In March 1995, the California Division of Mines and Geology released a Planning Scenario in Humboldt and Del Norte Counties, California for a Great Earthquake on the Cascadia Subduction Zone. It describes the possible effects of a very large earthquake along the southern segment of the Cascadia megathrust.
Earthquake planning scenarios model the expected strength of shaking, distribution of liquefaction and landsliding hazards, and damage to roads and utilities caused by a particular earthquake. They assist in planning emergency response and in reducing hazards before the earthquake occurs. The damage assessments are general in nature and are not intended as site-specific evaluations. Seven previous scenarios for earthquakes in metropolitan areas within the San Francisco Bay Area and southern California have been published by the Division of Mines and Geology since 1982.
The subduction zone planning scenario differs from previous scenarios in several important ways:
A magnitude 8.4 earthquake ruptures a 150-mile-long, 50-mile-wide segment of the megathrust, extending from Cape Mendocino to just north of the Oregon border. The fault plane dips at an angle of 11 degrees to the east beneath the coast. The postulated fault surface is about six miles beneath Petrolia, nine miles beneath Eureka and twelve miles beneath Crescent City. The scenario assumes concurrent movement and surface rupture along the Little Salmon fault. Potentially damaging ground shaking continues for about one minute, and aftershocks occur for several months, including a few in the magnitude 6 to 7 range. Expected shaking is generally greatest near the coast, with Eureka, Arcata, Fortuna and Crescent City likely to experience shaking similar to that experienced by Northridge residents in January 1994. The earthquake damages highways and utilities, restricting the arrival of emergency supplies for 2 weeks.
Distribution of scenario earthquake and tsunami effects, Humboldt Bay area. Pink shading shows areas of strongest ground shaking. Blue line is the expected extent of tsunami flooding. Figure from CDMG.
Map showing the extent of Crescent City flooding (shaded green) from the 1964 Alaska earthquake tsunami. The postulated extent of flooding from the scenario tsunami is shown by the blue line. Figure from W. H. Griffin and CDMG.
The earthquake will cause the surface of the sea floor to deform, producing a series of waves which will begin arriving within minutes of the earthquake. Expected wave heights were modeled for the Humboldt Bay and Crescent City areas only. The scenario tsunami is expected to inundate much of the Samoa Peninsula and, to a lesser extent, the community of King Salmon which faces the opening of Humboldt Bay. The peninsula takes the brunt of the wave, minimizing flooding in Eureka. The highest dunes along the Samoa Peninsula may offer refuge from flooding. Tsunami destruction at Crescent City exceeds that from the 1964 tsunami generated by the great Alaskan earthquake. The period of potentially dangerous wave activity is expected to last many hours.
An earthquake like the scenario earthquake has not occurred during the approximately 150 years of written history on the North Coast. There is, however, a growing body of scientific literature that shows earthquakes of this magnitude or larger occurred as recently as 300 years ago and recur on intervals of several hundreds of years. The 1992 Cape Mendocino earthquake near Petrolia demonstrated that the Cascadia subduction zone is both capable of producing large earthquakes and generating tsunamis. Although the planning area is lightly populated, the very large area affected by the earthquake and the potential for a destructive tsunami justified the preparation of the planning scenario. The scenario earthquake may not be the largest event which could occur along the megathrust. However, it is likely to produce within Humboldt and Del Norte Counties about as much damage as a rupture of the entire subduction zone.
No. The planning scenario discusses the likely effects of such an earthquake; it does not predict when the earthquake will happen or discuss the likelihood of such an event. However, the earthquake is considered sufficiently credible to justify planning in advance.
The planning scenario is a public document and is available in many local libraries and public agencies. Copies can be purchased from the California Division of Mines and Geology. If you have questions about the scenario or its expected effects in your area, contact your local Office of Emergency Services.
GROUND FAILURE due to lateral spreading during liquefaction, Kobe earthquake, January 1995. Photo by Carol Prentice.
When loosely packed, wet sand is shaken during an earthquake, it may flow like a liquid. Anyone who has walked along the beach has probably seen a small-scale version of this process. Stamp your foot in the sand near the water's edge and suddenly the area of your foot print vibrates like shaky gelatin.
When a soil liquefies, it is unable to support the weight of any structure above it. Bridges and buildings may lose their footings even though they may have been designed to withstand strong ground shaking. If the liquefaction area is on a slope, massive landslides may result.
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