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Poster Abstracts
necks in the immediate area of the fumaroles. The Calipatria fault is subparallel to the San Andreas and Imperial faults and
only one of many verified or suspected faults (including cross faults) in the complex tectonic setting of the Salton Trough.
Mud from several volcanoes was analyzed using scanning electron microscopy (SEM), and energy dispersive X-ray
spectroscopy (EDS). One sample contained boussingaultite, (NH4)2Mg(SO4)2·6(H2O), a rare mineral that is known to sublime
under fumarolic conditions, possibly by ammoniation of hydrated MgSO4.
As the Salton Sea level continues to drop, the third fumarole field is expected to surface in the next couple of years. It is also
likely that as more land emerges, many of the CO2 gas seeps currently under water will begin to form mud volcanoes and
mud pots, most of them along the same NW trending axis as the others. At about the same time a land bridge will form to
Mullet Island, the first in about 60 years.
IBEM3D: A REMOTE STRESS INVERSION PROGRAM HONORING MECHANICAL RELATIONSHIPS
BETWEEN STRESS AND FAULT GEOMETRY (A-112)
E.H. Madden, F. Maerten, and D.D. Pollard
Aftershocks have been correlated spatially with changes to the Coulomb shear stress on optimally orientated planes (e.g. Stein
and Lisowski, 1983; King et al, 1994) and static stress change on nodal planes inferred from focal mechanisms (e.g. Hardebeck
et al, 1998; Seeber and Armbruster, 2000) due to slip on planar mainshock faults. The orientations of individual aftershocks
have been mechanically related to the perturbed stress field from slip on non-planar faults, with distinct fault geometries
producing the best-fit model Coulomb planes at aftershock hypocenters, as well as honoring the recorded slip distribution at
Earth’s surface (Madden and Pollard, 2011, submitted). However, stress inversions generally overlook these connections
between aftershocks, fault geometry and stress perturbations. New inversion software, iBem3D (Maerten, 2010), uses a
homogeneous remote stress with one principal axis vertical to drive slip on non-planar faults and determines the resulting
stress state at all aftershock locations. The orientation and remote principal stress magnitudes that minimize the ‘cost’ or misfit
between aftershock focal mechanisms and model Coulomb plane orientations or slip vectors are converged upon iteratively,
over multiple model runs. The cost of individual aftershocks may be analyzed for systematic effects on inversion results of
aftershock characteristics such as magnitude, distance in time and space from the mainshock, and focal mechanism
uncertainty, or to determine locations where mainshock fault geometry may be incorrect, producing a cluster of aftershocks
with large misfits. The sum of all individual costs quantitatively assesses the relative performance of inversions executed with
different model geometries or subsets of aftershocks. In addition, relative costs are used to select one nodal plane, from the
two provided by the aftershock focal mechanism, for inclusion in the inversion. Other observed data, such as the orientations
of slickenlines, veins, or secondary faults, may be used to constrain the inversion, either in addition to, or instead of,
aftershock focal mechanisms; the only change to iBem3D procedure is adjustment of the cost function applied.
COMPILATION OF SLIP IN LAST EARTHQUAKE DATA FOR HIGH-SLIP RATE FAULTS IN CALIFORNIA
FOR INPUT INTO SLIP DEPENDENT RUPTURE FORECAST (A-129)
C.L. Madden Madugo, J.R. Arrowsmith, D.E. Haddad, J.B. Salisbury, and R.J. Weldon
Slip in the last earthquake along a fault, in conjunction with the application of appropriate recurrence models, can be used to
estimate the timing and size of future ground-rupturing earthquakes. Surface slip measurements are relatively easy to acquire
along highly active faults because offsets from the last event are often well preserved by geomorphic features in the landscape.
We present a comprehensive database of slip measurements for high slip rate strike-slip and dip-slip faults in California for
input into the slip-dependent 2011 Uniform California Earthquake Rupture Forecast (UCERF 3). Our database includes
historic, paleoseismic, and geomorphic data on slip in the last event and multi-event offsets. Faults were prioritized by highest
slip rates and longest time since the last event relative to average recurrence interval. Slip rate, timing of the last event, and
recurrence interval were obtained from past reports by the Working Group on California Earthquake Probabilities, unless
more recently published data were available. A literature search determined the availability of offset data for the highest
priority faults. We contacted authors of published slip studies to ascertain whether additional data exist in unpublished
archives, gray literature, or publications in preparation. Lack of consistency in existing schemes to rate offset quality led us to
develop a new semi-quantitative method to asses feature and tectonic quality for new and existing data. Recent analyses of
newly available, high-resolution LiDAR topography for micro-geomorphic offsets have substantially increased the number of
slip measurements available for our compilation. For faults with LiDAR coverage, but limited offset data, we identified
reaches with high potential to preserve geomorphic offsets and calculated slip measurements. The methodology for our
geomorphic analyses has been developed and implemented successfully in recent studies along the central San Jacinto Fault
and 1857 earthquake reach of the San Andreas Fault. Last, we compiled data collected from our literature search and LiDAR
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