Geologic Hazards and Disasters Research Group

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Abstracts: Rupture history of the San Andreas Fault in the Carrizo Plain

2008

2007

2006

Paleoseismological investigations of the San Andreas Fault in the Carrizo Plain have greatly influenced models of fault behavior and seismic hazard. Early geomorphic analysis of channel offsets near Wallace Creek led to development of a hypothesis that the Carrizo Plain segment is unusually strong and produces only large earthquakes (>7.8 M) every 240 to 450 years (Sieh and Jahns, 1984) unlike the segments that bound it to the north and south. Investigations of the Bidart Fan site, approximately 5 km southeast of Wallace Creek in the Carrizo Plain, have provided some of the best data on large surface rupturing earthquakes (Grant and Sieh, 1994). The Bidart site has good, reliably datable stratigraphy for discriminating individual earthquakes. Previously published results based on 14C analysis of 14 samples suggested that five surface ruptures occurred in a "cluster" of earthquakes between 1218 and 1857 A.D, culminating in the 1857 Fort Tejon earthquake. In 2005 and 2006 we excavated 5 additional trenches at the Bidart Fan and found evidence for additional earthquakes during the same time period, yielding an even shorter average recurrence interval of approximately a century (Grant et al., 2005; Akciz et al., 2006; Akciz et al., in prep). The significance of these results warrants thorough vetting prior to publication. Therefore, we have done additional radiocarbon dating of 39 archived samples to improve age constraints on the earthquakes from Grant and Sieh's (1994) original work, and compare with dates of earthquakes from our 2005-2006 trenches. The radiocarbon ages and probabilistic analysis using OxCal provide better constraints on the dates of individual earthquakes. The youngest surface rupture was the historic 1857 earthquake (Event A). Two earthquakes (Events B and C) occurred in the time interval 1540-1840. Event D occurred 1370-1425, and Event E occurred 1285-1340. Dates of 2 older events (F and G) remain poorly constrained as after 200 BC. The five younger events occurred between 1285 and 1857 AD, leading to an average recurrence interval of 143 years.

Paleoseismological investigations of the San Andreas Fault in the Carrizo Plain have greatly influenced models of fault behavior and seismic hazard. Early geomorphic analysis of channel offsets near Wallace Creek led to development of a hypothesis that the Carrizo Plain segment is unusually strong and produces only large earthquakes (>7.8 M) every 240 to 450 years (Sieh and Jahns, 1984) unlike the segments that bound it to the north and south. 14C dates of mainly detrital charcoal samples from the Bidart Fan site, approximately 5 km southeast of Wallace Creek in the Carrizo Plain, have provided some of the best timing constraints on these large surface rupturing events (Grant and Sieh, 1994; and our work in progress). The Bidart site has good, reliably datable stratigraphy for discriminating individual earthquakes. Earlier results based on 14 14C analyses have suggested that five large earthquakes have occurred since 1218 AD, indicating that large earthquakes in the Carrizo Plain are more frequent than previously thought. Paleoseismological evidence for four surface rupturing earthquakes from three new fault perpendicular trenches excavated in 2005 and 2006 include a well-preserved sag pond, buried mole tracks, fissure infills, and upward terminating fractures. Analyses of new 14C data from these new trenches, in addition to the incorporation of new 14C analyses on charcoal samples collected from three earlier trenches reveal that the last four of these earthquakes occurred since 1475 +or- 50 AD, confirming the possibility of a much shorter recurrence interval, at least during the time period between circa 1400 AD and 1857 AD. Whether all of these events were similar in magnitude to the M7.9 1857 Fort Tejon earthquake or were smaller remains to be determined. We currently do not have slip per event data from the Bidart Fan site to address this problem. Our future work will focus on replicating these short recurrence interval data as well as trying to get slip measurements associated with each of these events. Our continuing efforts of extending the paleoseismological record to 10+ events will also enable us to determine how "characteristic" these shorter recurrence intervals are in the late Holocene history of the San Andreas Fault in the Carrizo Plain as these preliminary results have major implications for the San Andreas and general fault behavior.

References:
Sieh, K. E., and Jahns, R.H., 1984. Holocene activity of the San Andreas Fault at Wallace Creek, California: Geological Society of America Bulletin, v. 95, p. 883-896.

Grant, L. B. and Sieh, K. E., 1994. Paleoseismic evidence of clustered earthquakes on the San Andreas Fault in the Carrizo Plain, California: Journal of Geophysical Research, v. 99, p. 6,819-6,841.

