coasts Sunday night, causing frayed nerves, little damage, and a
renewed respect for a shifting, postage-stamp patch of offshore crust
known as the Gorda plate.
The magnitude 6.8 temblor struck at 10:18 p.m. Pacific Daylight Time
some 52 miles west of Eureka, Calif., just north of Cape Mendocino.
The offshore region, known as the Mendocino Triple Junction, is a
hotbed of earthquake activity thanks to the motions of the three
tectonic plates that meet there.
The temblor and at least 13 aftershocks were triggered by a rupture
along a fault on the Gorda plate, which is slowly sliding underneath
the northern California and southern Oregon coasts — a process that
over the last 600,000 years built California’s Mt. Shasta and Mt.
More than 4,000 people reported feeling the quake, according to shake
reports submitted to the U.S. Geological Survey. The most intense
shaking occurred around Cape Mendocino’s immediate area. But reports
also came from as far north as Eugene, Ore., as far south as the
northern end of San Francisco Bay, and as far east as Lake Tahoe.
As the seismic waves traveled from the epicenter, they triggered a
swarm of tiny quakes underneath a geothermal power plant 70 miles
north of San Francisco, according to David Oppenheimer, with the U.S.
Geological Survey office in Menlo Park., Calif.
The Mendocino Triple Junction has a long history of triggering strong
to major earthquakes. Since 1900, five quakes with a magnitude of 7.0
to 7.3 have occurred within about 60 miles of the epicenter of
Sunday’s quake. In addition, in 1992, the Gorda plate’s movement
beneath the North American plate triggered a magnitude 7.2 temblor
beneath Petrolia, on Cape Mendocino, as well as quakes father inland.
Indeed, the Gorda plate — often considered the southern section of the
Juan de Fuca plate, which is sliding beneath the Oregon and Washington
coasts — is one stressed piece of underwater real estate.
Since 1976, the plate has been the site of 20 earthquakes with
magnitudes of 5.9 or higher, the most active site for large
earthquakes in the contiguous United States, according to researchers
John Rollins of the University of Southern California and USGS
colleague Ross Stein.
As the Pacific Plate creeps north toward Alaska along the San Andreas
Fault, it collides with the Gorda plate. There, the San Andreas
abruptly ends. The collision, which has fractured the sea floor for
more than 1,400 miles west of Cape Mendocino, puts the squeeze on the
Gorda plate. The plate’s eastward movement as it dives beneath the
North American plate adds additional stress.
As if to add insult to injury, the Gorda plate is sliding beneath a
North American plate headed southwest.
The combined stresses build on faults that lace the Gorda plate and
find release in quakes like the one that occurred Sunday.
The nature of the triple junction and the epicenter’s distance from
the northern end of the San Andreas suggest that Sunday’s quake is
likely to have had little or no effect on the northern end of the
fault, Mr. Oppenheimer notes.
Indeed, stress on the northern end “might even have been decreased,” he said.