The Significance of Recent Earthquakes in Northern California and Southern Oregon

The past five years have seen the highest level of regional earthquake activity this century. Nine earthquakes of magnitude 6 or larger have struck the coastal and offshore areas. Seven earthquakes were close enough to coastal communities to cause at least moderate damage. Damaging earthquake activity was not restricted to the coast. Klamath Falls, Ore., experienced its strongest earthquake in historic times. Recent earthquakes demonstrate both the high activity and the diversity of seismic source areas.

• The Eureka Earthquake: December 26, 1994, 6:10 a.m. PST, magnitude 5.4. The largest earthquake to occur within a 15-mile radius of Eureka since 1932. The earthquake caused an estimated $5 million in property losses and illustrated how a moderate-sized earthquake near a city is capable of causing more damage than a larger magnitude earthquakes in remote or offshore areas.

• The Mendocino Fault Earthquake: September 1, 1994, 8:15 a.m. PDT, magnitude 6.9. The largest earthquake ever recorded along the Mendocino fault. The earthquake was felt from south of San Francisco to Roseburg, Oregon but caused little damage due to its far offshore location. The widely felt magnitude 6.6 earthquake from the same region on February 18, 1995, is believed to have been triggered by the Mendocino fault earthquake.

• The Klamath Falls Earthquakes: September 21, 1993, 8:28 p.m. PDT, magnitude 5.9; 10:45 p.m. PDT, magnitude 5.9. The most damaging far-inland earthquakes of the century in the California-Oregon border region. These earthquakes occurred along faults which are part of the northernmost Basin and Range geologic province. Significant damage occurred in older unreinforced brick buildings in the Klamath Falls area. Rockfalls caused the death of a motorist. This sequence illustrates that inland communities, although not as frequently hit as coastal regions, are also vulnerable to strong earthquakes.

• The Cape Mendocino Earthquake Sequence (also called the Petrolia, Ferndale or Lost Coast earthquake): April 25, 1992, 11:06 a.m. PDT, magnitude 7.1; April 26, 12:41 a.m. PDT, magnitude 6.6; 4:18 a.m. PDT, magnitude 6.7. The first large historic earthquake generally thought located along the Cascadia subduction zone megathrust, and the largest historic onshore earthquake of the century. The April 25 earthquake produced some of the strongest ground shaking ever recorded. It uplifted a 12-mile stretch of coastline near Cape Mendocino by one to four feet, killing intertidal communities of barnacles, sea urchins and algae. The motion of the sea floor produced a tsunami that reached coastal communities within minutes and reached a maximum height 1 1/2 feet at Crescent City. The mainshock and large aftershocks caused on the order of $60 million in damages and resulted in a federal disaster declaration.

A TALE OF TWO EARTHQUAKES:

TOP - Ferndale after the 1906 San Francisco earthquake. Bricks from parapet wall were thrown forward into street at the General Mercantile building. Photo by Andrew Larson.

BOTTOM - History repeats itself as bricks from the same building (now the Valley Grocery) were once again downed by the April 1992 earthquake. It will not happen a third time; the building has since been demolished. Photo by Kevin Bayless.

Do You Wish to Return to Beginning?