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Temblor Gave N. Coast a Hazardous Lift : Wildlife: A 15-mile stretch of beach was pushed up about four feet in the April 25 earthquake, raising coastal rocks and stranding tide-pool animals.

August 03, 1992|MARIE GRAVELLE | SPECIAL TO THE TIMES

PETROLIA, Calif. — Rows of dead sea urchins, mussels and sea snails lying on the beach a few miles north of this tiny North Coast town are stark reminders of the powerful earthquakes that shook up this region in April.

Geologists, marine biologists and just plain curious people are flocking to the beach to see and study the effects of the first recorded coastal uplift in Northern California.

Scientists believe that a 15-mile stretch of beach along what is known as the Lost Coast, from Sea Lion Gulch north to Cape Mendocino, was pushed up as much as four feet in the magnitude 7.1 earthquake April 25 and the two large aftershocks that followed.

The last earthquake to create coastal uplift in the United States was in 1964 in Alaska, where the sea floor rose an estimated 25 feet. So the interest here has been intense.

"My first reaction was: 'Oh wow! Look at this wonderful experiment set up for me,' " Bob Rasmussen, a local marine biology professor, said of the Petrolia uplift. "You can't just go out there and jack up 15 miles of coastline for an experiment."

Rasmussen, from nearby Humboldt State University, has documented how the uplift raised the coastal rocks and stranded tide-pool animals and plants above the life-giving ocean. Like fish out of water, they began to die. Now their shriveled bodies and empty shells are washing up onshore.

The first quake hit at 11:06 a.m. April 25 and was centered just onshore near Petrolia, an isolated small town south of Eureka. The quake measured 7.1, as strong as the Loma Prieta quake that hit the Bay Area during the 1989 World Series.

Strong aftershocks, measuring 6.6 and 6.7, struck Petrolia the next day. Damage was widespread in the lumber and tourist towns south of Eureka, but it was the changes to the coastline that made the quakes notable.

Humboldt State geologist Lori Dengler said that in the main quake, a segment of a tectonic plate under Petrolia plunged beneath another plate, thrusting the surface land mass vertically about four feet.

She said studies are under way to find where the corresponding sinkage has occurred, probably inland near the towns of Rio Dell and Scotia. "You always have both," Dengler said. "If you have uplift, you have subsidence."

More than 1,100 homes and businesses were damaged, about 200 of them destroyed. Several small aftershocks continue to jiggle Petrolia each week.

"I'm not edgy now," said John McAbrey, 47, a Petrolia resident. "Most people are calming back down though there's still some people that won't sleep in their houses. Of course, some still don't have houses. Their houses got hurled."

A small tsunami, an earthquake-generated wave of about two feet, was recorded soon after the jolt. McAbrey, a general contractor, owns a cabin on the ocean near Petrolia. He was home during the first quake, but said he was "just holding on" rather than watching the beach. Others have said they saw the ocean water recede.

McAbrey said he has noticed a difference in the height of the rocks. Fishermen also report seeing taller rocks, and at low tide there may be as many as a dozen new visible rocks off the coast of Cape Mendocino, said Humboldt State professor and biologist Jeff Borgeld.

About midway along the 15-mile area of uplift is a group of rocks called Mussel Rock, named for the abundant mussel population in the area. It is now thought to be the site of the greatest uplift, though the beach has not been fully surveyed.

One of the first signs of the changed environment was the sight of mussels with their shells gaping.

"They waited for the water to come and it never did," Rasmussen said. Once a mussel gets too hot, it must open, and when that happens, the sea gulls set to work.

Other tide-pool animals, such as urchins, were entombed in their pools as the water dried, never to be replaced. Then came the smell and the flies.

"One of our first clues that something was going on were reports from the locals about a strong smell of decay," Rasmussen said.

Some marine life, such as sea slugs the size of small platters, have not "had sense enough to move," Rasmussen said. "They're still sitting out there and their food supply (seaweed) is gone.

"The question now is whether they will all starve to death or last until the seaweed comes back in."

Scientists estimate that it will take three to five years for all the tide-pool and intertidal organisms to return to their newly shifted locations. Already, bright green early stage seaweed is coming back, mixing with the dead white and black patches on the rocks.

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