Quake Rattles Eureka Region

A magnitude 5.1 earthquake centered under the Pacific Ocean 76 miles west of Eureka awakened residents along California’s North Coast early Friday, but was too far out at sea to cause any damage or injuries.

The U.S. Geological Survey said the temblor struck at 2:22 a.m. two miles under the ocean floor and was felt lightly from Crescent City to Cape Mendocino. One person told the survey that he felt it in San Francisco.

The quake occurred in a seismically active area where two tectonic plates intersect with the northern end of the San Andreas fault in what is called the “triple junction.”

The locale is at the southern end of the Cascadia subduction zone, where the Gorda tectonic plate is slowly diving underneath the continental North American plate. Quakes offshore are indirectly related to volcanic activity in the Cascade range.

The most recent major quakes in the vicinity occurred on April 25 and 26, 1992, when three quakes under the ocean off Eureka registered magnitudes of 7.1, 6.5 and 6.2, causing 356 injuries and $48 million in damage.

Scientists said they have uncovered evidence that about 300 years ago, a quake ranging in magnitude from 7.6 to 8.4 shook the same region, and that quakes as powerful as 9.5 are possible farther north, off the Oregon and Washington coasts.

Friday’s quake was only 1% as powerful as the strongest in 1992, and it was farther off the coast, so there was little danger.

The Geological Survey said there was a 10% chance of a strong magnitude 5.0 aftershock in the next seven days and a 5% to 10% chance of a larger quake than Friday’s in the same period.

It is common for quakes to occur in a series in the area.