Tuesday, March 11, 2008

Shake Up != Wake Up

The Inland Empire apparently had three magnitude 3+ earthquakes on Sunday night. I felt none of them, which I find disappointing, since it's been almost three months since the last one I noticed. I'm sure I would have noticed them if I had been awake, since I tend to notice even the 2.9s, and since I felt two 3.9s from the same area (near Devore) in May. I literally just missed these three, though - I went to sleep fifteen minutes before they happened, and was too exhausted to be woken up, even by the magnitude 4.0. I suppose that finishing the draft of my thesis on Sunday should have been satisfaction enough to make me feel less disappointed about missing these, but such is not the case.

I'm particularly disappointed, though, that my computer wasn't running to catch these. I've been alpha testing the Quake-Catcher Network software since the beginning of February, and I have not yet had a chance to see how it picks up a genuine earthquake or three (as opposed to the motions caused by cats named after faults rocketing around my apartment). Also, by having one less of the alpha test computers running at the time (not that I know how many were running), that's less data with which to test the collecting and analyzing part of the QCN software. I know one computer isn't going to screw it up all by itself, but since I'm trying to help test this thing, I know my computer will be more useful if it was turned on to pick up quakes, even if I'm not awake to feel them. I'm hesitant to leave the computer running 24/7 for the sake of my hard drive, but I may have to just not think about that.

Looking at this little sequence (4.0, 3.4, 3.2) and one from 23 May 2007 (3.9, 3.9), both centered in pretty much the same place near Devore, I have to wonder if there's a small asperity in the fault there. In both sequences, all of the events happened within a couple of minutes - if not seconds - of each other, without many aftershocks later. I admittedly don't know enough about fault mechanics yet to tell whether or not my thought here can hold any water, but it seems at least possible that in these sets of events, the rupture could start, be briefly paused by the little asperity, then continue after stress makes the asperity give way. Does this actually make sense? Now that there have been two little sets of events there, I plan on reading more about that aspect of fault mechanics once I'm done with my comprehensive exams.


Silver Fox said...

Hey there, I was wondering if you turned in a "did you feel it" report to the usgs site? They actually like reports from people who didn't feel it, like if they were asleep, because that can tell them about the quake (intensity). I think you can just follow your links, and the bottom of the page takes you to the usgs reporting site for each earthquake.

It's neat that you have a program to pickup earthquakes. I would guess you'd want to shut your computer down on occassion? Or at least restart it sometimes?

It looks like the quakes were on some offshoot of the San Andreas? I couldn't really tell from the maps I saw. Can't help much with your ideas. I guess you mean some little bend or rough spot in the fault plane that could hang up the motion?

Julian said...

I've filled out Did You Feel It reports for all ten quakes I have felt (and I've made some of the other music TAs fill them out for things like Loma Prieta), but I never thought to do one for the ones I haven't noticed, I guess because I figured they'd equate no response with not having felt it. Now that I think about it, they likely equate no response with not knowing about the site! I'll go back and fill the forms in for those three, as well as for a 3.6 I missed earlier last week.

I shut down my computer every night, in order to keep the hard drive from running itself down. I don't know if I should put more strain on the HD for a program like this, and I'm sure it'll pick up a daytime quake eventually, but considering how many night ones there were this month, it was enough to make me consider leaving it on for longer.

Devore is right at the point where the San Jacinto branches off of the San Andreas. Considering how close together the two faults are there, I'm not sure how one would tell which fault these quakes were on, considering epicenter locations are n't precise to the meter. Another thing I wonder is, instead of an asperity/rough spot on one fault, if these little sequences take place on both faults. That is, the first quake is on the San Andreas (or the San Jacinto), and the brief pause between it and the second has to do with the stress transfer onto the other fault.