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Earthquake Geology
SCEC Research Accomplishments | Report
The SCEC geology disciplinary group coordinates diverse field-based investigations of the Southern California natural
laboratory. The majority of Geology research accomplishments fall under two categories: (1) focused studies of the southern
San Andreas and San Jacinto faults in coordination with the SoSAFE (Southern San Andreas Fault Evaluation) special project;
and (2) studies of other portions of the southern California fault network aimed at a better understanding fault system
behavior. Geology also contributes to earthquake response efforts and supports field observations related to several focusgroup
activities (e.g., USR, WGCEP, FARM, GMP, LAD, CDM). Additional goals include longer-term slip rates and long,
multi-event paleoseismologic records that have a high impact on seismic hazard assessments. In support to these efforts the
Geology group coordinates geochronology infrastructure resources that are shared among various SCEC-sponsored projects.
SoSAFE Special Project
The primary goal of the SoSAFE project is to document the timing of large paleoearthquakes and amount of slip released by
the southern San Andreas and San Jacinto faults over the past 2000 years. Additional goals include examination of longer-term
slip rates and modeling studies that directly impact seismic hazard assessments. The SoSAFE project has become the
dominant focus of geological investigations under SCEC3. The numerous accomplishments of this program are described in
detail in the Special Projects section, below.
Fault System Behavior
A major emphasis of the Geology group has been to characterize patterns in fault system behavior that could significantly
affect earthquake hazards. Recent efforts, focused on the San Cayetano-Ventura reverse fault system, aim to understand fault
linkage and potential earthquake magnitude along the rapidly converging northern margin of the Ventura basin. Analysis of
well data, industry seismic reflection profiles, and four high-resolution seismic profiles acquired in 2010 have allowed
Hubbard, Shaw, Dolan, McAuliffe and Pratt to construct a more complete 3D model of the system (Figure 11). These data
suggest that the Ventura Fault, previously interpreted by some as a shallow, bending-moment fault that does not pose a
seismic hazard, is in fact a major, through-going thrust ramp that extends to seismogenic depths. Slip on the Ventura ramp has
generated the prominent Ventura Avenue anticline, which
Hubbard et al. (in prep.) interpret as a fault-propagation
fold. Moreover, the structural similarity of the Ventura
Avenue anticline and the Dos Cuadras structure offshore
to the west suggests that these major structures form an en
echelon system that could potentially link together to form
large-magnitude thrust earthquakes. Similarly, the
southern, blind San Cayetano Fault to the east of Ventura
is a north-dipping thrust ramp overlain by a large anticline
that appears to be part of this same system. Rupture of this
entire system could generate a very large-magnitude event,
similar to the 2008 Mw 7.9 Wenchuan earthquake.
Significant portions of such a rupture would lie offshore,
resulting in the potential for generation of large, nearshore
tsunamis that could affect coastlines throughout
southern California. Ongoing analysis of the 2010 highresolution
seismic reflection data, coupled with shallow
Figure 11. High-resolution seismic profile acquired across the active
fold limb and synclinal axial surface associated with the tipline of the
blind, southern San Cayetano Fault along the northern edge of the
Ventura basin. The upper figure shows the locations of continuously
cored boreholes and cone-penetration tests drilled across the zone of
active folding during Summer 2011 to define precisely the geometry
of strata folded in recent earthquakes and to sample dateable
material to define paleo-earthquake ages.
borehole data collected during Summer 2011 across the
locus of active folding above the Ventura and western San
Cayetano anticlines, should reveal the ages and
displacements of recent earthquakes on these major thrust
ramps, which will in turn allow the evaluation of possible
multi-segment ruptures and correlation to the uplifted
Holocene marine terrace record documented by T.
Rockwell.
2011 SCEC Annual Meeting | 49