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.
Coastal wetlands provide the most promising environment in Southern California for the deposition, preservation, and recognition of sandy/gravely tsunami deposits. The wetlands sediment normally consists of organic-rich mud and/or fine sandy deposits. When a tsunami washes over a low lying wetlands, it leaves behind an exotic deposit of coarse sand and/or gravel that contains material such as shell fragments or microfossils that were derived from the beach or offshore environment. To begin a search for paleotsunami deposits, we focused on, from south to north, Oneonta Slough south of San Diego, Los Penasquitos Marsh within the Torrey Pines Reserve, San Elijo Lagoon just south of Encinitas, and the Seal Beach Wetlands and Wildlife Refuge within the Seal Beach Naval Weapons Station. These sites were chosen based on the relative lack of disturbance by human activity, their easy accessibility, and the ability of obtaining a research permit. Fieldwork in all of these sites is limited to 3-4 months in the fall and winter due to the long breeding seasons of endemic endangered species. Fieldwork is also limited to lowest tides. Our search began by visiting these wetlands and collecting and describing nearly 100 gouge cores of between 1 and 5 m depth. The cores were collected using narrow, open-sided steel tubes that are pushed into the wetlands surface and extracted, trapping a core of sediment inside the tube. The open side of the core is scraped off to allow the examination of the core’s interior. These cores were then described in detail in the field and discarded. Cores from Oneonta Slough, Los Penasquitos, and San Elijo generally contain a single organic rich (peaty) zone extending 10-30 cm downward from the surface. This organic zone represents the modern soil of the marsh. Beneath this soil lies layers of mud and fine sand. We interpret this simple sequence of layers as recording the infilling of tidal lagoons that formed within valleys that became submerged following post ice-age sea-level rise. Cores from Seal Beach contain a much more complicated sequence of layers that includes multiple organic-rich zones interlayered with mud. The multiple (and now buried) organic zones represent marsh surfaces that were subsided and buried by muddy marine sediment. These buried organic-rich zones suggest that the subsidence of the marsh occurred during discrete events. We are currently investigating the hypothesis that this sequence of layers developed due to subsidence related to ancient earthquakes along the Newport-Inglewood fault zone. If this is true, we will be able to provide a history of prehistoric earthquake activity along this fault that, up until now, consists only of the historic Long Beach earthquake of 1933. In Los Penasquitos and Seal Beach, our gouge cores penetrated distinctive shell-hash layers up to 10 cm thick. Larger diameter cores, obtained using specialized, custom made coring equipment, were collected to analyze these layers. Preliminary laboratory data suggest that the shells were derived at least in part from quiet water within a lagoon, or seaward from the beach. These layers at both sites show a significantly higher proportion of magnetic minerals than the lagoonal sediments, thus favoring an offshore source for the shells. Shells brought in from offshore must be transported by high-energy waves – either very large storms, or tsunami waves. Shell samples from this layer at Los Penasquitos yielded C-14 dates of 1795, 1835, and 1895 years before present. We are tentatively interpreting this shell-hash as a tsunami deposit. Future work will involve additional coring at Los Penasquitos and Seal Beach to delineate the extent of the fossil hash layers, and to further investigate the subsidence at Seal Beach.
