The original posting of the earthquake on the 15th off the coast of Oregon was for two earthquakes. It was then made one quake. See the details here
Despite the fact that the tensor solutions both seem to suggest a lower magnitude quake (5.8 or 5.9) it is still listed as Magnitude 6.0
PNSN have now relisted it as two earthquakes:
The image links to the PNSN site but obviously after a few days those earthquakes will not show.
I decided to take a look myself to see if I could confirm this analysis. Those who know me are aware that I spend a great deal of time listening to earthquakes. Yes listening. My software (under development but free to a good home converts SAC files to WAV files amongst other things so I can then hear the earthquakes and see the spectrum using Audacity (also free to a good home)
I picked the seismographic station UW.JEDS at random and downloaded the data for the earthquake.
The waveform (this is the BHE channel) is heavy at the front end and a little more modulated possibly than normal, but I would not have immediately jumped up and said “double quake” looking at that.
For the technorati who spot it these are at 2000 samples per second against the station rate of 40 sanples per second thus 50 x
Next I looked at the spectrum without doing any processing on the file.
This one does seem to show something that looks like a double quake. I low pass filtered for 6 Hz (300Hz at the multiplication I was using) and the amplified the signal as much as possible. Now the ‘doubleness’ was clearly visible.
So finally I listened to it. I have to direct you to my 4shared facility I am afraid as I don’t have extensions here that enable it. UW.JEDS.Oregon-20120215.WAV
If you listen straight you hear two thumps. There is a slight echo feeling which may be because of the two quakes (echo being a delay to a signal added to the original). Now a close quake like this will pretty much always have a double thump, but not as long a gap as this. The second thumps is usually the arrival of the S-wave after the P-wave and there is very little gap on a close quake so it comes across as a quick double thump. If you turn up the treble you can hear the S-wave sound immediately after the first thump.
Double trouble? Yes I concur with the PNSN analysis that this was indeed two earthquakes one second apart.