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An uncommon, but not unexpected seismic event

Frank Pazzaglia and his students study the Appalachian Mountains, a region whose structural grain strikes from southwest to northeast, as did the Aug. 23 earthquake in Virginia.

The 5.9-magnitude earthquake on Aug. 23 that sent vibrations up and down the East Coast of the United States was especially interesting to Frank Pazzaglia, chair of the department of earth and environmental science.

Pazzaglia studies the geology and tectonics of eastern North America and also conducts research in the Rocky Mountains, in Italy and on the Mediterranean island of Crete.

Even before the earthquake hit, Pazzaglia and Anne Meltzer, professor of earth and environmental science, had proposed to densify a network of seismometers currently being deployed in the eastern U.S. to better understand earthquake distribution, frequency and hazards in the mid-Atlantic region.

Pazzaglia discussed the earthquake with The Express-Times of Easton, Pa., and with The Wall Street Journal, and answered the following questions for Lehigh’s website:

Q: Why are earthquakes more common in western than in eastern North America?

A: Earthquakes are a natural phenomenon; they happen all the time. The West Coast is an active plate boundary so earthquakes are very common there. The East Coast is no longer an active plate boundary; it’s rather quiet, so earthquakes are not as frequent.

Yesterday’s earthquake served as a reminder that the effects of geological processes on the East Coast, though commonly slow and small, do accumulate and are capable of generating earthquakes.

Q: Why was the earthquake felt more strongly in some areas than in others?

A: The structural grain of the Appalachian Mountains strikes southwest to northeast. The earthquake waves of the Aug. 23 rupture seemed to have preferentially followed that grain. The U.S. Geological Survey (USGS) published a map that shows that most of the reported shaking occurred in a northeast-southwest alignment; although it is also true that there is denser population, and more reporting, in the same alignment.

Q: In terms of size, how did this earthquake rank with previous seismic events in eastern North America?

A: Yesterday’s earthquake was big, but not the biggest. A representative, but not exhaustive list of other events includes the 1663, 7.0-magnitude earthquake in the Charlevoix region of Quebec. In 1775, a 5.9-magnitude earthquake hit Massachusetts. In 1886, a 7.3-magnitude earthquake caused a lot of damage in Charleston, South Carolina, and in 1929, a 7.2-magnitude earthquake struck the Grand Banks of Newfoundland.

Q: Is the East Coast as well-prepared as the West Coast for an earthquake?

A: In terms of building codes, understanding seismic hazards and risks, and emergency response training and preparedness—not as well as other places in the U.S., such as California. The people on the West Coast know what to do in the event of an earthquake simply because they experience them more regularly.

Q: What role did Lehigh’s seismometer (located on South Mountain near Sayre Drive) play in tracking yesterday’s event?

A: Our seismometer is part of a regional network maintained by the Lamont-Doherty Earth Observatory (LDEO). The LDEO network was used by the USGS to help pinpoint the earthquake’s location and size. Within three to four minutes of the quake, thanks to this network, the USGS had pinpointed its location and published this information to its website.

 

Story by Kurt Pfitzer

Posted on Wednesday, August 24, 2011

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