Moderate Earthquake Rumbles Through Northern California

A moderate earthquake rumbled through northern California Tuesday night at 8:03pm local time. According to the US Geological Survey, the earthquake's epicenter ranged some eleven miles (18km) ESE of Willow Lake, California, about 307 miles northwest of Sacramento. San Francisco's CBS television affiliate website (channel five) quotes witnesses, saying that the quake was short-lived but powerful.

The epicenter fell within the borders of the Shasta-Trinity National Forest, some eleven miles below ground along an unmapped fault line.

There were no reports of damage, though the event was felt as far away as San Francisco, some 320 miles away. The epicenter fell in a fairly remote mountainous region of the state.

The same region was struck by a 7.2-magnitude quake on April 25, 1992. The 1992 temblor set off a tsunami, albeit a minor one.

Tuesday's quake in Northern California is considered moderate, registering 5.2. A quake of that magnitude can cause slight damage to well-built structures and take a moderate to heavy toll on less well-designed buildings. In comparison, the largest earthquake of a swarm that recently struck the Reno, Nevada area was a 4.7.

A little northwest of Tuesday's temblor, another swarm of earthquakes has been shaking things up off the Oregon coast.

This is the latest of several earthquakes, of magnitude 4 or above, striking along or near the western coastlines of North America over just a few days' time. A 5.8 earthquake struck the vicinity of Mexico City on Monday, with a near-twin 5.4 followup on Monday, centered a short distance to the north. A 5.1 magnitude quake rumbled offshore near Vancouver, BC, Canada, on Sunday, and a couple of 4-plus rated temblors rattled offshore Oregon over the past week, the most recent of those also taking place on Tuesday. (The USGS site reflects all quakes worldwide of 4.0 or above, and those of 2.0 and above in the US.)

Seismic experts find the trend more than a little disconcerting. Earlier in the week, warnings went out to say that the Reno, Nevada area might expect an event of 6.0 or greater, since quakes there were running contrary to the typical pattern. The series of earthquakes shaking northern Nevada were growing stronger over time, rather than the expected large quake followed by lesser aftershocks. An April 25, 2008 article in The Oregonian online warns that the "big one" along the Cascadia Subduction Zone could strike at virtually any time, the resulting quake "likely" a magnitude 9 or greater. (The quake that triggered the Indonesian tsunami in 2004 registered 9.2.)

Such an event, associated with a process known as Episodic Tremor and Slip, or ETS, is expected to generate heavy damage to adjacent cities and to produce catastrophic tsunamis that could sweep across the Pacific in all directions. ETS is a cyclical process in a line that ranged northward to Vancouver, BC. Scientists have identified a pattern, with intervals of approximately 14-15 months between episodes. ETS are not considered true earthquakes, though they are believed to indicate stress buildup that may lead to a catastrophic quake. Canada's earthquake-tracking site, www.earthquakescanada.nrcan.gc.ca, addresses concerns there about the earthquake risk to Vancouver Island and its surroundings.

Tuesday's quake near Eureka falls at what's believed the end of an ETS sequence.