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Introduction to the Special Issue on Paleoseismology of the San Andreas Fault System

Department of Environmental Analysis and Design
University of California
Irvine, California 92697-7070
lgrant{at}uci.edu
(L.B.G.)

William Lettis & Associates, Inc.
1777 Botelho Drive, Suite 216
Walnut Creek, California 94596
lettis{at}lettis.com
(W.R.L.)

	Purpose

Top Purpose Summary Acknowledgments

 
The purpose of this special issue of BSSA is to provide a state-of-the-science compendium of data on the rupture history of the San Andreas fault from recent paleoseismic investigations. The San Andreas fault is the primary boundary fault between the Pacific and North American plates and one of the most thoroughly studied faults in the world. Fourteen articles prepared by 74 authors provide dates of surface ruptures and/or measurements of slip or slip rate at 10 paleoseismic investigation sites along the main trace of the San Andreas fault, at one site on the Hayward fault, and along the northern San Jacinto fault. This issue includes documentation of the longest multicycle earthquake sequence in California—and possibly in North America (14 ruptures at the Wrightwood site). Geographic coverage extends almost along the entire San Andreas fault system, from Bodega Bay in northern California to the Mecca Hills in the Coachella Valley. The issue also includes articles linking paleoseismic data with modern instrumental recordings of ground motion and seismicity.

Data on the earthquake history of the San Andreas fault form the basis of numerous models of fault behavior and calculations of seismic hazard. Despite the relatively large amount of data available for the San Andreas fault, there remain unresolved questions about segmentation, the rupture patterns of large earthquakes, and characteristics of earthquake recurrence. The data presented in this special issue contribute toward understanding the spatial and temporal rupture history of the San Andreas fault over multiple rupture cycles during the last few hundred to few thousand years. This is the timescale of greatest interest for seismic hazard assessments and fault modeling.

	Summary

Top Purpose Summary Acknowledgments

 
This introduction provides an overview of the issue and summarizes results in simplified, tabular form. Table 1 contains dates of surface ruptures and/or average recurrence intervals reported from 10 sites discussed in this issue. Table 2 includes measurements of slip or slip rate at seven of the locations. The summary tables highlight the data presented in each article and are not intended to be substitutes for the detailed presentation and discussion of data in individual articles. Articles are ordered geographically from north to south, with the exception of the first two.

The first two articles by Toppozada et al. and Brune provide a bridge between the instrumental record of earthquakes along the San Andreas fault system and the paleoseismic observations in the rest of this issue. Toppozada et al. present a compilation of M 5.5 earthquakes along the entire San Andreas fault zone. Most of the San Andreas fault, Hayward fault, and a portion of the San Jacinto fault ruptured between A.D. 1812 and 1906 in large earthquakes with minimal or no instrumental records (Toppozada et al. and Brune.). Toppozada et al. discuss the incompleteness of the early historic record for M 6 events in key areas such as Parkfield and present an interpretation of the location of the December A.D. 1812 southern California earthquakes based on felt reports. Several other articles in this issue also address the location of the 1812 ruptures based on paleoseismic trench excavations (Lindvall et al., Weldon et al., Biasi et al., Fumal et al., and McGill et al.). The A.D. 1868 earthquake on the Hayward fault is discussed by both Toppozada et al., from historic records, and by Lienkaemper et al., from paleoseismic excavations.

Brune analyzes precarious rocks to estimate ground motions from prehistoric, early historic, and recent earthquakes. He provides constraints on shaking near the San Andreas and San Jacinto faults from historic earthquakes in A.D. 1812, 1857, 1899, and 1912, as well as prehistoric ground motions at Banning Pass between the Plunge Creek trench site of McGill et al. to the northwest, and the Thousand Palms site of Fumal et al. to the southeast.

In northern California, Knudsen et al. studied relative sea level changes at Bodega Bay and Bolinas Lagoon. They interpreted several episodes of earthquake-induced subsidence along the north coast section of the San Andreas fault. When combined with results of other paleoseismic investigations in the north coast region, the results suggest that ruptures occurred approximately 700 years ago and 350 years ago, as well as in A.D. 1906.

