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Paleoearthquakes on the southern San Andreas Fault, Wrightwood, California, 3000 to 1500 B.C.: a new method for evaluating paleoseismic evidence and earthquake horizons

Katherine M. Scharer, Ray J. Weldon, Tom E. Fumal and Glenn P. Biasi
Paleoearthquakes on the southern San Andreas Fault, Wrightwood, California, 3000 to 1500 B.C.: a new method for evaluating paleoseismic evidence and earthquake horizons
Bulletin of the Seismological Society of America (August 2007) 97 (4): 1054-1093

Abstract

We present evidence of 11-14 earthquakes that occurred between 3000 and 1500 B.C. on the San Andreas fault at the Wrightwood paleoseismic site. Earthquake evidence is presented in a novel form in which we rank (high, moderate, poor, or low) the quality of all evidence of ground deformation, which are called "event indicators." Event indicator quality reflects our confidence that the morphologic and sedimentologic evidence can be attributable to a ground-deforming earthquake and that the earthquake horizon is accurately identified by the morphology of the feature. In four vertical meters of section exposed in ten trenches, we document 316 event indicators attributable to 32 separate stratigraphic horizons. Each stratigraphic horizon is evaluated based on the sum of rank (Rs), maximum rank (Rm), average rank (Ra), number of observations (Obs), and sum of higher-quality event indicators (Rs (sub >1) ). Of the 32 stratigraphic horizons, 14 contain 83% of the event indicators and are qualified based on the number and quality of event indicators; the remaining 18 do not have satisfactory evidence for further consideration. Eleven of the 14 stratigraphic horizons have sufficient number and quality of event indicators to be qualified as "probable" to "very likely" earthquakes; the remaining three stratigraphic horizons are associated with somewhat ambiguous features and are qualified as "possible" earthquakes. Although no single measurement defines an obvious threshold for designation as an earthquake horizon, Rs, Rm, and Rs (sub >1) correlate best with the interpreted earthquake quality. Earthquake age distributions are determined from radiocarbon ages of peat samples using a Bayesian approach to layer dating. The average recurrence interval for the 10 consecutive and highest-quality earthquakes is 111 (93-131) years and individual intervals are + or -50% of the average. With comparison with the previously published 14-15 earthquake record between A.D. 500 and present, we find no evidence to suggest significant variations in the average recurrence rate at Wrightwood during the past 5000 years.


ISSN: 0037-1106
EISSN: 1943-3573
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 97
Serial Issue: 4
Title: Paleoearthquakes on the southern San Andreas Fault, Wrightwood, California, 3000 to 1500 B.C.: a new method for evaluating paleoseismic evidence and earthquake horizons
Affiliation: Appalachian State University, Department of Geology, Boone, NC, United States
Pages: 1054-1093
Published: 200708
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 40
Accession Number: 2007-095007
Categories: Quaternary geologySeismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 5 tables, strat. cols.
N34°22'10" - N34°22'10", W117°40'04" - W117°40'04"
Secondary Affiliation: University of Oregon, USA, United StatesU. S. Geological Survey, USA, United StatesUniversity of Nevada-Reno, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200742
Program Name: USGSOPNon-USGS publications with USGS authors
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