23 August 2011

My first earthquake

Yes, I was one of the many who felt the 5.8 to 5.9 earthquake centered in Virginia.  The final scoring and location will come from the US Geological Survey -- http://www.usgs.gov/  This is the first time I've noticed an earthquake in my life.  Growing up around Chicago and then living around here doesn't give you a lot of exposure.  The strongest here that I've been present for was 4.3 to 4.4, as I recall it.  I didn't notice it at all, not even in the sense of 'oh, so that's what was happening'.

This time, it was noticeable.  The initial stage was some fairly minor, fairly high frequency vibration -- similar to having some heavy trucks drive past, or some kinds of construction drilling.  Since there is construction going on in my area, I figured this was it.  For the first few seconds.  Then there was a rapid increase in the size of the vibrations, and a decrease in their frequency.  Rather than having some vibrations passing through the building, we moved to having the building itself swaying back and forth.  Earthquake!

By my count, I felt the quake for about 30 seconds.  (If that's too long compared to the USGS figures, blame my inner ears.)  Call it 20-30.  One friend estimated the swaying as being 4 cycles per second in the real quake phase.  My guess is closer to 2.  I'll have to see what the USGS has.  There are probably confounders from the engineering of my building.  So far, all seems ok at work and home.  Some of the more precariously perched items fell off book cases at home, but that's about it.  Fingers crossed that this is true for everybody.

A question popular in our parking lot at work (we'd been evacuated post-quake) was to wonder what fault line caused the quake.  My early answer, which was supported by a snippet on the radio (i.e., don't place a lot of confidence here) is that there was no fault line involved.  This is simply a much stronger version of the usual earthquake for this region -- adjusting to the fact that the Laurentide ice sheet is gone.  In areas that the ice sheet occupied -- down to southern Illinois or in to Pennsylvania -- the land sank under the weight of the ice sheet.  This squeezed some of the more fluid parts of the mantle out from there, and over to the areas in front (south) of the ice sheet.  That elevated the areas around, say, Virginia.  Once the ice sheet was removed, the fluid started oozing back to its original location.  As it does so, the crust creaks its way back in to position.  Creaking = earthquake.

9 comments:

Mitchell Coffey said...

In California I've slept through worse.

muoncounter said...

Glacial readjustment? I'm ok with that in Maine, but I'd check into that possibility more deeply for an earthquake as far south as central Va.

The shallow thrusts in the central Va seismic zone do indeed 'creak' every once in a while. With a significant recent flooding event, this might (speculating here) just turn out to be an example of hydroseismicity.

Robert Grumbine said...

Braggart! :-)

muon:
The thing about the VA earthquakes is not that they can be from having been under an ice sheet -- they weren't -- but that they (at least sometimes) are from the area being in front of the ice sheet. You may be entirely correct, of course. It's good to have more ideas on the table.

One datum to keep in mind, however, is that the quake seems to have had a shallow focus -- only 6 km (3.6 miles). That says something, though I'm not entirely sure what, about the mechanisms involved.

yea-mon said...

Good to hear you're ok.

Living in Tohoku I've become sadly used to earthquakes now...

muoncounter said...

Prior quakes (magnitude ~4) in the area (south of Mineral, along the James River) were also shallow. It's an area of low angle thrust faults that sole out at 5km or so below surface. The question is why was this one so much bigger than prior earthquakes? The USGS mentions the 1875 quake, also along the James River.

Groundwater's lubricating effect on fault zones is well-documented (see Rocky Mountain arsenal). I don't know enough about the recent rain/snowmelt runoff in the area (other than the May rainstorm) to speculate whether enough there was enough of a change in groundwater levels to put this hypothesis in play.

Aargh. My old days in earthquake seismology are coming back to haunt me.

R said...

@ Mitchell Coffey

I can see where the West Coast is coming from in taking good humor out of the East Coast's panic about our "minor" earthquake, but consider this. Tens of millions of people, for the first time, felt their first earthquake. I would equate that to a West Coast or Southern person seeing snow for the very first time. It would be a huge deal for them to witness snow as would it be for a first timer, like myself, witnessing an earthquake.

Robert Grumbine said...

muon:
Looks like you've got the right of it. The USGS write up on the quake is http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/se082311a.html and they definitely favor fault lines.

Thanks for the links. Some good things there. I did know about the Rocky Mountain Arsenal earthquakes (readers who don't know about it, take a look at http://en.wikipedia.org/wiki/Rocky_Mountain_Arsenal). But had always thought of the MD/VA area's quakes as being postglacial -- some certainly are, and being a glaciologist, I'm biased.

yea-mon:
One of the thoughts I had during and shortly after the quake was about how much worse the major quakes are. This was 5.8. A 7.3 is 30 times as severe; rather than knocking off a few unstable items off the bookcases, it would have knocked over the bookcases. An 8.8, not even as powerful as the one that took out the Fukushima reactors earlier this year, is another 30 times more powerful than that. Given the lack of quakes around here, that'd be throwing the houses over, not just interior bookcases.

For that reason, I'm quite happy to leave Mitchell's earthquake experience unchallenged. Or maybe he's just a very sound sleeper.

@R
I'll mention snow and cold to Mitchell if he pushes the quake expertise too much.

Hank Roberts said...

Thoughts from experience both with little quakes in California often, and old building construction generally:

Those little building-swaying motions you noticed may only have been an eighth or a quarter of an inch -- but if you applied the same motion between parts with a crowbar you'd see things separate.

Remember the building motions happened across everything.

I always go 'round and look for little stuff like cracks in paint/plaster, separation of brickwork from flashings around roof penetrations at pipes and chimneys, and, oh, yes, check any place you have a combustion flue for gaps.

It's worth considering putting in carbon monoxide and gas detectors alongside the smoke detectors.

Aging wood shrinks, loosening the nails; watch for places that nails have pulled a bit out of place. Hangers for pipes and heating ducts for example.

It's often better to replace nails with screws rather than just hammer them back in if something is pulling out.

And when walking under cornices remember they're all likely just a bit looser now than they were before, and apt to have a bit more exposure to rainwater.

Rainwater leads to rusty fasteners and to wintertime freezing in cracks, loosening things further later on.

Little stuff that likely never shows up in anything statistically.

Robert Grumbine said...

@hank:
Some good pointers. Thank you.

On the other hand, with the 10 inches (25 cm) rain we're expecting to accumulate in the next day or two, I'm pretty sure we'll find out every loosened area in the house without any great effort on our part :-(