By Robert Leeper
Paleotsunami data along the California coast are rare and the data that do exist are limited to northern California. For a better understanding of the tsunami threat throughout central and southern California, prehistoric data need to be unearthed and documented in these regions. To accomplish this, the USGS Multi-Hazards Demonstration Project (MHDP) has brought together scientists from the USGS, California Geological Survey (CGS), Humboldt State University, and California State University, Fullerton, to identify traces and develop a prehistoric tsunami inundation chronology. These data will provide geologic evidence to support the next MHDP scenario, which will consider the effect on California of a tsunami generated by a major earthquake in the eastern Aleutian Islands.
Research sites were determined by reviewing previous paleotsunami reconnaissance and research studies, considering coastal geomorphology and historic site-disturbance, and use of tsunami run-up data from the latest numerical tsunami models. These models were developed collaboratively by the CGS, California Emergency Management Agency, and the Tsunami Research Center at the University of Southern California. In response to a major earthquake in the eastern Aleutian Islands, one scenario results in wave elevations in excess of 9 m at some locations along the central and northern California coast, and 4 m in portions of southern California. In late July 2011, reconnaissance fieldwork commenced in northern California south of Point Arena and will continue southward along California’s coast throughout 2011.
Results of the paleotsunami reconnaissance will set the stage for more in-depth research, which will get underway in early 2012. The final results of the study will be used by the State to better assess its tsunami hazard, especially for probabilistic tsunami hazard assessment, and incorporated by the MHDP into the next scenario.
Collaborating scientists and institutions: Graehl, N. (HSU), Hemphill-Haley, E. (HSU), Hoirup, D. (CDWR), Jaffe, B. (USGS), Kelsey, H. (HSU), Kirby, M. (CSUF), Leeper, R. (USGS/CSUF), Peters, R. (Santa Cruz, CA), Rhodes, B. (CSUF), Richmond, B. (USGS), and Wilson, R. (CGS)
What exactly are we looking for during our reconnaissance?
Paleotsunami deposit identifiers:
1. Anomalous coarse-grained material overlying peat or mud: usually composed of sand or coarse silt and may contain gravel.
2. Deposit geometry: laterally continuous or semi-continuous, i.e., mappable deposits that thin landward and away from channels.
3. Provenance: deposits containing sediment consistent with a seaward source.
4. Evidence for event-driven deposition: sharp or erosional basal contacts are characteristic of tsunami deposits. Lack of a sharp basal contact is evidence for gradual change in depositional environment.
Identifiers that may be evident in sediment cores:
1. Evidence for turbulent flow: deposits displaying erosional basal contacts or containing rip-up clasts. In narrow-diameter gouge cores, rip-up clasts may be difficult to discern except in thicker tsunami deposits.
2. Evidence for multiple waves (multistage deposit): could include 2 or more coarse-grained beds separated by capping mud or debris.
3. Normal grading: normal grading is common in tsunami deposits.
4. Relatively isolated occurrence: sand deposits in a stratigraphic section, i.e., stratigraphic section not containing numerous other sandy deposits indicating frequent flooding by other means.
Variability in paleotsunami deposit preservation and characteristics:
Depositional environment, geomorphic processes, and historic site-disturbance control how well paleotsunami deposits are preserved. Diffuse, abrupt, irregular contacts and lateral continuity may or may not be seen in paleotsunami deposits.
Below are photos of what we’ve seen in different depositional environments:
California’s coastline (estuaries/marshes):