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Poster Abstracts
In an emergent view, this transition region has heterogeneous frictional properties, and is composed of frictionally unstable,
velocity-weakening patches embedded in a frictionally stable fault region. Tremor swarms are viewed as the collective
response of brittle asperities interacting through transient aseismic slip in their surroundings. A hierarchy of migration
patterns of tremors has now been observed in the Cascadia subduction zone, including large-scale along-strike tremor
propagation at ~10km/day and rare swarms that propagate 10 times faster in the opposite direction ('rapid tremor reversals').
A cascade of brittle asperity failures mediated by transient creep is an appealing model to explain these migration patterns.
We performed a quantitative study of this model through numerical simulations of heterogeneous rate-and-state faults under
the quasi-dynamic approximation, solved by a Boundary Element Method. We conveniently generated SSEs, propagating at
~10 km/day, by adopting an ad hoc rate-and-state friction law with a transition from velocity-weakening to strengthening.
Brittle asperities are defined as small patches of velocity-weakening friction with shorter characteristic slip distance Dc, larger
friction parameters a and b or higher effective normal stress (sigma) than their surroundings. The state variable is governed by
the 'slip law', which allows conditionally stable behavior. A collection of brittle asperities is distributed along the fault. We
studied the effect of their size and spacing and of the contrast of a*sigma and Dc with respect to the background. We
successfully simulated both observed migration patterns. The slow forward migration is obviously due to tremor triggering
near the leading front of the propagating SSE pulse. Less trivially, our model also produces RTRs with similar characteristics
as in Cascadia: spatially scattered swarms back-propagating at fast speed ~100 km/day. These RTRs are rare because they
nucleate at the asperities with largest Dc. While our model reproduces key features of the complex spatio-temporal
organization of tremors as observed in Cascadia and Japan, this comes at the cost of tuning some model parameters. We will
report the results of our parametric study.
ABSOLUTE STRESS IN SOUTHERN CALIFORNIA CONSTRAINED BY EARTHQUAKE FOCAL
MECHANISMS AND MODELS OF STRESS CONTRIBUTIONS FROM TOPOGRAPHY AND FAULT LOADING
(A-060)
K.M. Luttrell, D.T. Sandwell, and B.R. Smith-Konter
Earthquake focal mechanisms in southern California may be used as in situ indicators of the 3-D orientation of the stress field.
This stress field is heterogeneous, as reflected by the presence of strike-slip, reverse, and normal faulting mechanisms in close
proximity to one another. We attempt to reproduce these observations by accounting for the stress fields from three sources: 1)
the 3-D crustal stress associated with the support of local topography (i.e., wavelengths less than ~350 km) at both the surface
and Moho, as constrained by gravity observations; 2) the 3-D earthquake cycle stress accumulated on locked fault segments;
and 3) a 2-D regionally uniform stress field representing plate boundary scale tectonic driving stress. The magnitude and
orientation of the regional stress field are varied to obtain the best possible fit to the stress orientations indicated by
earthquake focal mechanisms. For southern California, we find the non-lithostatic component of regional driving stress must
consist of at least 30 MPa compression oriented at N15E. However the magnitude of the orthogonal component must be varied
along the fault in order to simultaneously reproduce both the strike-slip and thrust mechanisms. This indicates that there is an
additional E15S compressional stress in the big bend region that is not accounted for by the weight of topography and the
earthquake cycle stress.
OBSERVATIONS OF RECENTLY-EXPOSED FUMAROLE FIELDS NEAR MULLET ISLAND, IMPERIAL
VALLEY, CALIFORNIA (A-138)
D.K. Lynch, K.W. Hudnut, P.M. Adams, and L.S. Bernstein
New field observations, lidar measurements, aerial imaging and preliminary laboratory measurements of mud samples are
reported of three formerly submerged fumarole fields in the Salton Trough near Mullet Island in southeastern California, USA.
The fumarole fields have recently been exposed as the Salton Sea level has dropped. The largest of the three fields visited in
January 2011 is irregular in outline with a marked northeast elongation. It is roughly 400 meters long and 120 meters wide.
The field consists of approximately one hundred warm to boiling hot (100° C) mud volcanoes (0.1 – 2 m in height), several
hundred mud pots, and countless CO2 gas vents. Unusual shaped mud volcanoes in the form of vertical tubes with central
vents were observed in many places. Lidar measurements were obtained in the time period Nov 9-13, 2010 using an Optech
Orion 200M lidar from an elevation 800 m AGL. They reveal that the terrain immediately surrounding the two fields that are
above water level reside on a low (~0.5 m high) gently sloping mound about 500 m across that shows no evidence of
lineaments indicative of surface faulting. With other geothermal features, the fumaroles define a well-defined line marking the
probable trace of the Calipatria fault. Although the precise locations is uncertain, it appears to define a straight line 4 km long
between the Davis-Schrimpf mud volcanoes and Mullet Island. Mullet Island is one of five late Quaternary rhyolitic volcanic
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