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The Cape Mendocino Earthquakes of April 25-26, 1992 (20 Images)

This presentation is based on a 35mm slide set with the same title published by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Geophysical Data Center, Boulder, Colorado 80303, USA (Product No. 647-A11-016

STATISTICS

On April 25, 1992, at 11:06 A.M. a magnitude 7.1 main shock near Petrolia, California, initiated a series of shocks in the Cape Mendocino area. Two additional shocks of magnitudes 6.6 and 6.7 occurred the next morning. The first shock was located six miles north of Petrolia in a sparsely populated part of southwestern Humbolt County. The second and third earthquakes were located offshore, about sixteen miles west of the first earthquake. The three quakes were felt over an approximate area of 86,000 km2. The region of strongest earth shaking is bounded by the Eel and Mattole River valleys. Five small communities are located within a 50 mile radius of the three earthquakes: Honeydew, Petrolia, Rio Dell, Scotia, and Ferndale.

The main shock came immediately after a Saturday morning parade at Ferndale. Horses were reportedly spooked during the parade, but riders attributed it to the flags fluttering in the wind along the parade route.

Seismicity and Geology:

The Cape Mendocino region of California's north coast is one of the most seismically active regions in the conterminous United States. This seismic zone accounts for twenty-five percent of the seismic energy release in California in the last 50 years. It coincides with the Mendocino Triple Junction, a tectonically unstable area where three tectonic plates that make up the Earth's outer shell join together. Paleoseismicity studies of two major zones of faulting in the southern Cascadia subduction zone fold-and-thrust belt near Eureka have produced a record of at least five large earthquakes during the past 1,700 years.

The main shock on April 25 is exceptional for several reasons: It generated a tsunami, which was well recorded along the West Coast and measurable in the Hawaiian Islands. It also produced distinct uplift (maximum uplift = lm) along a 25-km-long section of the coastline. Ground shaking during the quake reached at least intensity VIII on the Modified Mercalli scale. The April 25 main shock is the largest historic earthquake in this region with an epicenter clearly located on land. It is also the only damaging event that may have resulted from thrust faulting along the Cascadia subduction zone.

Damage:

Collectively these shocks caused significant damage to older structures in the sparsely populated, mountainous, epicentral region. The earthquake caused 98 injuries and moderate damage in Humboldt County. Losses could top $66 million. Single- and double-story, wood-frame houses and commercial buildings were the most common type of structures in the epicentral region, where lumbering is the principal industry. Many foundations were the pier and post type which provide little resistance to lateral ground shaking. The earthquakes jarred many older homes with this type of underpinning off their foundations.

Earth Effects:

A broad zone of ground fractures, coastal uplift, and elevation changes are the only surface changes to indicate the location and tilt of the fault plane. Strong ground shaking during the earthquakes triggered numerous landslides in steep mountainous areas. A landslide at Scotia Bluffs (south of Scotia) temporarily interrupted traffic on the North Coast Railroad. Tension cracks, due to soil compaction and downhill slumping, restricted traffic on the Mattole Road between Honeydew, Petrolia, and Ferndale.

Liquefaction took place in scattered localities in modern deposits of the Mattole River between Petrolia and Honeydew and in the flood-plain deposits of the Eel and Salt Rivers. Soils liquefy when ground water near the surface is forced between the grains of sand during an earthquake. The sandy soil behaves like a very thick liquid. Structures then settle or tip in the liquefied soil or are ripped apart as the ground spreads laterally or flows. Sandblows result from the eruption of liquefied sediment from the subsurface onto the ground. The eruption of liquefied sediment is facilitated by fissures in the ground, which are also produced by strong shaking during an earthquake. Many geologists and seismologists believe that the main shock in this sequence may be a forerunner of a much more powerful earthquake in the Pacific Northwest.

RIO DELL (Population 2,900): Twelve homes were destroyed, and 57 sustained major damage. On Main Street fifteen out of twenty of the older one- and two-story buildings incurred damage including broken glass, fallen bricks, and cracked plaster. Buildings shifted as much as two feet from their pre-earthquake positions. Earthquake damage contaminated water supply systems, broke water mains and sewer pipes, and interrupted telephone, gas and electricity services. The total earthquake damage was estimated at $8 million.

SCOTIA (Population 1,000)

This 123 year old town lies south of Rio Dell across the Eel River. The two towns are joined by a bridge across the Eel River. Fifty homes were damaged and a shopping center consisting of five stores burned to the ground. As in Rio Dell, water mains and sewer pipes were broken and water supply systems contaminated. Telephone, gas, and electricity services were interrupted. Total damage was estimated at between $10 and $15 million.

HONEYDEW (daytime population 2,500, but only 2 Persons after 5 pm)

In this town, close to the epicenter, houses were shifted on foundations, cupboards were emptied, and brick chimneys fell. The single-story, wood-frame building where the post office and general store were located showed no signs of movement on its foundation

PETROLIA (Population 100)

Downtown Petrolia consisted of a post office, a 100-year-old general store, a gas station, and a volunteer fire station. The post office, gas station, and the general store burned to the ground in an earthquake-related fire. The door of the fire station was jammed by the first large shock. Several firemen were required to raise the door before the fire equipment could be removed. The fire station was later condemned. Buildings that had been recently constructed performed well during the earthquakes with little or no damage. However, damage was severe to poorly-built or poorly-designed structures. One- and two-story, wood-frame houses and commercial buildings were the most common type of construction. A total of five homes were destroyed, 28 sustained major damage, and 25 required minor repairs. Total damage was estimated at $1 million. As in other towns in Humboldt County, services including water, sewer, telephone, gas, and electricity were interrupted.

FERNDALE (Population 1,450)

This Victorian tourist town is listed in the National Registry of Historic Places. Thirty-six homes were seriously damaged. Foundations collapsed on at least a dozen homes; and there was sporadic fracturing of water lines. Forty businesses (80% of the total) were damaged in the four-block business district. Utilities were also interrupted in this town. Losses were estimated to be $10.4 million.