Magnitude 6.5 Earthquake off Northern California (as recorded at MJC)

A large earthquake has shaken the ocean floor off the coast of Northern California. It took place about 100 miles west of Cape Mendocino (below) and the town of Ferndale.  The 6.5 magnitude event took place on the oceanward extension of the San Andreas fault roughly halfway between the Gorda Ridge and the south end of the Cascadia Subduction Zone. The first motion diagram for the event is consistent with a right lateral strike-slip fault. The lateral motion of the shaking minimized the chances of producing a tsunami, and none was reported. The quake was felt over a large part of the North Coast, but I expect there was little damage. A hundred miles is a good distance to be away from an epicenter.

Cape Mendocino, the closest land feature to the 6.5 magnitude quake on Thursday.

We got a good record of the event on our instructional seismometer at Modesto Junior College. I would have posted earlier, but found that although the event was recorded, the monitor had fizzled out and had to be replaced so I could see what I was doing to preserve the seismograph of the event.

Source: http://earthquake.usgs.gov/earthquakes/eventpage/us20007z6r#map

Earthquakes are not unusual in this area, and indeed there have been some destructive events over the years, including a 7.2 magnitude event in 1992 that caused some serious damage. It is a complex region, with a divergent plate boundary offshore of a subduction zone, and the transform boundary of the San Andreas fault.

All in all, the best kind of earthquake: big and noisy, but far enough away that no one gets hurt. For more details, check out the details at the USGS: http://earthquake.usgs.gov/earthquakes/eventpage/us20007z6r#executive

Magnitude 6.5 Quake Offshore of Northern California

A magnitude 6.5 quake has struck offshore of northern California, about 25 miles offshore of Ferndale and Eureka, at 4:27 PST. It was apparently felt widely across the northern part of the state. Seven aftershocks in the range of 30-4.5 magnitude have followed the main shock so far. This area is quite close to the Triple Junction where three plates, the Gorda, Pacific and North American come together. The fault motions were primarily horizontal so a tsunami is not considered likely (and it would have hit onshore by now anyway). Andrew Alden at About.com:Geology provides a nice perspective of the quake, the biggest in the lower 48 states in a couple of years.

Update, 1/10: 23 aftershocks thus far, including three that exceeded magnitude 4. Some minor damage reports, in the form of fallen chimneys and broken windows. A nice history of seismicity in the region by the Berkeley Seismological Laboratory can be seen here.