Two Earthquakes Rattle North Coast of California

by Heather Muller , 5/9/2007
The Eureka Reporter

“I thought I was on a boat in the ocean,” said Renee Jacobsen, manager of the Petrolia Store and former chief of the remote town’s volunteer fire department.

The magnitude 5.2 earthquake that struck 39 miles west of Petrolia shortly before 1 a.m. Wednesday “pushed you up like on top of a wave,” she said.

“And then there was the aftershock,” she said, a magnitude 4.5 temblor 17 minutes later and two miles closer to shore.

“That one was more like a sharp jolt.”

According to the U.S. Geological Survey, people reported feeling the initial earthquake as far north as Trinidad and as far south as Victorville — more than 900 miles away.

Brenda Godsey, public information officer for the Humboldt County Sheriff’s Office, said her office received only one earthquake-related inquiry and no reports of injury or damage.

The Office of Emergency Services was not activated, said OES Program Coordinator Dan Larkin.

In situations like this, Larkin said, “there’s nothing we do, because there’s nothing we can do.”

Had the incident been serious, he said, any of a range of responses would have been triggered, possibly including an emergency declaration and the activation of the Emergency Alert System, as well as various local, state and federal emergency response protocols.

Humboldt State University geology professor Lori Dengler said the earthquakes were “of interest and a low level of excitement,” adding that the two events occurred along the Mendocino Fault, which is the region’s single most active fault.

“Since 1980, we’ve had about 150 earthquakes of magnitude 4 or larger along the Mendocino Fault,” she said.

Dengler described the fault as “kind of our Mason-Dixon Line. It really divides the West Coast of the United States into two completely different tectonic areas.”

To the south is the San Andreas Fault with its side-to-side “transform” movement. To the north are the Gorda and the Juan de Fuca plates that converge along the coast and dive down to create the vertical movement characteristic of the Cascadia Subduction Zone.

The Mendocino Fault itself, where Wednesday’s earthquakes occurred, moves horizontally, Dengler said.

“So if you had been standing south of the Mendocino Fault, and if there were no water in the ocean and you could have actually seen what was going on, you would have seen the land north of the fault move maybe half a foot closer to the coast,” she said.

But it could have been worse.

“In the 1906 earthquake, had you been on the San Andreas Fault and looked across to the other side, you would have seen it move more than 20 feet in some places,” she said.

Dengler has long said the North Coast is overdue for a damaging earthquake.

“The point I really like to make is that this is the time to take action to reduce your hazards. It can be something as simple as that huge potted plant on a bookshelf above your bed. Maybe there’s a better place for it. Develop an emergency plan for your family. Make sure your hot water heater is secured to a structural element of your house.

“They’re small things, but they can make a big difference.”


On shaky ground: Significant North Coast earthquakes of the past 30 years

+ June 14, 2005 6:50 p.m.
Magnitude 7.2, centered 97 miles west of Trinidad
Felt lightly but resulted in tsunami warning for the entire West Coast
No reports of injury or damage

+ Feb. 18, 1995 8:03 p.m.
Estimated magnitude 6.5, centered 80 miles west-southwest of Eureka
No reports of injury or significant damage

+ Dec 26, 1994 6:10 a.m.
The Eureka Earthquake
Estimated magnitude 5.4, centered 12 miles west-southwest of Eureka
Minor injuries reported and damages estimated at more than $2.3 million

+ Sept. 1, 1994 8:15 a.m.
The Mendocino Fault Earthquake
Estimated magnitude 6.9, centered approximately 90 miles west of Cape Mendocino
No reports of injury or significant damage

+ April 25, 1992 11:06 a.m.
The Ferndale Earthquake
Magnitude 7.1, centered near Petrolia
Followed by aftershocks of magnitude 6.6 and 6.7 near Cape Mendocino
Damage estimated at $60 million, resulted in federal disaster declaration

+ Sept. 17, 1991 12:29 p.m.
The Honeydew Earthquake
Estimated magnitude 6.1, centered 13 miles south-southeast of Cape Mendocino
More than 60 percent of Honeydew residents reported structural damage

+ Nov. 8, 1980 2:27 a.m.
The Trinidad Earthquake
Magnitude 7.1, centered 30 miles west of Trinidad
Freeway overpass south of Eureka collapsed
Regional damages estimated at $1.75 million

Source: Humboldt State University Geology Department’s Humboldt Earthquake Education Center