Many models of fault behavior and seismic hazard are based on a growing dataset of spatially and temporally well-constrained rupture history of the San Andreas Fault (SAF). However validity of the models is questionable because the earthquake record at a paleoseismic site may be incomplete at single or closely spaced trenches, and at many sites there is insufficient datable and chronologically significant organic material for age control of structurally relevant stratigraphic units. We are compiling a long chronology of surface rupturing earthquake events from the Carrizo Plain section of the SAF by placing multiple spatially-spread trenches across the Bidart Fan site where an alluvial fan is cut by the SAF and has good, reliably datable stratigraphy for discriminating individual earthquakes. The Bidart Fan site, approximately 18 km southeast of Wallace Creek in the Carrizo Plain, offers an opportunity to refine and extend the paleoseismic record of surface ruptures for the SAF. Analysis of data from 4 fault perpendicular trenches yields a composite chronology of at least 10 surface ruptures over the last 3000 years, which roughly correspond to a 300 year interval between large earthquakes. The Bidart Fan site is only the third major paleoseismic site on the SAF with a chronology of earthquakes spanning at least 10 ruptures. Preliminary results from a recently excavated fifth trench with excellent stratigraphy and over 90 detrital organic samples (of which 30 has been dated) reveal that at least 4, and possibly 6, of these large earthquakes occurred since 1475 +or- 50 AD, leading to a much shorter recurrence interval. In the next field season we will attempt to confirm this 500-year rupture history of the SAF at the Bidart fan site and refine the dates of events. Results will be crucial in determining the validity of average earthquake recurrence as a useful tool for estimating time dependent seismic hazard along this portion of the SAF.

2005

Chronologies of earthquakes spanning at least ten ruptures at multiple sites are required for developing robust models of fault behavior and forecasts of future earthquakes. Such a long chronology can be obtained by placing multiple trenches across the San Andreas fault at the Bidart alluvial fan paleoseismic site in the Carrizo Plain to capture the spatio-temporal record of earthquakes created by the interplay of surface rupture and spatially varying deposition. Exposures from one trench reveal evidence of at least 6 and probably 7 earthquakes since 3000 BP. Evidence of 7 earthquakes since 2200 BP has been interpreted from exposures in 3 other trenches. Analysis of exposures from two new trenches is in progress. Excavations reveal alternating sequences of depositional preservation and gaps in the record of earthquakes. The "gaps" are massive featureless zones caused by bioturbation of the fan surface while that portion of the fan was depositionally inactive. When the depositional record of 4 trenches is combined, it yields a composite chronology of at least10 surface ruptures over the last 3000 years, for a minimum average recurrence interval of 300 years if the most recent event exposed in all trenches is assumed to be the 1857 Fort Tejon earthquake. So far, the uncertainty in dates of pre-1857 ruptures ranges from decades to millennia, and at least 5 of the 10 recognized earthquakes are obscured by depositional gaps at one of the trench sites. Therefore, synchroneity of ruptures at different trench sites is difficult to establish, and there is the possibility that the existing record contains more than 10 earthquakes and/or additional ruptures may have occurred that are not preserved by deposition.

Data on the rupture history of the San Andreas Fault (SAF) form the basis of numerous models of fault behavior and seismic hazard. The Carrizo segment of the SAF is one of the best places to study the rupture history of the SAF because it has a proven paleoseismic record with excellent slip rate and slip per event measurements. We conducted a paleoseismic study along the San Andreas fault at the Bidart Fan site to lengthen and refine the record of recent surface ruptures. Previous work at the Bidart Fan site (Grant and Sieh, 1994) demonstrated that is an excellent place to develop a long chronology of earthquakes because: (1) It has good stratigraphy for discriminating individual earthquakes. (2) It has datable material. Detrital charcoal and other datable organic material is commonly embedded in the deposits. During the 2005 field season we excavated two 11-foot-deep trenches perpendicular to the SAF (BDT5 and BDT6) and collected 125 samples for radiocarbon dating. We here present the BDT5 trench log and our interpretation of the 6+ events. Age control is based on radiocarbon ages of detrital-charcoal samples. Our best 30 charcoal samples from BDT5, which should help us constrain the ages of the 4 surface rupturing events prior to the penultimate earthquake, are currently being processed at UCI's new Keck AMS facility. A longer record of surface ruptures at the Bidart Fan site will be helpful for correlating ruptures between the Carrizo Plain and sites on the adjacent Mojave and Cholame segments and therefore estimating the magnitude of earthquakes previously documented at other sites.