Recent Earthquakes:

WHERE EARTHQUAKES OCCURRED IN THE PAST, THEY WILL HAPPEN AGAIN

The North Coast region and adjacent offshore area is the most active seismic region in the continental United States. The size, location, and frequency of past earthquakes give an indication of what to expect in the future. The instrumental record of earthquakes on the North Coast extends back only into the early 1900s. However, it is possible to learn about older earthquakes from written accounts in newspapers, church logs, and diaries. From these accounts, seismologists can reconstruct the pattern of ground shaking and estimate the location and magnitude of early earthquakes. Since 1853, more than sixty earthquakes have caused some damage to North Coast communities.

The majority of North Coast earthquakes have been centered offshore in the southeastern portion of the Gorda plate. These earthquakes recur frequently, causing some damage to North Coast communities, particularly in the Cape Mendocino area, almost every two years. Northern Humboldt County, Del Norte County and coastal communities of southern Oregon have only rarely been affected by earthquakes in historic times. However, the historic record only gives a short-term view of earthquakes. The 1993 Klamath Falls earthquakes demonstrated that even historically quiet areas are at risk.

EPICENTER MAP This map shows the location of the best-studied North Coast earthquakes. It includes all earthquakes since 1960 with magnitudes as large as 5.5 or that have produced damage. It also includes the larger earlier earthquakes since 1920.

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.

Significant Earthquakes

• 1906: Although centered near San Francisco, the rupture extended northward at least to Point Arena and perhaps as far as Shelter Cove. In Ferndale, not a chimney remained standing and brick buildings were badly damaged.

• 1922: Largest magnitude historic earthquake, but too far offshore to produce much damage. Felt from San Jose to Oregon and Nevada.

• 1923: Similar damage to 1992 earthquake. Knocked down all chimneys in Petrolia. Felt by three steamers offshore.

• 1932: Killed a woman in Eureka when a chimney from a neighboring building crashed through two floors.

• 1954: The only historic earthquake that may have been on one of the mapped faults northeast of Arcata. Killed a man when a lumber stack collapsed. Fishermen on the Mad River were knocked down and reported water splashing on banks and temporarily flowing upstream.

• 1980: Six persons seriously injured, five when a portion of Highway 101 overpass collapsed south of Eureka. Property losses were estimated at $1,750,000. Felt in the San Francisco Bay Area, northwestern Nevada and southern Oregon.

• 1991: Shaking strong enough near Honeydew to toss one foot diameter boulders into the air. Affected well water levels in southern Humboldt County.

• 1992: Three strong earthquakes in 19 hours. Over $60 million in property losses.

• 1993: Two strong earthquakes struck the Klamath Falls area. A motorist was killed by a rockslide. Damage to unreinforced masonry buildings and roadways.

• 1994: 90 miles off shore along the Mendocino fault. Did very little damage due to distance from population centers.

• 1994: Over $5 Million in damages in Humboldt County.

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