Continued aftershocks from Eureka quake likely

By Jane Stebbins, Pilot staff writer March 12, 2014 08:23 am

Few people in Curry County felt the ground move with the 6.9 magnitude earthquake that struck 50 miles offshore of Eureka Sunday evening — or the 6.1 magnitude temblor that struck at 5 p.m. Monday in the same area — but the odds are good they’ll get another chance.

The “Probability Report” of the U.S. Geological Society indicates there is a 90 percent chance of a “strong and possibly damaging aftershock” of 5.0 magnitude or greater in the week following the first quake.

 

Earthquake officials warn there is also a 5 to 10 percent chance or one the size of the original temblor — or larger — could occur as well. And 10 to 20 small aftershocks in the 3- to 5 magnitude range can be expected throughout the week; more than a dozen of those were reported Sunday night and Monday.

As a rule of thumb, a magnitude 6 mainshock might have aftershocks up to 10 to 20 miles away, while a magnitude 7 mainshock may have aftershocks as far as 30 to 50 miles away.

“I was alarmed enough to get up and stand in the doorway of my slider,” said Jane-Ann Phillips, who lives south of the Winchuck River.

“Chopper (her greyhound) raised his head and looked around to see what was happening; he looked at me to see if I was doing something to cause it. It felt like an amusement park ride … rolling. It felt like I was on a boat.”

The seaboard parallelling the Cascadia Fault — 30 miles offshore and stretching from northern California to Alaska — is overdue for a major quake, geologists now believe. The Juan de Fuca Plate, parallelling the coast about 30 miles offshore, is subducting under the North American Plate on which the continent sits. As it slides under the continental plate, it gets stuck in random locations. When it releases, or slips, energy is released in the form of an earthquake. If the earthquake is under the ocean, it can also trigger a tsunami.

Based on written records in Japan — and 300-year-old markers on the land indicating people should not build below that line — scientists were able to determine the Cascadia Fault had last ripped in January 1700, sending a tsunami to Japan.

Oral stories from Native Americans here tell of a “great shaking” at that time, backing the Japanese lore. Additionally, tree rings indicate a massive salt-water inundation killed trees and created “ghost forests” along the western coast of North America at the same time. 

Subsequent research assumes a major, devastating quake could strike here every 300 to 600 years.

The California coast crackled with aftershocks Monday.

That area is no stranger to earthquakes. In 1992, residents experienced a 7.2 magnitude quake that left 95 people injured and caused millions of dollars in damage. It was followed by a 6.5 magnitude temblor 12 hours later, and a 6.7 magnitude quake a few hours after that.

This one was felt as far north as Gold Beach and south to San Francisco.

“Scared the heck outta me,” someone from Crescent City wrote on a USGS site that solicits comments from people who feel quakes. “My whole house shook and it seemed to last a long time; it was rumbling then rolling then rumbling again. It also cracked a wall inside my house.”

Gaylene Henderson of Brookings said she thought the dog was shaking her bed.

“Then I realized stuff hanging from my ceiling was swinging and the dog was sound asleep,” she said.