4.5 Earthquake Off Ferndale

UPDATE 3/31: HSU professor and chair of the geology dept., Lori Dengler, did some research on why there were contradictory numbers for the quake. Here’s what she said,

I’ve sorted out some of the problem but don’t have the solution yet. There is an error on the USGS maps site that you have been looking at. They list the early epicenter coordinates (about 50 miles offshore) but use the later coordinates for the distance from coastal communities. If you use 40.453 and -125.233, the earthquake is only 52 miles from Ferndale, not the 99 miles they post. The magnitude estimate they are using is Berkeley’s moment tensor based on the preliminary epicenter location. My hunch is that is where the problem is – if you use the closer location, you end up with a smaller magnitude. This event illustrates the problem of moderate-sized offshore events. They are hard to get a good location, and there is a lot of noise in the signal. You can see the seismogram posted at www.facebook.com/rctwg. Keep up the good work with your reporting. It’s hard to be accurate when the sources aren’t.

UPDATE 8:48 P.M.: 

HSU professor and chair of the geology dept. Lori Dengler, commented on this post on LoCO’s Facebook page, You have it backwards – the earthquake was first estimated at 4.4, then 4.7 and closer to the coast (about 50 miles off the coast), then finally revised to 5.1 92 miles west of Cape Mendocino. There could still be some adjustments but I’d be surprised if it changes very much.

LoCO replied, “Lori, I know you know what you are talking about but could you tell me what I did wrong. I’m seeing on this site the 4.5 number. http://earthquake.usgs.gov/earthquakes/map/I’ve seen the site revise to the more correct number. So I assumed the other numbers given by other sites were from earlier information and the 4.5 is the most recent. Obviously, I was wrong. Where did I misunderstand?

Dengler responded, “This is definitely odd as this is the official USGS map – but the other USGS sites ( and the California Integrated Seismic System are still showing it as a 5.1 (http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/quakes_big.php). I’ll contact my USGS colleagues and see if I can find out the discrepancy. Far offshore eqs can be problematic – but this inconsistency on USGS sites is unusual.


Original post below:

The USGS reported a 4.5 earthquake about 99 miles west of Ferndale.  The earthquake struck a little before 3 P.M.  The quake was initially reported as a 5.1 but has been revised downward.


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