This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Tuesday, August 23, 2011

And, a magnitude 5.8 on the East Coast! Excitement in D.C.!!

National Cathedral in D.C. closed temporarily for damage inspection
Photo from CNN.com
Toppled statue
Courtesy of the National Cathedral

Thanks to the magnitude 5.8 earthquake in the D.C. area today, we now know that there is a town called Mineral, Virgina! It had a population of 424 in the 2000 census, and is probably going to have a lot more people there tonight! During its heyday in the early part of the 20th century, there were 15 gold mines located within two miles of the town, as well as a zinc and lead mine. Mineral is only 5 miles from the epicenter of today's earthquake. It is 38 miles northwest of Richmond. According to the USGS, the quake was at a very shallow depth, 6 km, 3.7 miles (other estimates go up to 7 miles).

There is a neat video of the seismic waves from the quake crossing the IRIS seismometer array here. Gives a good feel for how long this earthquake was (>20 seconds).

Washington Monument
National Park Service
Numerous buildings in D.C. were evacuated, the metro and trains are apparently running slowly as they check for track damage. From witnesses on the ground, there are reports of a lot of people with broken bones because they jumped out of windows during the quake. Traffic disruptions are severe. Buildings at the Smithsonian are closed; schools closed the next day. Washington Monument closed after cracks discovered toward the top. Many buildings erected in the mid- 1800's require inspection. One of the funniest stories that I heard on NPR came out of the Geology Department at William and Mary.  They were having a faculty meeting when the earthquake struck, and instead of taking cover under tables as advised during earthquakes, they just sat and started at each other, unable to believe that this was happening!  Californians, meanwhile have some very predictable comments, such as "a tiny 5-point something"!

Seismicity map of eastern North America showing epicenters of
quakes from 1928-1981. From Zoback and Zoback 1981.
The most earthquake prone areas of the U.S. are Charleston, South Carolina, eastern Massachusettes, the St. Lawrence River area, and the central Mississippi River Valley.  I remember sitting in my office in Bolton, Ontario (northeast of Toronto) when an earthquake struck in the northeastern U.S. Because of hard, cold bedrock, waves from earthquakes here tend to spread further than equivalent quakes on the west coast.

Earthquakes occur in the eastern U.S. because ancient plate boundaries are being reactivated by today's stresses. During the past half billion years, the crust underlying the east coast has been the site of two major geologic episodes. Between ~450-250 million years ago, Africa collided with North America to form Pangea, the supercontinent.  Then, beginning about 200 million years ago, the present day Atlantic ocean began to form as Pangea broke up. About 200 million years ago, further rifting led to the opening of the present-day Atlantic.  This episode is referred to as the Mesozoic rifting event. Mesozoic rifts are exposed in rocks in Connecticut and Massachusettes and around the New york City area, and are believed to be buried beneath sediments along the continental margins of the eastern U.S. This summary taken from here.

Here's an interview with Rowena Lohman of Cornell in which she speculates on the role that glacial rebound may have east coast earthquakes.

And, here's an interview about the animals in the Washington Zoo that addresses the question about whether or not animals anticipate these events.

For a technical overview, see Zoback and Zoback, "State of Stress and Intraplate Earthquakes in the United States," Science, 213p. 96, 1981. Updated in M.L. Zoback and M. Zoback, "Lithosphere Stress and Deformation, Treatise on Geophysics, 6, 353-273, Editor-in-Chief, Gerald Schubert, Academic press, in publication.

Added 8/27/11: Here's the USGS analysis of the earthquake and a good Wiki article.

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