On Monday, my Physical Geology students and I experienced a magnitude-2.0 earthquake during our final exam. The hypocenter was 1 km south and 6km down.

I pulled out the geologic map of the Annandale quadrangle (1986) by Avery Drake and A. J. Froelich to check for faults in that area. Here's a scan of the map:

You can see our campus in the northwest, and I've highlighted the epicenter of the quake with the red and green concentric circle. Interstate 495, the Capitol Beltway runs north-south through the center of this area. To the south and east of NOVA's campus, you can see that there's a mappable thrust fault (the Red Fox Thrust) which dips to the northwest, presumably under the epicenter and under the campus. However, the map provides no information on the angle of dip of this thrust. Is it steep enough to get to 6km depth a mere ~3km north of its surface trace? (The map's cross section shows it dipping at about 52 degrees, but that's pretty speculative.) Or if the dip is shallow, is there a deeper (perhaps parallel) thrust underneath it? (There is none shown within the map area, though there is one to the north of campus that dips to the north -- making it unlikely to be the culprit.) Alternatively, was Monday's quake caused by a new fault? Perhaps a normal fault which cross-cuts these Paleozoic thrusts?

No new answers -- only more questions...

Labels: earthquakes, news, nova

A quick update here on yesterday's "final exam" earthquake:

The USGS has upgraded the magnitude to 2.0, which seems more in line with what my students and I felt. They have also indicated the depth of the hypocenter as being about 6 km.

Talking to other faculty over the course of the day yesterday, I learned it was felt strongly across campus. Hardly surprising, considering we were essentially on top of it. My colleague Nancy Chamberlain wrote me an e-mail describing her experience: "It was a bang, a shake and a rattle. I flew out of my office exclaiming an earthquake... I was told someone dropped something upstairs but it really was an earthquake! I grew up in California and 10 miles off the New Madrid fault in St. Louis... I know an earthquake when I feel one!"

The Survey has also posted a few additional maps. For instance, to establish context, consider this map of historical seismicity in our area:

Labels: earthquakes, nova

Three-quarters of the way through my Physical Geology final exam this afternoon, the room shook and a low rumble propagated through our building. The students, many of whom were in the area on 9/11, instantly looked up with a very concerned "Was that a bomb?" look on their faces. To assuage their fears, and because I've previously heard similar rumbles coming from our HVAC ducts, I told them that it was just the cooling system, and not to worry.

Turns out I was wrong: it was a small earthquake with an epicenter about 1 km from here (we're about 1 km west of Annandale proper, and the USGS suggests it was 2km southwest of Annandale; see map below).

Almost on the epicenter of our own little quake! For us east-coasters, that's a big deal!

WTOP reported on it (Thanks to Ron Schott for forwarding me this link).

Here's the USGS quake page on the event.

Here's the Maryland Geologic Survey's seismometry. (source of image above, which is in Eastern Standard Time, not Daylight)

Here's the Washington Post's (brief) treatment of it.

I've gotta say, Ron Schott's up on it -- within an hour of the seismic event, he had e-mailed Tuff Cookie and I to ask about it. Pretty prompt! Thanks, Ron.


Update: Here's the Google Map showing the epicenter (green arrow). I've circled the building where I was giving the exam in the green circle:

Also: Here's the intensity map as it looked after I logged on and completed the "Did You Feel It?" survey:

Labels: dc, earthquakes, nova

Breaking News: Series Of Concentric Circles Emanating From Glowing Red Dot

Labels: earthquakes, humor, tv

Several websites are dedicated to monitoring Earth's seismicity over time. They make an interesting comparison in terms of graphical representation, considering that they are all reporting on the same information. Check them out and decide which one you think does the best job.

Earthquake Watch is a homemade site using Google Maps to show quake locations and magnitude. It shows magnitude, but only quakes in the last day, without differentiating between quakes in the past hour and longer times. Because it's done using Google Maps, it can be centered wherever you want -- or wherever the earthquakes happen. This is in contrast to the next two options, which have a fixed map centered on the Pacific Basin.

The USGS uses a nice physiographic map as the base for their information display. A series of colored squares of different sizes show magnitude and time of the quake. This is an advantage over the first site: more information revealed in the same amount of space. Still, like the first site, the map is small -- roughly 40% of the total "real estate" available on the screen.

The IRIS Seismic Monitor is my favorite of the bunch. It has a large map (~70% of the screen) with blinking circles of different sizes and colors show information about when a quake happened: last hour, last day, last two weeks, and the past five years. It also offers an option for a huge map (larger than the whole screen). Additionally, it offers a global night shadow -- so you can see which portion of the planet is in daylight, and which half is sleeping through the night.

Labels: earthquakes, graphics