The geology of the North Coast is distinct from the rest of California. Geologists and seismologists find this region of special interest because of the Mendocino triple junction, a place where three plates, the Gorda, the North American and the Pacific, are in contact. This unique geologic setting makes the North Coast vulnerable to earthquakes from a variety of sources.
THE GEOLOGIC SETTING Three geologic plates intersect along the North Coast at the Mendocino triple junction. Black arrows show the relative motion at plate boundaries. The San Andreas fault is the boundary between the Pacific and North American plates; the Mendocino fault separates the Gorda and Pacific plates. The Cascadia subduction zone is the boundary between the Gorda and Juan de Fuca plates offshore and the North American plate. The white triangles show the locations of active volcanoes, common along subduction zones.
A vertical slice along the southern part of the Gorda plate where it is being subducted beneath the North American plate. The megathrust is the locked boundary between them.
South of the triple junction, the Pacific plate moves northwest relative to the North American plate. Most of this motion takes place along the San Andreas fault system. The great 1906 San Francisco earthquake ruptured more than 200 miles of the fault from south of Santa Cruz to at least Point Arena and perhaps as far north as Shelter Cove. Southern Humboldt County and the Humboldt Bay area experienced as much or more damage from this earthquake as any other historic tremor. The average interval between 1906 type earthquakes is on the order of several hundred years.
North of the triple junction, the Gorda plate and its northern extension, the Juan de Fuca plate, move eastward on a collision course with the North American plate. The Gorda plate slowly descends beneath the North American plate along the Cascadia subduction zone. Most scientists believe that the upper 50 or so miles of the contact between the Gorda and North American plates is locked. This boundary is called the megathrust.
The April 1992 Cape Mendocino earthquake broke a small part of the megathrust. Rupture of a longer segment would produce a much larger earthquake. How big? Geologic evidence suggests that these earthquakes could have magnitudes of 8 or greater, produce strong ground motion lasting up to a minute or more, and generate tsunamis.
The last great subduction zone earthquake occurred about 300 years ago. There are no written records of past Cascadia earthquakes but their effects are recorded in sediments, trees and the oral history of native peoples. Studies suggest that these earthquakes recur with intervals on the order of hundreds of years. Scientific evidence increasingly suggests the eventual recurrence of such earthquakes. The enormity of their possible effects make planning and preparedness efforts essential for individuals, communities, and all levels of government.
Strong earthquakes centered onshore have recurred about every 20 years and, when centered near populated areas, have caused major damage. These earthquakes historically have had magnitudes of up to 7.1. Geologic evidence, however, suggests this type of earthquake may be even larger. The collision between the North American and Gorda plates has created a zone of folded and faulted crustal rocks. Nine major thrust faults are exposed along the Humboldt County coastline. A strong earthquake along one of these faults could cause surface rupture and produce stronger ground shaking than any historic earthquake.
Most North Coast earthquakes in historic times have been centered offshore. Offshore earthquakes come from two sources: the Mendocino fault which forms the plate boundary between the Pacific and Gorda plates, and faults within the Gorda plate itself. These earthquakes have had magnitudes of up to 7.5 and cause some damage to coastal communities every few years. The 1980 magnitude 7.1 Trinidad earthquake, the 1994 magnitude 6.9 Mendocino fault earthquake and the 1994 magnitude 5.4 Eureka earthquake were all offshore earthquakes.
A recent study shows how a Cascadia subduction zone earthquake is likely to affect our area.
Earthquakes of magnitude 3.0 & higher January 1975 - July 1995. Compiled by the U.S Geological Society
If the earth were the size of an egg, its outermost layer would be about the thickness of an eggshell. This shell is broken into a number of pieces called plates. These plates are at least forty miles thick and vary in width from a few hundred to many thousands of miles. A plate may contain oceans, continents or both. The plates move slowly relative to each other at rates between one and four inches per year. Most earthquakes occur along boundaries where plates are driven apart, slide past one another, or collide.
A thrust fault slants gently like a ramp relative to the earth's surface. Earthquakes along a thrust fault push the rock above the ramp up and over the rock beneath it. The Northridge earthquake in 1994 occurred along a thrust fault. In subduction zones, the boundary between the plates resembles a giant thrust fault, extending hundreds of miles in length. The locked part of the subduction interface is known as the megathrust. All of the worlds greatest earthquakes (magnitude 8.5 and larger) are associated with ruptures of megathrusts.
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