Lienkaemper et al. report the results of paleoseismic excavations at Tysoon's Lagoon (also called Tule Pond) along the Hayward fault, a major branch of the San Andreas fault system in the densely populated east San Francisco Bay area. In addition to describing evidence of the historic 1868 earthquake, they discuss evidence of creep, as well as three prehistoric ruptures.

Two articles discuss rupture characteristics of the San Andreas fault between Cholame Valley and the Carrizo Plain. Runnerstrom et al. report changes in survey section-line lengths and positions of survey monuments spanning the San Andreas fault before and after the A.D. 1857 earthquake. The right-lateral displacement measured by Runnerstrom et al. spans approximately 2 km of the fault zone and is significantly greater than the slip reported by Young et al. from hand excavations within the fault zone. Young et al. measured shorter aperture displacement from the 1857 earthquake and report the approximate date of the penultimate earthquake. They also describe enigmatic tectonic fractures that apparently formed after 1857.

Lindvall et al. identified two surface ruptures, including the 1857 earthquake, within the last 500 years at the Frazier Mountain paleoseismic site in the mountainous Big Bend region of the San Andreas fault. They conclude that the penultimate rupture in the Big Bend occurred several centuries earlier, rather than the A.D. 1812 date proposed by Toppozada et al. However, they cannot rule out the possibility of rupture on a nearby trace in 1812.

Three companion articles by Weldon et al., Fumal et al., and Biasi et al. report evidence of surface rupture and well-constrained dates of 14 earthquakes at the Wrightwood site in the San Gabriel Mountains. This is the longest chronology of large earthquakes from any site in California and possibly all of North America. The length of the record and the number of earthquakes provide a data set large enough for testing models of earthquake recurrence and calculation of seismic hazard with low uncertainties. Weldon et al. provide a detailed description of the near-surface structure of the San Andreas fault zone at Wrightwood, the average displacement in the most recent events, and the average slip rate. Fumal et al. describe evidence for the earthquakes and constraints on the age of each rupture. Biasi et al. present a quantitative analysis of rupture dates and their temporal distribution, as well as conditional probabilities of rupture during the next 30 years. They also apply statistical methods to previously published data from the Pallett Creek paleoseismic site and attempt to quantify the potential for joint rupture of Pallett Creek and Wrightwood in the last 1500 years.

The San Jacinto fault is a major branch of the San Andreas fault system that splays off the main fault southeast of the Wrightwood paleoseismic site. Kendrick et al. conducted a study of landscape evolution along the northern San Jacinto fault and applied their methods toward measuring the slip rate of the fault. Their results suggest that the slip rate of the northern San Jacinto fault may be as high as the slip rate of the San Andreas fault in the San Bernardino region.

McGill and colleagues examined the rupture history of the San Bernardino strand of the San Andreas fault at the Plunge Creek paleoseismic site. Much of the article discusses evidence for, and dating of, the most recent earthquake. They address the question of whether or not the A.D. 1812 rupture extended through the site and present evidence that supports a significantly earlier date of most recent rupture. Their results suggest that the 1812 rupture terminated to the northwest, between Plunge Creek and Wrightwood.

An article by Fumal and colleagues documents the occurrence of four or five earthquakes at the Thousand Palms Oasis site on the Mission Creek strand of the San Andreas fault, since A.D. 800. The most recent rupture occurred several centuries ago, prior to historic records. Fumal et al. also report a minimum slip rate for the Mission Creek strand of the fault.

The final manuscript by Shifflett et al. includes geodetic measurements, analysis of geomorphic features, and dating of offset shorelines and archeological features to estimate the slip rate and average recurrence time for large earthquakes along the southernmost San Andreas fault in the Mecca Hills, Coachella Valley.

	Acknowledgments

Top Purpose Summary Acknowledgments

 
This special volume is an outgrowth of discussions held at the 2000 and 2001 SSA meetings, at which preliminary results of many ongoing paleoseismic investigations on the San Andreas Fault System were presented. In particular, we thank David Schwartz for the inspiration and motivation for seeing this volume come to fruition and the many authors and reviewers who spent countless hours preparing and refining the articles presented herewith. We also thank the SSA Publications Committee and staff for their support and assistance.

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This Article Extract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Grant, L. B. Articles by Lettis, W. R. Search for Related Content GeoRef GeoRef Citation