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Evidence for Large Earthquakes on the San Andreas Fault at the Wrightwood, California, Paleoseismic Site: A.D. 500 to Present

U.S. Geological Survey
MS 377
345 Middlefield Rd
Menlo Park, California 94025
(T.E.F., T.E.D., W.T.F., D.P.S.)

Department of Geological Sciences
University of Oregon
Eugene, Oregon 97403-1272
(R.J.W.)

Seismological Laboratory, MS-174
University of Nevada–Reno
Reno, Nevada 89557
(G.P.B.)

Lawrence Livermore National Laboratory
Center for Accelerator Mass Spectrometry
7000 East Avenue, L-397
Livermore, California 94551
(G.G.S.)

We present structural and stratigraphic evidence from a paleoseismic site near Wrightwood, California, for 14 large earthquakes that occurred on the southern San Andreas fault during the past 1500 years. In a network of 38 trenches and creek-bank exposures, we have exposed a composite section of interbedded debris flow deposits and thin peat layers more than 24 m thick; fluvial deposits occur along the northern margin of the site. The site is a 150-m-wide zone of deformation bounded on the surface by a main fault zone along the northwest margin and a secondary fault zone to the southwest. Evidence for most of the 14 earthquakes occurs along structures within both zones. We identify paleoearthquake horizons using in-filled fissures, scarps, multiple rupture terminations, and widespread folding and tilting of beds. Ages of stratigraphic units and earthquakes are constrained by historic data and 72 ¹⁴C ages, mostly from samples of peat and some from plant fibers, wood, pine cones, and charcoal.

Comparison of the long, well-resolved paleoseimic record at Wrightwood with records at other sites along the fault indicates that rupture lengths of past earthquakes were at least 100 km long. Paleoseismic records at sites in the Coachella Valley suggest that each of the past five large earthquakes recorded there ruptured the fault at least as far northwest as Wrightwood. Comparisons with event chronologies at Pallett Creek and sites to the northwest suggests that approximately the same part of the fault that ruptured in 1857 may also have failed in the early to mid-sixteenth century and several other times during the past 1200 years. Records at Pallett Creek and Pitman Canyon suggest that, in addition to the 14 earthquakes we document, one and possibly two other large earthquakes ruptured the part of the fault including Wrightwood since about A.D. 500. These observations and elapsed times that are significantly longer than mean recurrence intervals at Wrightwood and sites to the southeast suggest that at least the southermost 200 km of the San Andreas fault is near failure.

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