Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Saturday: 15 February 2014

Amateur Night Earthquake  -  @ 11:09:38

Eleven years ago, on March 18, 2003, at 1:04 AM, right here in my bed in Oglethorpe County, I was awakened by a little earthquake, the first I'd ever felt. There was a vibration that, for about five seconds, rumbled through the bed and rattled the windows with a distinct hum. That was a little more than a year before I started this blog, so it never appeared here except in the form of comments. I went to the computer to explore reporting possibilities, and discovered earthquake.usgs.gov. There I discovered that the earthquake was centered about 30 miles southeast of us, 3 miles deep, and (at the time) about a magnitude 4.4. Since then it's been downgraded to a 3.5. I was also fascinated in the listing of reports from other locations.

Last night, at 10:23 PM, I felt a rumbling vibration under the chair as I sat at the computer downstairs. I took off the headphones and called up to Glenn, "Did you feel that?" Indeed he had! Windows rattled for about five seconds, and rippled the surface of a glass of wine. We brought up USGS and entered in the information: yes, we were awake; yes, we both felt it; here's what the shaking was like; we were *quite* excited; no, there was no damage; and we did not move to a doorway, drop and cover, or run outside. We ran to the USGS website and had entered our info before the website had produced a preliminary presentation page.

As did 13,600 other respondants in 930 zip codes and 56 cities.



In the above figure, we're just above the little "a" in Athens-Clarke. Reports came in from Georgia, South Carolina, North Carolina, with a few from Alabama, Tennessee, and even Virginia.

Initial reports were of a 4.4 magnitude earthquake centered near Edgefield, South Carolina, somewhere around 60 miles east of us, and 3 miles deep. Not very exciting to those on the active margins of the West US coast, of course, but as a once in a decade event, pretty neat to us.

I took a look at the USGS response page. There were 301 responses from Athens-Clarke County, roughly 0.16% of the population. There were 24 responses from Oglethorpe County, including us, and amounting to somewhere around 0.17%. Augusta, GA, about 30 miles from the center, could only boast 342 contributers, again 0.17%. On the other hand Edgefield, SC, at the epicenter itself, managed 324 observations out of a population of 4750, or 6.8%, 40 times more than Athens, Oglethorpe County, or Augusta. Oddly, given its history, Charleston, SC, could muster only 44 reports out of 125,000 Charlestonians. These locations, including Edgefield's, averaged a 3 on the response scale you see above, generally a weak to light earthquake with no damage.

How then to explain the 40-fold difference in responses from the immediate zone compared to most of those 60 miles away? The intensity wasn't all that different. We don't get a summary of the length of time of the event, so I can't compare our 5 second duration with that of others closer. And we can't see when those responses were entered at USGS, but many of them were made hours after the event.

There were a few tweets from folks around Athens almost immediately, and of course I texted a couple of nearby acquaintances to ask if they'd felt it. Comments on a blog that features a few north Georgia readers began appearing, and apparently FaceBook featured quite a few posts almost immediately. Too Much Information appeared via the inevitable Valentine's Day Quake jokes. There must be some number of contributors who only learned of the USGS page later.

That repeated percentage of reporting from populations 30-100 miles away, amounting to about 0.16%, is kind of interesting too. It's just the number who reported, not the number who felt it. That number must be considerably higher, though I'm sure a lot of people didn't notice it at all. Maybe a lot of those who did notice it just didn't care all that much. But even so, *just* 1 in 600? When use of the internet in the US is now around 80%?

Glenn thinks, in an optimistic way, that the report rate of 1 in 600 is high. Certainly that 13600+ absolute number is great, and provides enough observational data to make that detailed figure above. And it's true that a lot, maybe even most of my acquaintances use the internet in only a minimal sort of way. They're not particularly adept at searching, nor do they show much interest outside of professional or social network use. Maybe Glenn's right.

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