may still define a discrete panel deforming by other mechanisms.
STRAIN LOCALIZATION DRIVEN BY THERMAL DECOMPOSITION DURING SEISMIC SHEAR (A-092)
J.D. Platt, N. Brantut, and J.R. Rice
De Paola et al. [2008] analyzed a series of faults in the Northern Apennines, Italy, hosted in anhydrite and dolomite rocks.
They found a highly localized band of less than 100 microns, contained within a broader damage zone. Recent High-Velocity
Friction (HVF) experiments on kaolinite-bearing gouge samples (Brantut et al. [2008]) have also shown extreme localization in
samples undergoing thermal decomposition. They performed microstructural analysis on HVF samples and found an
“ultralocalized deformation zone”, less than ten microns wide, interpreted to be the main slipping zone in the experiment. By
measuring relative humidity in the sample chamber they were also able to observe the thermal dehydration of kaolinite. These
laboratory and field observations indicate that straining is extremely localized in fault materials where thermal decomposition
reactions may occur.
During thermal decomposition reactions pore fluid is released, leading to increases in pore pressure, and a corresponding
drop in frictional heating. The reactions are endothermic, so heat is also absorbed as the reactions progress. Previous work by
Sulem and Famin [2009] has investigated how these effects influence the evolution of pore pressure and temperature in a
uniformly sheared gouge layer. They found that accounting for thermal decomposition reactions leads to significant pore
pressure increases, and that the endothermic nature of the reaction acts to cap the maximum temperature achieved.
In previous work (Platt, Rudnicki and Rice [2010]) we investigated strain localization using a model for shearing of a fluidsaturated
gouge material, finding a formula for the localized zone width as a function of physical properties of the gouge. We
now extend this model to include thermal decomposition. Using linear stability methods and an idealized reaction kinetic we
infer a new localized zone width when decomposition is accounted for. Numerical simulations then allow us to compare this
prediction to results obtained using a realistic Arrhenius kinetic relation for the reaction. We find qualitative agreement
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