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Poster Abstracts
between our two methods and find that thermal decomposition is important when attempting to predict the severity of strain
localization. The presence of the reaction localizes the deformation to zones as narrow as a few microns. This extreme
localization leads to dramatic dynamic weakening, suggesting thermal decomposition reactions may play an important role in
earthquake mechanics.
UPDATES FOR THE CVM-H INCLUDING NEW REPRESENTATIONS OF THE OFFSHORE SANTA MARIA
AND SAN BERNARDINO BASINS AND A NEW MOHO SURFACE (B-128)
A. Plesch, C. Tape, R. Graves, J.H. Shaw, P. Small, and G. Ely
We deliver an updated version of the Community Velocity Model to the SCEC UCVM as version 11.9. Major improvements
include a newly compiled Moho surface, addition of the offshore Santa Maria basin, a new detailed representation of the San
Bernardino basin, and a much smoother transition between the low resolution area and high resolution area at the southern
border of the Los Angeles basin.
The new Moho surface was compiled by Tape et al. from a large number of data sources, including global data sets, receiver
functions and and active-source studies. In replacing this surface care was taken to ensure that the depth intervals around the
revised Moho level both in the crustal and in the mantle data grids of the model were properly assigned to crust or mantle,
and consequently parametrized with velocities extrapolated from the underlying mantle model or from the crustal
background model.
In the offshore Santa Maria basin we extended the definition of the basement surface using reflection seismic data (McIntosh
et al., 1991) to the very western margin of the model, as well as to the north. This offshore basin addition removed an abrupt
transition from the basin to the background model. We smoothly extrapolated sediment velocities from the onshore model
into this new offshore basin area.
We also added a basin representation for the San Bernardino basin to the model. The basement surface is based on gravity
data (Anderson, 2000) and seismic reflection data (Stephenson et al., 2004). To the west the relatively small basin is connected
by a thin veneer of sediments to the Los Angeles basin whereas to the ESE the basin is represented as gradually tapering out
keeping in mind that the data there allow for other interpretations. The velocity structure (Vp) in the basin is defined by
stacking velocities (Stephenson et al., 2004) and a 1D velocity profile (Graves, 2008) combined to a basin thickness-depthvelocity
function. Vs is derived from Vp using Brocher (2004).
Finally, we revised the transition from the high to the low resolution models, removing a discrepancy in the velocity structure
between the southern margin of the high resolution model in the Los Angeles area and the surrounding low resolution model.
We modified the high resolution area by introducing a smooth N-S gradient in the previously existing delta between the low
and high resolution models in a region at its southern margin.
WHAT’S COOKING? EVALUATING FRICTIONAL STRESS USING EXTRACTABLE ORGANIC MATERIAL
IN FAULT ZONES (A-073)
P.J. Polissar, H.M. Savage, R.E. Sheppard, E.E. Brodsky, and C.D. Rowe
Determining the absolute stress on faults during slip is one of the major goals of earthquake physics as this information is
necessary for full mechanical modeling of the rupture process. One indicator of absolute stress is the total energy dissipated as
heat through frictional resistance. The heat results in a temperature rise on the fault that is potentially measurable and
interpretable as an indicator of the absolute stress. We present a new paleothermometer for fault zones that utilizes the
thermal maturity of extractable organic material to determine the maximum frictional heating experienced by the fault. As a
rock is heated, temperature-sensitive molecules degrade, increasing the abundance of refractory organic molecules. On the
short timescales involved in fault heating, these reactions are strongly temperature dependent and therefore track the
maximum temperature achieved during fault slip. Furthermore, because there are no retrograde reactions in these organic
systems, the maximum heating signature is preserved. We investigate four fault zones. According to a variety of organic
thermal maturity indices, the thermal maturity of the wall rocks falls within the range of heating expected from the bounds on
burial depth and time, indicating that the method is robust and in some cases improving our knowledge of burial depth.
Only the Pasagshak Point Megathrust, AK, which is also pseudotachylyte-bearing, shows differential heating between the
fault and off-fault samples. Our finding confirms that this rapid heating is sufficient to measurably alter the thermal maturity
of organic molecules. However, most of the faults did not get hotter than the surrounding rock during slip. Simple
temperature models coupled to the kinetic reactions for organic maturity let us constrain certain aspects of the fault during
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