The quake took place in a complicated region where the San Andreas fault system intersects with the Cascadia subduction zone. It is close to the Mendocino Triple Junction where the Pacific, North American, and Gorda Plates are in contact with each other. Moderate quakes are not at all unusual in the region, with quakes measuring between 6.4 and 7.2 in 1923, 1932, 1954, 1980, 1992, and 2010. The Cascadia subduction zone generated a quake of around magnitude 9 in 1700, although it was farther north in Washington and Oregon.
A tsunami warning was not issued, presumably because the ocean floor was not lifted enough to cause one. Tsunamis are most commonly produced during compressional earthquakes in subduction zones, events that cause hundreds or thousands of square miles of seafloor to suddenly rise or subside. A quake the size of this one was either on a predominately strike-slip fault, or just didn't disrupt enough of the seafloor a sufficient distance.
As I write this, there have been half a dozen aftershocks recorded, mostly around magnitude 3, with one magnitude 4.6 event. There were also a few magnitude 3 events that could be considered foreshocks. Latest information from the U.S. Geological Survey can be found here: http://earthquake.usgs.gov/earthquakes/eventpage/nc72182046#summary, although I like the SCEC earthquake maps at http://www.data.scec.org/recenteqs/Maps/125-41.html as well.
I'll post updates if I get more information. I can hardly wait to see if the new seismograph at school was working this weekend!
UPDATE: A good synopsis of the geological circumstances of the quake has been posted at http://seismo.berkeley.edu/blog/seismoblog.php/2014/03/10/the-strongest-california-earthquake-in
Thank you, Gerry Hayes for your update. When I first learned of the quake this morning, I wondered, Cascadia? I worked for years in earthquake info and emergency preparedness in Del Norte Public Health, so am very familiar with the Subduction Zone, on a very deep gut level. I'll be watching for more information.
ReplyDeleteI've been trying to determine the fault system on which this occurred - it certainly felt different than other quakes we've experienced from that area. No tsunami warning makes me think transform rather than subduction. Keep us posted, and thanks!
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