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

Abstract

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov 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., Chague-Goff, C., Goff, J., Caldwell, D., and Loofbourrow, C., 2014, The search for geologic evidence of distant source tsunamis using new field data in California, chap C of Ross, S.L., and Jones, L.M., eds., The SAFRR (Science Application for Risk Reduction) tsunami scenario: U.S. Geological Survey OpenFile Report 2013-1170-C, 122 p., http://dx.doi.org/10.3133/ofr20131170c. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. ISSN 2331ISSN -1258 iii fig. 57 for location) STATE OF CALIFORNIA EDMUND G. BROWN, JR. GOVERNOR THE NATURAL RESOURCES AGENCY JOHN LAIRD SECRETARY FOR RESOURCES DEPARTMENT OF CONSERVATION MARK NECHODOM DIRECTOR CALIFORNIA GEOLOGICAL SURVEY AbstractA 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 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 Al...

Changing impacts of Alaska-Aleutian subduction zone tsunamis in California under future sea-level rise

“…The 1960The , 1964, and 2011 tsunamis coincided with differing stages of low tide at the ports; therefore, their maximum nearshore tsunami heights (MNTH; defined here as the maximum amplitude of the tsunami wave time series relative to mean sea level (MSL)) were dampened. Nevertheless, strong currents and inundation caused by MNTH of 0.27 m in 1960 and 0.41 m in 1964 caused ~$17 M and $149 M (amounts referenced in this paper are in 2019 inflationadjusted U.S. dollars) of damage in Los Angeles, respectively 19,21 . In 2011, MNTH of 0.13 m created strong currents at the ports and disrupted operations, and higher MNTH (>1 m) along the northern California coast caused ~$115 M in damage 19,21 .…”

“…The economic impacts of future distant-source tsunamis are estimated to be in the billions of dollars for parts of the Pacific Rim, including the southern California coast 18 . Over the past two centuries, at least 14 distant-source tsunamis have damaged the California coast, with 8 of the 14 occurring in the last ~70 years 19 . The economic impacts of distant-source tsunamis in California have increased as coastal populations and infrastructure have grown.…”

“…At the low-lying, densely populated, and economically important Ports of Los Angeles and Long Beach, recent maximum tsunami amplitudes (defined as the absolute value of the difference between a particular peak or trough of the tsunami and the undisturbed sea level at the time) recorded at tide gauge 1 20 (TG1; Fig. 1c) are associated with distant-source tsunamis from earthquakes originating in Chile (1960; M w 9.5; maximum tsunami amplitude = 0.72 m), Alaska (1964; M w 9.2; maximum tsunami amplitude = 0.56 m), and Japan (2011; M w 9.0; maximum tsunami amplitude = 0.31 m) 19 . The 1960The , 1964, and 2011 tsunamis coincided with differing stages of low tide at the ports; therefore, their maximum nearshore tsunami heights (MNTH; defined here as the maximum amplitude of the tsunami wave time series relative to mean sea level (MSL)) were dampened.…”

Tsunamis as Paleoseismic Indicators

“…Indicative for washover fans are a tapering and fining landward, a common height above mean sea level, and the content of marine fossils. These observations can be easily achieved by shallow drillings (e.g., Wilson et al 2014) and GPS control or ERT (electric resistivity) of the drill points for large strips along a coast. For boulder deposits, the orientation of the long axis of min.…”

Evidence for coseismic subsidence events in a southern California coastal saltmarsh

“…Submergence of coastal wetlands due to coseismic subsidence results in sharp changes in lithology and paleoecology that commonly reflect a deepening of the depositional environment 4 5 6 8 9 . Here, we evaluate the Seal Beach saltmarsh in southern California that contains multiple buried peaty and organic-rich mud layers, which is different when compared to other southern California coastal wetlands 3 10 11 . We hypothesize that peaty and organic-rich intertidal deposits, buried by coarse-grained sediments containing mostly sparse allochthonous freshwater diatoms, demonstrates that the saltmarsh coseismically subsided three times during the late Holocene.…”