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SCEC researchers are also attempting to
determine how representative geologic slip
rates are of the true slip rate of fault slip at
depth. Although surface fault slip
measurements are commonly used to infer
seismic hazard, it has long been noted that
surface slip during earthquakes is often
smaller than slip at depth determined from
inversion of geodetic and seismologic data. If
this shallow slip deficit is due to significant
off-fault deformation in the near-surface
region, as suggested by earlier SCECsupported
studies (e.g. Shelef and Oskin, 2010),
then geological surface slip measurements will
systematically underestimate fault slip at
depth. Consequently, seismic hazard inferred
from these measurements will also be
underestimated. In an effort to determine if
this shallow slip deficit varies as a function of
measureable fault characteristics such as total
fault slip or near-surface geology, Haravitch
SCEC Research Accomplishments | Report
and Dolan (in prep.) have systematically compared the co-seismic surface displacements in six, large-magnitude (Mw ≥7)
strike-slip earthquakes with static inversions of geodetic data for slip at 3-6 km depth. Their study reveals a striking
correspondence between the degree to which fault slip remains localized to the surface and the structural maturity of the fault,
as defined by cumulative fault displacement. For example, ruptures on faults with ≤10 km of cumulative fault slip typically
manifest only ~40-60% of slip at 3-6 km depth as discrete surface fault slip, in contrast to ruptures on structurally mature
faults, which manifest 80-95% of slip at depth as discrete surface fault slip (Figure 13). This pattern extends to smaller scales,
and comparisons of slip at the surface with slip at depth for any point along these ruptures reveals the same basic trend when
structurally simple and structurally complicated sections of the surface ruptures are examined separately. These observations
have basic implications for probabilistic seismic hazard assessment, which depends strongly on geologic fault slip rates as a
primary input, and provide a way forward to potentially defining a multiplier based on measurable fault parameters (e.g.,
cumulative fault slip) that can be used to
correct geologic slip rates to account for the
amount of deep fault slip that is distributed in
the near surface.
Geochronology Infrastructure
The geochronology infrastructure program
pools dating funds at various participating
laboratories. This approach leads to greater
efficiency in allocating resources, as most
projects cannot fully anticipate their dating
requirements ahead of time. Nearly 900
Figure 13. Comparison of Surface Slip Ratio (SSR) with cumulative displacement of
six faults that produced earthquakes M≥7. Comparisons are based on geologic
surface offset data, geodetic estimations of slip at depth and geologic estimation of
cumulative displacement.
Figure 14. Sampling profile and shape model for Grass Valley Rock GV-2. Modeling
the 3-D exposure history reveals that this rock has been precarious for 18.0 ± 2.5 kyr.
geochronology analyses have been completed thus far under the SCEC3 program. 600 of these have been 14C, spread roughly
equally between Lawrence Livermore National Lab (LLNL) and U.C. Irvine labs. Cosmogenic 10Be (measured at LLNL)
comprises most of the remaining analyses (227 dates) followed by optically stimulated luminescence (52 OSL dates, measured
at University of Cincinnati and Utah State). 76% of 14C, 65% of 10Be, and 60% of OSL dates have been expended on SoSAFE
projects, reflecting the high impact this special program has had on SCEC3 geology. The geochronology program has also
spawned new collaborations to advance dating methodologies. One rapidly advancing technique melds the 10Be and U-series
dating approaches to date alluvial fans. This has been applied to date offsets along the San Andreas Fault at Biskra Palms
(Behr et al., 2010; Fletcher et al., 2010) and at several sites along the southern San Jacinto Fault (Blisniuk et al., in preparation).
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