Holocene Seismicity of the Northern San Andreas Fault Based on Precise Dating of the Turbidite Event Record

Oregon State University, Corvallis OR

Investigators

Abstract

The Northern San Andreas Fault last ruptured in the great 1906 earthquake that destroyed San Francisco. Efforts on land to determine the frequency of such earthquake and their magnitudes and rupture lengths have been hampered by few suitable localities where a geologic record has been created. Offshore, new core data show that continental margin channels in Northern California have recorded a Holocene history of regional submarine landslides possibly triggered by San Andreas great earthquakes. This project is applying marine paleoseismologic techniques to the turbidite history along the Northern San Andreas transform margin using cores collected in 2002 during a cruise of the Scripps vessel R/V Roger Revelle. The goals of this project are to test models of full margin and segmented rupture along the North Coast segment of the San Andreas Fault. A Holocene event chronology along the margin from San Francisco to Cape Mendocino is being established using offshore turbidites and turbidite events are being tested for synchronous (and therefore earthquake) triggering. The methods include direct dating of each turbidite, biostratigraphy, and direct physical property correlation using density, magnetics, mineralogy and visible and X-ray imagery to construct a stratigraphic framework along the margin. Correlation of events between separate sites along the fault using radiocarbon, and fingerprints of each event are being used to test the linkages between deposits along the fault. Direct linkages point to an earthquake origin, since other triggers of submarine landslides happen in only one canyon at a time. Using this information, it may be possible to establish both rupture length and timing of past earthquakes along the Northern San Andreas. This information will be valuable for analysis of seismic hazards in San Francisco, as well as shed light on the long-term behavior of large fault systems that does not presently exist.