When It Comes to Earthquakes, the North Coast’s Plate Is Full

California’s two North Coast counties, Humboldt and Del Norte, face seismic and tsunami hazards as great as any in California, and a series of temblors over the last month is just the latest chapter of their turbulent history.

The reason for the heavy seismic activity is the North Coast’s close proximity to the convergence of three tectonic plates. Three moderate offshore earthquakes shook the region in recent weeks: a magnitude 5.1 on Aug. 15, a 4.4 on Aug. 25 and a 4.3 on Wednesday of this week. Earthquakes in 1932 and 1954 caused single fatalities, and a series of quakes ranging in magnitude up to 7.1 injured 356 people in 1992.

Humboldt State geologist Lori Dengler said Thursday that a quake strong enough to knock items off shelves affects the two-county area every two to three years.

But the most devastating natural event in modern times was a series of tidal waves that swept into Crescent City just hours after the great Alaskan quake of March 27, 1964, killing 10 people in the Crescent City port area and another person further south in the town of Klamath.

Tsunamis are caused by sudden displacements of water and can be the result of earthquakes, undersea volcanic eruptions or landslides. The waves speed across the sea at speeds as high as 500 mph.

The deaths of unwary people in Crescent City who went down to the docks to view damage from the first waves, only to be killed later by the 12-foot-high fourth wave, has inspired a tsunami warning system and educational campaign.

Last summer, the National Oceanic and Atmospheric Administration designated Crescent City “the first tsunami-ready community in the state of California.”

Allen Winograbov, emergency services coordinator for Del Norte County, said Thursday: “We’ve put out evacuation signs and signs indicating tsunami danger zones. We’ve also been able to procure a second tsunami siren and are getting information out to schools and hotel rooms, telling people that the waves frequently arrive 45 minutes or an hour apart.”

In Eureka, John Lovegrove, a National Weather Service warning coordinator, said plans have been made to alert radio and television stations to broadcast a tsunami alert within minutes of a warning. “They will tell everyone to get back from the water and stay back until an all-clear is given,” he said.

Tsunamis, of course, are not particularly frequent. But Dengler said that 21 had been recorded on the North Coast since 1855. Only four of these were generated by nearby offshore quakes. The rest came from geologic events as far away as Chile. A tsunami from the great Chilean earthquake of 1960 flooded the Crescent City harbor, although it did not push inland as far as the 1964 Alaskan tsunami.

Depending on the terrain and the direction from which the tsunami comes, certain areas can be hit while others can be left untouched, although the 1964 tsunami that hit Crescent City also caused some damage when it sent a tidal surge into Los Angeles County harbors.

The earthquakes that shake Humboldt and Del Norte counties are mostly related to the Cascadia Subduction Zone, which in the prehistoric past has generated quakes as strong as magnitude 9 off the coasts of Washington, Oregon and Northern California.

Dengler said that core samples taken near Crescent City showed a five-inch deposit of sand presumed to have been generated by a tsunami caused by such a mammoth quake in 1700. That quake also caused deadly tsunamis to strike Japan.

In a subduction zone, one tectonic plate wedges or “dives” under another over a vast amount of time. The process melts magma deep underground and sends it rising to the surface, perhaps 100 miles inland, finally causing volcanic eruptions.

Off the North Coast, the Gorda, the Pacific and the North America plates intersect, in what is known as the “triple junction.” Studies show the Gorda plate is wedging under the North American, while, further south, the San Andreas fault forms a boundary between the Pacific and North American plates.

The subduction process between the Gorda and North American plates is punctuated by numerous earthquakes and also ultimately causes such eruptions as have taken place at Mt. Shasta in 1786 and Lassen Peak in 1914-17. Both peaks are well east of either Humboldt or Del Norte counties, however.

A paper updated last spring by Dengler on the earthquake and tsunami hazards in the two counties said that both soil amplification and liquefaction have compounded the damage from “triple junction” earthquakes off the coast.

Amplification is the process by which faults and other formations under the surface can intensify shaking generated by a quake occurring on another fault.

Liquefaction occurs when water-saturated soil, often in a flood plain, shakes like jelly in an earthquake.

Dengler said such processes have contributed to significant damage to bridges in the Humboldt Bay area off Eureka in modern times.

An investigation in the 1990s of the Samoa bridge linking Eureka to a nearby community across Humboldt Bay showed that the bridge would be unlikely to stand up to the shaking of even a moderate nearby quake.

Dengler said that the 1932 and 1954 quakes, both less than 20 miles from Humboldt Bay, measured between magnitude 6 and 6.5, and were closer to land than many of the triple junction earthquakes.

Further quakes in the same vicinity, she suggested, could reach magnitude 7, causing heavy damage and casualties.