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The search for geologic evidence of distant-source tsunamis using new field data in California

Open-File Report 2013-1170-C
By: , , , , , , , , , , , , , , and 
https://doi.org/10.3133/ofr20131170C

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Abstract

A statewide assessment for geological evidence of tsunamis, primarily from distant-source events, found tsunami deposits at several locations, though evidence was absent at most locations evaluated. Several historical distant-source tsunamis, including the 1946 Aleutian, 1960 Chile, and 1964 Alaska events, caused inundation along portions of the northern and central California coast. Recent numerical tsunami modeling results identify the eastern Aleutian Islands subduction zone as the “worstcase” distant-source region, with the potential for causing tsunami runups of 7–10 m in northern and central California and 3–4 m in southern California. These model results, along with a review of historical topographic maps and past geotechnical evaluations, guided site selection for tsunami deposit surveys. A reconnaissance of 20 coastal marshlands was performed through site visits and coring of shallow surface sediments to determine if evidence for past tsunamis existed. Although conclusive evidence of tsunami deposits was not found at most of the sites evaluated, geologic evidence consistent with tsunami inundation was found at two locations: Three marshes in the Crescent City area and Pillar Point marsh near Half Moon Bay. Potential tsunami deposits were also evaluated at the Carpinteria Salt Marsh Reserve in Santa Barbara County. In Crescent City, deposits were ascribed to tsunamis on the basis of stratigraphic architecture, particle size, and microfossil content, and they were further assigned to the 1964 Alaska and 1700 Cascadia tsunamis on the basis of dating by cesium-137 and radiocarbon methods, respectively. The 1946 tsunami sand deposit was clearly identified throughout Pillar Point marsh, and one to two other similar but highly discontinuous sand layers were present within 0.5 m of the surface. A tsunami-origin interpretation for sand layers at Carpinteria is merely consistent with graded bedding and unsupported by diatom or foraminiferal assemblages. Additional studies, including age dating, grain-size, and microfossil analyses are underway for the deposits at Crescent City, Pillar Point marsh, and Carpinteria, which may help further identify if other tsunami deposits exist at those sites. The absence of evidence for tsunamis at other sites examined should not preclude further work beyond the reconnaissance-level investigations at those locations.

Suggested Citation

Wilson, R., Hemphill-Haley, E., Jaffe, B., Richmond, B., Peters, R., Graehl, N., Kelsey, H., Leeper, R., Watt, S., McGann, M., Hoirup, D.F., Chague-Goff, C., Goff, J., Caldwell, D., Loofbourrow, C., 2014, The search for geologic evidence of distant-source tsunamis using new field data in California: U.S. Geological Survey Open-File Report 2013-1170, viii, 122 p., https://doi.org/10.3133/ofr20131170C.

ISSN: 2331-1258 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title The search for geologic evidence of distant-source tsunamis using new field data in California
Series title Open-File Report
Series number 2013-1170
Chapter C
DOI 10.3133/ofr20131170C
Year Published 2014
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Science Application for Risk Reduction (SAFRR)
Description viii, 122 p.
Larger Work Type Report
Larger Work Subtype USGS Numbered Series
Larger Work Title The SAFRR (Science Application for Risk Reduction) tsunami scenario (Open File Report 2013-1170)
Country United States
State California
Datum North American Datum of 1983
Projection Universal Transverse Mercator Projection
Online Only (Y/N) Y
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