Many places in California are linked to a history of powerful earthquakes — San Francisco, struck in 1906 and again in 1989 with the rest of the Bay Area and Santa Cruz. Northridge near Los Angeles in 1994. The Monterey County town of Parkfield, dubbed the Earthquake Capital of the World.

But the Humboldt County coast where a magnitude 6.4 earthquake struck offshore early Tuesday might be among the state’s most regular if unheralded quake customers. It’s been home to more than three dozen magnitude 6 or 7 quakes in the last century. In fact, a year to the day before Tuesday’s earthquake, a magnitude 6.2 struck in the same area.

“It’s not an area where they’re strangers to earthquakes,” Mark Ghilarducci, director of the California Office of Emergency Services, said at a news conference Tuesday. “They’ve had pretty good sizeable earthquakes up in this area in the past, and we’re happy that this one wasn’t as large as it could have been and as we’ve seen in the past.”

Sarah Minson, a research geophysicist at the U.S. Geological Survey’s Earthquake Science Center at Moffett Field in Mountain View, said the seismic activity is due to a mashup of tectonic plates in the earth’s crust.

The main actors, the North American plate and the Pacific Plate, meet along the notorious San Andreas fault, which runs from Mexico east of Los Angeles and along the coast from San Francisco to Cape Mendocino, Minson said.

But from Cape Mendocino, the smaller Juan de Fuca plate is nestled offshore between the Pacific and North American plates, stretching northward along southwestern Canada. And the plate pileup, Minson said, produces a host of active fault lines.

“This is actually a really interesting area with really high seismicity,” Minson said. “There’s an awful lot going on in that area.”

One of the most powerful recent earthquakes in the area struck April 25, 1992, a magnitude 7.2, followed the next day by powerful 6.5 and 6.6 magnitude aftershocks. The strong shaking damaged buildings, roads and bridges and triggered landslides in many of the same small towns affected Tuesday — Ferndale, Fortuna, Petrolia, Rio Del and Scotia, the California Department of Conservation said.

On Nov. 8, 1980, a magnitude 7 quake struck northwest of Eureka, with strong shaking for half a minute. It was felt from San Francisco to Salem, Oregon, though only six people were injured, and damage was relatively minor, given the magnitude, the USGS reported.

The USGS noted moderate earthquakes, including a magnitude 5.2 south of Eureka on June 7, 1975, that produced more damage than the 1980 quake because its epicenter was onshore and closer to populated areas. A magnitude 6.5 was reported on Dec. 21, 1954.

“The area over the last century has had about 40 earthquakes that are magnitude 6 to 7, so it’s not unusual for us to have earthquakes this size in this region,” said Cynthia Pridmore, senior engineering geologist for earthquake hazard communication at the California Geological Survey, at Tuesday’s news conference.

She said strong aftershocks are a concern — by Tuesday afternoon there already had been about 80 aftershocks, the strongest magnitudes 4.6, 4 and 3.9 — and there’s a 13% chance of a magnitude 5 or greater striking in the near future.

“So people do need to be prepared especially if they’re in weakened structures,” Pridmore said.

Despite the high potential for earthquakes, Minson said that unlike the Bay Area, the fault lines and earthquake epicenters along the Humboldt County coast tend to be far out to sea, lessening their severity. Ghilarducci said Tuesday’s temblor was about eight miles from shore.

“Most of these earthquakes are happening offshore,” Minson said, “so the amount of shaking onshore is less.”

And the way the faults tend to move — side to side rather than up and down — lessens the potential for an earthquake related hazard, such as tsunamis or seismic sea waves that can produce devastating flooding.

“If you don’t move the water vertically, you don’t make a wave,” Minson said. “So it’s just not going to do anything that’s going to excite a tsunami, which is good. Because otherwise, this could be very hazardous.”

Does the region’s seismic activity signal more earthquake risk for the Bay Area? Minson said “the answer is slightly complicated.”

“Everything is part of the global tectonic structure of the plates,” Minson said. Shaking from one quake can potentially spur motion on other nearby faults, “but large earthquakes are less likely than smaller earthquakes.”


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