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Poster Abstracts | Group 2 – Earthquake Geology
2-035
HIGH GEOLOGIC SLIP RATES SINCE EARLY PLEISTOCENE INITIATION OF THE
SAN JACINTO AND SAN FELIPE FAULT ZONES IN THE SAN ANDREAS FAULT
SYSTEM: SOUTHERN CALIFORNIA, USA Janecke SU
The San Jacinto fault system is crucial for understanding dynamics and seismic hazards in southern
California, yet its age of initiation and long-term average slip-rate are controversial and little is
known about its structural evolution. We synthesize four studies in the Salton Trough that provide
evidence for initiation of structural segments of the San Jacinto fault zone at or slightly before the
1.07-Ma base of the Jaramillo subchron and creation of new fault segments after ~ 0.6 Ma. Strands
of the San Jacinto and adjacent San Felipe fault zones began to cut, fold, and deactivate the older
West Salton detachment fault ~1.3-1.1 Ma. The San Felipe, Coyote Creek and Clark faults all show
evidence of major structural adjustments after about 0.6 to 0.5 Ma.
The Clark and Buck-Ridge-Santa Rosa faults accumulated about 14.4 km of right separation in its
lifetime near Clark Lake and at least 5.6 ± 0.4 km in the last 0.5 to 0.6 Ma in its San Felipe Hills
segment. The Coyote Ridge segment of the Coyote Creek fault has a displacement of ~3.5 ± 1.3 km
since before about 0.8 to 0.9 Ma, whereas the Borrego Mountain and Borrego Badlands segments
have only 1-2 km of right slip since ~0.6 Ma. The San Felipe fault accumulated between 4 and 13.5
km (~6.5 km preferred) of right slip in the last 1.1 to 1.3 Ma at Yaqui and Pinyon ridges. Combining
these five estimates of displacement with ages of fault initiation indicate a lifetime geologic slip rate
of 17.8 +2.6/-2.3 mm/yr across the San Jacinto fault zone (sum of Clark, Buck Ridge and Coyote
Creek faults) and about ~5.4 +6.9/-2.3 mm/yr across the San Felipe fault zone at Yaqui and Pinyon
ridges. This suggests that the San Jacinto and San Felipe fault zones accommodated slightly more
plate motion than the southern San Andreas fault since their inception roughly 1-1.3 Ma.
The Coyote Creek fault zone is narrower and more continuous than adjacent faults yet it is slightly
younger, has < 1/4 the total displacement, and slipped at a much slower rate than the Clark fault.
The San Felipe fault zone displaces late Pleistocene deposits, has ~ 6.5 km of displacement, and has
a slip rate comparable to that of major adjacent faults. Its lifetime slip rate exceeds those of the
central and southern Elsinore and the Coyote Creek faults. The San Felipe fault zone may have
slowed in the recent past or may be experiencing a lull in activity. Our structural and stratigraphic
data do not support steady state assumptions of slip rate.
2-036
REVISED DATES OF 5 LARGE EARTHQUAKES AT THE BIDART FAN SITE ALONG
THE CARRIZO SECTION OF THE SAN ANDREAS FAULT, CALIFORNIA, SINCE A.D.
1310±30 Akciz SO, Grant Ludwig LB, and Arrowsmith JR
Precise knowledge of the age and magnitude of past earthquakes is essential for characterizing
models of earthquake recurrence and key to forecasting future earthquakes. We present 28 new
radiocarbon analyses that refine the chronology of the last five earthquakes at the Bidart Fan site
along the Carrizo section of the San Andreas Fault. We first present redrafted versions of Grant’s
(1993) original trench logs and include a table containing a systematic evaluation of the quantity
and the quality of paleoearthquake evidences. Note that this makes the complete logs available to a
broader audience, given that Grant and Sieh (1994) only published critical portions of the logs for
brevity. The new data show that the penultimate earthquake in the Carrizo Plain occurred not
earlier than A.D. 1640 and the modeled 95-percentile ranges of the three earlier earthquakes and
their mean are A.D. 1540-1630 (1585), A.D. 1360-1425 (1393), and A.D. 1280-1340 (1310), indicating
an average time interval of 137±44 years between large earthquakes since A.D. 1310±30. A robust
earthquake recurrence model of the Carrizo section will require even more well-dated earthquakes
156 | Southern California Earthquake Center