R. J. Weldon

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

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.nnComparison 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.

Estimating Surface Rupture Length and Magnitude of Paleoearthquakes from Point Measurements of Rupture Displacement

We present a method to estimate paleomagnitude and rupture extent from measurements of displacement at a single point on a fault. The variability of historic ruptures is summarized in a histogram of normalized slip, then scaled to give the probability of finding a given displacement within a rupture for any magnitude considered. The histogram can be inverted assuming any magnitude earthquake is as likely as another, yielding probability density functions of magnitude and rupture length for any given displacement measurement. To improve these distributions we include a term to account for the probability that the earthquake would cause ground rupture and two alternative distributions of earthquake magnitude. The Gutenberg- Richter magnitude distribution predicts shorter rupture lengths and smaller magnitudes than does a uniform distribution where any magnitude earthquake is considered equally likely. Longer ruptures and larger magnitudes than the uniform model are predicted by an alternative magnitude distribution designed to return site average displacement. This model is a generalization of the characteristic earthquake model, and reasonably describes paleoseismic findings on the southern San Andreas fault, where slip is accommodated by average displacements of a few meters and earthquake recurrence times of 100–250 years. Our results should increase the value of paleoseismic displacement measurements for hazard assessment. In particular, they quantify probability estimates of earthquake magnitude and rupture length where point observations of rupture displacement are available, and so can contribute to probabilistic seismic-hazard analyses.nnOnline material: Rupture profiles used to sample rupture variability.