EAGER: Search for a Paleotsunami Record in the Wetlands of Southern California

Csu Fullerton Auxiliary Services Corporation, Fullerton CA

Investigators

Abstract

In 2004, a magnitude 9.3 earthquake off Sumatra triggered a tsunami that devastated an unaware and unprepared country, resulting in more than 150,000 Indonesian deaths. Seven years later, an only slightly smaller magnitude 9.0 earthquake off the densely populated East coast of Japan triggered a tsunami of similar size, but with a much lower death toll. Japan has a rich record, both geological and historical, of past tsunamis. This record has raised awareness in the population of Japan and fostered arguably the most stringent building codes and best coastal engineering in the world--both contributing to the reduced loss of life in Japan. In Southern California, historical written records extend back only 150-200 years, with no confirmed physical evidence for past tsunamis in the geological record. Yet Southern California has a densely populated coastal zone, and is home to three large ports, including the Port of Los Angeles and Long Beach Harbor and ten power plants, including the San Onofre nuclear power plant. Even a modestly sized tsunami would have an enormous impact of the economy of Southern California. In the past ten years, mapping and seismic reflection profiling of off-shore Southern California has revealed a dense network of potentialy seismogenic faults and large submarine landslides. Modeling of these hypothetical seismic and landslide sources suggests that they could generate a tsunami with local wave heights of up to 20m or more. However, without a geologic record of tsunamis, these studies can only be regarded as speculative. Coastal wetlands of Southern California, formed during the Holocene sea-level rise, provide an ideal, predominantly muddy sedimentary environment to capture a sandy tsunami deposit. This project is a systematic search of these wetland sediments for tsunami deposit candidates. Finding a record of tsunamis in Southern California will verify and validate much of the research of the last ten years on modeling local tsunamis from submarine landslides and offshore earthquakes. The scale and frequency of inundation along the coast has a direct impact on emergency management policy and development, especially in densely populated, economically important Southern California. Modeling can only provide an estimated scale of inundation, but a geologic record of tsunamis represents a transformative scientific discovery that would help verify the occurrence, scale, and frequency of inundation--critical information for planning tsunami hazard mitigation. Data from this project provides a robust record of the Holocene history of the California wetlands, including environmental changes through time, and places constraints on Holocene sea-level change. This project also adds to paleoclimate data for the region. Many of the wetlands in the area are ecological preserves with active on-site laboratories and/or popular visitors centers. Several of the organizations overseeing these areas have requested that the results from this project be integrated with their on-site displays. Tsunami information derived from the project will reach a broad cross-section of the Southern California community. This information will also be also shared with local governments and media to ensure the public is apprised of the implications of these results. The project also has a strong educational component, training three undergraduate research assistants, who will be included in all aspects of the project, including fieldwork, laboratory analyses, interpretation, and the presentation of the results both orally at conferences and through co-authorship of scholarly papers.