Great Cascadia Earthquake Tricentennial
48 June 2000
- John J. Clague
- Dept. of Earth Sciences, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada, and Geological Survey of Canada, 101-605 Robson St., Vancouver, BC V6B 5J3, Canada
- Brian F. Atwater
- U.S. Geological Survey at Dept. of Geological Sciences AJ-20, University of Washington, Seattle, WA 98195
- Kelin Wang
- Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
- Yumei Wang
- Oregon Department of Geology and Mineral Industries, 800 NE Oregon St., No. 28, Portland, OR 97232
- Ivan G. Wong
- URS Greiner Woodward Clyde Federal Services, 500 12th St., Ste. 200, Oakland, CA 94607
• U.S. Geological Survey, Geological Survey of Canada, and
• Oregon Department of Geology and Mineral Industries
The year 2000 marks the tricentennial of the last great (magnitude 8 or larger) earthquake at the Cascadia subduction zone, which is located along the Pacific coast of North America from British Columbia to northern California. Coastal and offshore work has confirmed that many great plate-boundary earthquakes have struck this region in the Holocene, and geodetic studies have shown that the subduction zone is accumulating strain that will be released in a future earthquake.
To commemorate the tricentennial, almost 100 geologists, geophysicists, engineers, and public officials gathered in Seaside, Oregon, in the first week of June 2000 to critically review current knowledge about great Cascadia earthquakes, clarify the hazards posed by these earthquakes, discuss appropriate strategies for reducing earthquake losses, and identify priority research directions. Further understanding of the great earthquake potential of the Cascadia subduction zone is required for seismic hazard characterization, engineering design, emergency planning and response, and other mitigation efforts in a region with a population of nearly 10 million people. Seaside was an appropriate place to hold the conference because much of the community, including the conference hotel, lies within the inundation zone of the tsunami of the 1700 earthquake!
The conference consisted of three days of indoor sessions, a field trip, and a public forum on Cascadia earthquakes and tsunamis. Sessions on the first day dealt with earthquake hazards and their mitigation. These initial sessions provided focus for subsequent sessions on regional earthquake histories, tectonics, and present-day seismicity and strain accumulation. Evidence for past Cascadia earthquakes was examined and discussed during a canoe trip along the Niawiakum River in southwestern Washington and at a nearby park where lake, tidal marsh, and deep-sea cores collected during previous paleoseismological investigations were displayed. The public forum, held on the first evening of the conference, attracted more than 200 people, including two state senators, and allowed conference participants to hear concerns of local residents. The forum and the conference as a whole were covered extensively in local newspapers.
A huge amount of progress has been made in understanding the behavior of the Cascadia subduction zone over the past two decades. Fifteen years ago, scientists were debating whether great earthquakes occur at the subduction zone. Today, few scientists doubt that great earthquakes occur in this region; rather, the discussion has shifted to questions such as the magnitude of the earthquakes and attendant tsunamis, the location and width of the seismogenic zone, and the involvement of crustal structures in plate-boundary rupture. These issues were topics of discussion and debate at the conference. Consensus was achieved on several important issues; major points of consensus are summarized below.
- Damage, injuries, and loss of life from the next great earthquake at the Cascadia subduction zone will be great and widespread, and will impact the national economies of Canada and the United States for years or decades. Increased research, information exchange, public education, mitigation, and planning are needed to reduce risk. Damage from historical earthquakes and results of predictive damage and loss studies suggest that disastrous future losses will occur in the Pacific Northwest from a great Cascadia earthquake. Mitigation efforts in other seismically active regions and cost-benefit studies indicate that mitigation can effectively reduce these losses and help with recovery. Accurate data acquired through geological and geophysical research, followed by information and technology transfer to key decision makers, will reduce risk to citizens of the coastal Pacific Northwest.
