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Geophysics seminar: Katherine Guns (University of Arizona) "New geodetic constraints on Southern San Andreas Fault slip rates in and around San Gorgonio Pass, CA"

02/04/2020 - 12:00pm
Munk Conference Room
Event Description: 

Talk Abstract:

Assessing fault slip rates in diffuse plate boundary systems such as the San
Andreas Fault in southern California is critical both to characterize seismic hazards and
to understand how fault strands work together to accommodate plate boundary motion. In
places like San Gorgonio Pass, where one fault diverges into three separate strands,
added geometric complexity presents an extra obstacle to understanding the rupture
potential and behavior of each individual fault. To better understand partitioning of fault
slip rates in this region, we build a new set of elastic block models that test fifteen
different model fault geometries for the San Gorgonio Pass area. These models invert a
newly estimated, postseismic-reduced, GPS velocity field in order to calculate long-term
fault slip rates. To calculate this velocity field we assess ongoing viscoelastic postseismic
displacements from 217 Mw≥6.0 earthquakes across California, Nevada, and northern
Mexico and determine that 12 events are likely measurably contributing to the modern
day deformation field in southern California. We remove these forward modeled
displacements from observed time series and variance reduction assessments indicate that
our methodology removes up to 60% of measured postseismic signal. Inverting this
postseismic-reduced GPS velocity field to estimate long-term fault slip rates improves χ 2
misfits to the data by over 35%. Eight of our fifteen model geometries estimate geodetic
slip rates that sum across Eastern California Shear Zone faults to match published sums
of geologic slip rates (within uncertainties); however, our lowest χ 2 misfit model
geometry produces a summed slip rate across the Eastern California Shear Zone in the
Mojave Desert of ~11-12 mm/yr, which is higher than geologic rates, but in agreement
with other geodetic estimates. Only one model geometry produces geodetic slip rates that
match preferred geologic rates along the Mojave section of the San Andreas Fault, an
area of ongoing discrepancy. This model requires a fault geometry that has zero motion
on all faults east of the main San Andreas trace, suggesting that modern GPS data may be
detecting a lull in San Andreas Fault activity.

For more information on this event, contact: 
Yehuda Bock
Event Calendar: