Relative Hazards of Near- and Far-field Tsunami Sources
The 2005 National Science and Technology Council report describes the nation’s mix of tsunami threats in terms of sources termed “local” and “distant”:
United States coastal communities are threatened by tsunamis generated by both local sources and distant sources. Local tsunamis give residents only a few minutes to seek safety. Tsunamis of distant origins give residents more time to evacuate the threatened coastal areas, but require timely and accurate tsunami forecasts of the hazard to avoid costly false alarms. Of the two, local tsunamis pose a greater threat to life because of the short time between generation and impact. The challenge is to design a tsunami hazard mitigation program to protect life and property from two very different types of tsunami events.1
The relative tsunami hazard of local and distant sources varies with the region according to a nationwide assessment prepared a few years ago for the National Tsunami Hazard Mitigation Program (NTHMP).2 Distant sources account for most of the tsunami hazard in Hawaii, while local sources predominate in Alaska. Washington, Oregon, California, and the Caribbean face a mix of local and distant. So does the U.S. Atlantic seaboard, from its nearby landslides and its exposure to hypothetical tsunamis from the Puerto Rico Trench.3
Below are summaries of tsunami hazard studies in Alaska, Oregon, and California that allow direct comparison between local and distant.
Nearby tsunami sources dominate the hazard depicted on tsunami inundation maps of Kodiak, Homer, and Seldovia—communities in the vicinity of the rupture area of the giant 1964 Alaska earthquake.
The Kodiak maps4 depict seven scenarios: four of them for partial or complete breakage of the 1964 rupture area, one for surface rupture of a thrust fault that extends offshore from Kodiak Island, and two tsunamis of distant origin. The scenarios with the greatest inundation result from repetition of 1964-style earthquakes, and the scenarios with the least inundation result from distant earthquakes off the Aleutians and at Cascadia.
The smallest of the modeled inundations in the Kodiak area corresponds to a distant earthquake on the Aleutian-Alaska subduction zone west of Kodiak Island. The starting assumption here is a break that extends across the so-called Shumagin seismic gap and includes rupture areas of earthquakes in 1938 and 1946. Such a hypothetical earthquake is among the
seismic sources adopted in the most recent U.S. Geological Survey seismic-hazard assessment for Alaska.5 A geophysical speculation has such an earthquake recurring at intervals of 750 years or more.
The Homer and Seldovia maps6 show two scenarios: a repeat of the 1964 earthquake, and a hypothetical break on a local fault believed inactive in the past 2 million years. Recurrence intervals for 1964-style earthquakes have averaged about 600 years during the past 5,000 years.7
Far-field tsunamis pose the most expectable source of tsunami hazards in Cannon Beach and Seaside, Oregon. However, as in Alaska, it is the near-field tsunamis that dominate the hazard in terms of tsunami size.
Worst-case inundation extends more than twice as far inland for a near-field (Cascadia) tsunami than for a far-field (eastern Aleutian) tsunami, according to an inundation map prepared by the State of Oregon in 2008 for the tourist town of Cannon Beach.8 A similar contrast is apparent in maps, of the nearby town of Seaside, that were prepared a few years earlier by a group of federal, state, and local scientists.9 The Seaside maps show contrasting observations (inundation limits and sedimentary deposits of the 1964 Alaskan tsunami, versus sedimentary deposits of the 1700 Cascadia tsunami), as well as tsunami heights depicted in terms of probabilities that are tied to estimated recurrence intervals.
Far-field tsunamis are the most expectable in Oregon because they can beam toward that state from multiple parts of the Pacific Rim. Thus, in aggregate, they happen more often than do tsunamis from Cascadia sources alone. The Seaside mapping accordingly shows far-field tsunamis as the dominant source of hazard for flooding that would lap onto the edges of town. Only at lower probabilities, commensurate with Cascadia recurrence intervals that average about 500 years, do the waters cover the entire town.
A Cascadia rupture that includes the California part of the subduction zone produces a simulated tsunami that, at Crescent City, runs inland for double the inundation distance of the 1964 Alaskan tsunami. This Cascadia tsunami, moreover, begins with a positive (leading elevation) wave that arrives in less than a half hour.10
1. National Science and Technology Council. 2005. Tsunami Risk Reduction for the United States: A Framework for Action. Subcommittee on Disaster Reduction and United States Group on Earth Observations, National Science and Technology Council, Washington, DC.
2. Dunbar, P.K. and C.S. Weaver. 2007. U.S. States and Territories National Tsunami Hazard Assessment: Historical Record and Sources for Waves. National Tsunami Hazard Mitigation Program, National Oceanic and Atmospheric Administration, Silver Spring, Maryland.
3. Geist, E.L. and T Parsons. 2009. Assessment of source probabilities for potential tsunamis affecting the U.S. Atlantic coast. Marine Geology 264(1-2):98-108.
4. Suleimani, E.N., R.A. Hansen, R.A. Comebellick, G.A. Carver, R.A. Kamphaus, J.C. Newman, and A.J. Venturato. 2002. Tsunami Hazard Maps of the Kodiak Area, Alaska. Alaska Division of Geological and Geophysical Surveys and the Geophysical Institute, University of Alaska, Fairbanks, Alaska.
5. Wesson, R.L., O.S. Boyd, C.S. Mueller, and A.D. Frankel. 2008. Challenges in making a seismic hazard map for Alaska and the Aleutians: Active tectonics and seismic potential of Alaska. Geophysical Monograph 179:385-397.
6. Suleimani, E.N., R.A. Comebillick, R.A. Hansen, A.J. Venturato, and J.C. Newman. 2005. Tsunami Hazard Maps of the Homer and Seldovia Areas, Alaska. State of Alaska Department of Natural Resources, Fairbanks, Alaska.
7. Carver, G. and G. Plafker. 2008. Paleoseismicity and neotectonics of the Aleutian Subduction Zone: An overview. In Active Tectonics and Seismic Potential of Alaska, Freymueller, J.T., P.J. Haeussler, R. Wesson, and G. Ekstrom (Eds.). American Geophysical Union, Washington, DC.
8. Priest, G.R., C. Goldfinger, K. Wang, R.C. Witter, Y. Zhang, and A.M. Baptista. 2009. Confidence levels for tsunami-inundation limits in northern Oregon inferred from a 10,000-year history of great earthquakes at the Cascadia subduction zone. Natural Hazards 1-47.
9. González, F.I., E. Geist, C.E. Synolakis, D. Arcas, D. Bellomo, D. Carlton, T. Horning, B. Jaffe, J. Johnson, U. Kanoglu, H. Mofjeld, J. Newman, T. Parsons, R. Peters, C. Peterson, G. Priest, V.V. Titov, A. Venturato, J. Weber, F. Wong, A. Yalciner. 2006. Seaside, Oregon Tsunami Pilot Study—Modernization of FEMA Flood Hazard Maps. National Oceanic and Atmospheric Administration, U.S. Geological Survey, and Federal Emergency Management Agency, Washington, DC.
10. Uslu, B., J.C. Borrero, L.A. Dengler, and C.E. Synolakis. 2007. Tsunami inundation at Crescent City, California generated by earthquakes along the Cascadia subduction zone. Geophysical Research Letters 34:L20601.