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Poster Abstracts
safety zones for the maritime community; 3) completion of a tsunami deposit and tsunami source database; and, 4) formation
of pre-tsunami field teams to observe and collect data during and after a tsunami. A pilot project for the land-use planning
maps, which will require a probabilistic tsunami hazard analysis, has been initiated in the City of Huntington Beach. The
maritime community maps, which will include in-harbor tsunami hazard maps and offshore safety zone, have also been
initiated with likely “pilot” locations in Ventura Harbor, the Ports of Los Angeles and Long Beach, and San Diego Bay. The
two tsunami databases for deposits and sources will be completed by 2012 to help evaluate existing tsunami hazard mapping
products and guide future products for both emergency response and land-use planning. During and after the March 11, 2011
tsunami, which caused over $50-million in damage to two dozen harbors in the state, data collected by the pre- and posttsunami
field team will also help evaluate the tsunami modeling and mapping methods used by the state, especially as they
relate to strong tsunami currents within harbors.
CORRELATION OF PEAK DYNAMIC AND STATIC COULOMB FAILURE STRESS WITH SEISMICITY
RATE CHANGE AFTER THE M7.2 EL MAYOR-CUCAPAH EARTHQUAKE (B-102)
K.B. Withers and K.B. Olsen
We have investigated the relation between the April 4 Mw7.2 El Mayor-Cucapah earthquake and seismicity rate changes in
southern California and northern Baja California in the months following the mainshock. Specifically, we use dynamic rupture
models with observational constraints for the event simulated in the SCEC 3D CVM4.0 (Roten and Olsen, 2009) to calculate the
resulting static (ΔCFS) and dynamic Coulomb failure stress changes, ΔCFS(t), parameterized by its largest positive amplitude
(peak ΔCFS(t)). The seismicity rate changes are estimated using the Z-value (Haberman, 1983) using both undeclustered and
declustered datasets (with a magnitude of completeness equal to 1.5), and show varying goodness of fit values with different
time periods after the mainshock (ranging from a day to 1 year). We employ different methods to measure the correlation
between the seismicity rate change and both ΔCFS and peak ΔCFS(t) in time and space, in order to estimate the most efficient
triggering parameter for the aftershocks, including a ternary map comparison (as used by Kilb et. al, 2002) and crosscorrelation.
We perform this analysis using both CVM-4 and CVM-H, investigating, in particular, which model better
describes the increased seismicity NW of the rupture. We have rotated the stress changes onto focal mechanisms of several
Mw5 aftershocks as well as onto optimum oriented planes (King, 1994). We find that increases in static stress changes closer to
the fault can explain nearby aftershocks, while the triggering of those farther away are more likely attributed to dynamic stress
changes, where static stress change is nearly zero. We attempted to define an estimate of potential threshold level of static and
dynamic stress change required to trigger earthquakes/aftershocks of different magnitude, but thus far have found no
definitive relation.
THERMAL FLUID AND FAULT INTERACTIONS AT THE INTERSECTION OF TWO FAULTS, AGUA
CALIENTE, CALIFORNIA (A-099)
R.E. Wood and J.P. Evans
Agua Caliente Springs lies at a unique intersection between the NNW-trending Elsinore fault and the 40° northeast-dipping,
likely inactive West Salton detachment fault; it provides an opportunity to study damage zone geometry, fault behavior in
crystalline rocks, a left-stepover, microseismicity, and the influence of thermal fluids on rock deformation. The Elsinore fault
bounds the northwestern flank of the Tierra Blanca Mountains with strike-slip and normal motion; the detachment fault wraps
around the northernmost portion of the mountains. Damage along the Elsinore ranges in thickness from a narrow slip plane to
> 100 m along the eastern flank of the Tierra Blanca Mountains. Subsidiary faults trend northeast and southeast, and slip
orientations vary from normal to strike-slip horizontal motion. Thermal fluids (~30°C) emerge at the intersection of the West
Salton detachment and Elsinore faults actively alter the 94 Ma La Posta tonalite pluton, already fractured and crushed during
fault slip, to a fine-grained white to orange powder through mineral re-equilibration. Grain sizes decrease with closer
proximity to the faults. Fault cores contain thin dark green zones of chlorite ± epidote, and fault surfaces are coated with a thin
layer of the same. Origin of the mineralization may be from reworked biotite crystals. We present water chemistry data from
the hot springs at Agua Caliente in conjunction with geochemical and petrographic analysis of the surrounding rock. Water
analyses include cation and anion measurements, bicarbonate, stable isotopes, tritium, and a multi-month recording of spring
conductivity, water level, and temperature fluctuations. Cation geothermometry shows the fluids are enriched in Na, Ca, Mg,
K, and Si from broken down quartz, plagioclase, and orthoclase. Water level and temperature data are compared to seismicity
during the logging interval; temperatures so far have diurnal fluctuations indicating air temperature plays a larger role than
anticipated for the subsurface fluids. Conductivity also displays daily cycles. We propose a larger scale map of the intersection
of the two faults and the continuation of the Elsinore farther south showing the current extent and probable growth of the
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