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Group 1 – CDM | Poster Abstracts
Bernardino Mountains from San Clemente Island to southwestern Nevada. Published Holocene to
late Pleistocene slip rates (Peterson et al., 1996) for faults within the transect do not fit the GPS
velocity data from CMM3. The CMM3 data are best fit by rates in the Eastern California shear zone
(ESCZ) that are higher than rates estimated over geologic time scales, and by rates for the San
Andreas and San Jacinto faults that are lower than published geologic rates. Locking depths of13-
18 km were used in the modeling (Peterson et al., 1996). Slip rates that fit the CMM data well are as
follows: the Eastern California shear zone, 20 mm/yr; San Andreas fault, 11 mm/yr; San Jacinto
fault, 6mm/yr; the Elsinore fault, 5 mm/yr; the NIFZ, 2 mm/yr, and the Palos Verde fault, 3
mm/yr. The discrepancy between geologically and geodetically estimated slip rates for the San
Andreas and San Jacinto faults and for the ESCZ could be a result of variations in strain
accumulation rates at different stages of the earthquake cycle (for a viscoelastic medium). The low
rate of geodetically measured strain accumulation on the San Andreas fault could be because it is
in the late stage of the earthquake cycle. The high rate of strain accumulation modeled in the ECSZ
could be a result of recent activity--the Landers and Hector Mine earthquakes. An alternative
hypothesis could simply be that the San Andreas fault is no longer the primary fault in the vicinity,
but movement has been shifted to the ECSZ within the last few decades.
1-114
B4-AUGMENTED IDENTIFICATIONS OF SAN ANDREAS FAULT SURFACE
FEATURES: SPATIAL STRUCTURE OF THE PLATE BOUNDARY BETWEEN
BOMBAY BEACH AND DESERT HOT SPRINGS Lynch DK, and Hudnut KW
We present preliminary results of a hyper-accurate (± few m) inventory of SAF features between
Bombay Beach and Desert Hot Springs based on overhead imagery. The ultimate goal is to identify
fault features that can be used to determine offset distance and slip rates. Many fault structures
have been previously reported and about two dozen new ones have been found in this study.
Using approximately 80 fault components we defined a map view piecewise continuous trace of
features (and interpolations when no structure could be discerned) that we are calling the
provisional plate boundary (PPB). The resulting PPB trace closely matches the faults reported by
Clark (1984) and those in the qfaults data base. We analyzed the PPB and present evidence of
spatial structure on scales of a few hundred meters.
1-115
SMALL-SCALE UPPER MANTLE CONVECTION AND CRUSTAL DYNAMICS IN
SOUTHERN CALIFORNIA Fay NP, Bennett RA, Spinler JC, and Humphreys ED
We present numerical modeling of the forces acting on the base of the crust caused by small-scale
convection of the upper mantle in southern California. Three-dimensional upper mantle shear
wave velocity structure is mapped to three-dimensional density structure that is used to load a
finite element model of instantaneous upper mantle flow with respect to a rigid crust, providing an
estimate of the tractions acting on the base of the crust. Upwelling beneath the southern Walker
Lane Belt and Salton Trough region and downwelling beneath the southern Great Valley and
eastern and western Transverse Ranges dominate the upper mantle flow and resulting crustal
tractions. Divergent horizontal and upward directed vertical tractions create a tensional to
transtensional crustal stress state in the Walker Lane Belt and Salton Trough, consistent with
transtensional tectonics in these areas. Convergent horizontal and downward directed vertical
tractions in the Transverse Ranges cause approximately N–S crustal compression, consistent with
active shortening and transpressional deformation near the ‘‘Big Bend’’ of the San Andreas fault.
Model predictions of crustal dilatation and the forces acting on the Mojave block compare
favorably with observations suggesting that small-scale upper mantle convection provides an
important contribution to the sum of forces driving transpressional crustal deformation in southern
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