- The Cascadia subduction zone produces great earthquakes, the most recent of which occurred in 1700 and was of moment magnitude (Mw) 9. Geologic evidence from a large number of coastal and offshore sites from northern California to southern Vancouver Island and historical records from Japan show that most or all of the 1100 km length of the subduction zone ruptured about 300 years ago. Japanese accounts of a correlative tsunami suggest that this rupture occurred in a single earthquake, possibly of Mw 9, on January 26, 1700. The sizes of earlier Cascadia earthquakes are unknown. It is possible that some of them ruptured adjacent segments of the subduction zone over periods ranging from hours to years, as has happened historically in Japan and Colombia.
- Great Cascadia earthquakes generate tsunamis, the most recent of which was probably at least 10 m high on the Pacific coast of Washington, Oregon, and northern California, and up to 5 m high in Japan. These tsunamis threaten coastal communities all around the Pacific Ocean but would have their greatest impact on the U.S. and Canadian west coasts, which would be struck 1540 minutes after the earthquake. Deposits of past great Cascadia tsunamis have been identified at numerous coastal sites in California, Oregon, Washington, and British Columbia. The distribution of the deposits and computer-based simulations of tsunamis indicate that many coastal communities in the region are partially to largely within the inundation zone of past Cascadia tsunamis. These communities are threatened by future tsunamis from great Cascadia earthquakes. Tsunami arrival times depend largely on the location of the rupture zone, specifically its distance from the coast.
- Strong ground shaking from a Mw 9 plate-boundary earthquake will last three minutes or more and will be dominated by long-period ground motions. Damaging ground shaking will probably occur as far inland as Vancouver, Portland, and Seattle. The large cities of Cascadia are 100150 km from the nearest point on the inferred plate-boundary rupture zone. Although ground shaking at these locations will be less than that of a nearby large (Mw greater than or equal to 7) crustal earthquake, the shaking will last much longer and the long-period waves could damage many tall or long engineered structures. Shaking will be strongest along the Pacific coast, resulting in significant damage to coastal communities.
- The mean recurrence interval for great plate-boundary earthquakes in Cascadia is 500-600 years, but some of the past earthquakes had intervals less than the time that has elapsed since the 1700 earthquake. Intervals between successive great earthquakes range from a few centuries to about one thousand years. The number of well measured recurrence intervals is small-rarely more than five. The data show that great earthquakes have occurred at irregular intervals, but they do not show whether the earthquakes cluster or are randomly distributed in time. Because the recurrence pattern is poorly known, probabilities that the next earthquake will occur within particular intervals have broad ranges.
- The Cascadia plate boundary is currently locked, and the locked zone is offshore and widest off northwest Washington. The maximum area of seismogenic rupture is 1100 km long and 50150 km wide. The location and size of the seismogenic portion of the plate boundary are critical for determining earthquake magnitude, tsunami size, and the strength of ground shaking. The landward limit of the "locked" portion of the plate boundary, where no slip occurs between the Juan de Fuca and North American plates during periods between earthquakes, has been delineated from geodetic measurements of the deformation of the land surface. However, few or no data constrain the seaward limit of the locked zone. In addition, the transition zone, which separates the locked zone from the zone of continuous sliding to the east, is also poorly constrained. Earthquake rupture may extend an unknown distance from the locked zone into the transition zone.
- Movement on some crustal faults near the coast may accompany plate-boundary earthquakes and increase the size of the tsunami and the intensity of local ground shaking. Detailed mapping along the coast of Cascadia on the adjacent continental shelf has revealed the presence of numerous folds and faults that were active during the Quaternary and perhaps remain active today. The question of whether some crustal faults slip during plate-boundary earthquakes, and thus are independent seismic sources, was debated at the conference. Movement on crustal faults does not explain the coastal coseismic subsidence evidence in some areas, which can only be interpreted as resulting from plate-boundary rupture. The evidence, however, does not disprove that some faults ruptured before, during, or immediately after great earthquakes. This issue is important for seismic risk assessment because moderate or large earthquakes might occur on crustal faults close to urban areas, and displacements of the seafloor along such faults could trigger tsunamis with very large waves.
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Kenneth R. Aalto
Jonathan F. Hughes