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Monday, June 17, 2024

Column: Shaking coasts

To many Californians, the magnitude 5.8 earthquake that shook the East Coast on Aug. 23 probably didn’t sound that impressive. According to the United States Geological Survey (USGS), California experiences an average of one earthquake above magnitude 5.0 every year, while Virginia experiences an earthquake of the same magnitude about every hundred years.

However, writing the East Coast earthquake off as minor can obscure some interesting facts that make it different than what is felt on the West Coast.

First of all, the earthquake on the East Coast shook people much further away from its epicenter than a similar earthquake on the west coast would.

According to the USGS, “a magnitude 5.5 eastern U. S. earthquake usually can be felt as far as 300 miles from where it occurred, and sometimes causes damage as far away as 25 miles.”

How could this be, since the East Coast lacks anything comparable to the San Andreas Fault in California?

“The eastern United States don’t have a plate boundary, but it still has many, many faults,” said Donald Turcotte, a geologist at the UC Davis W. M. Keck Center for Active Visualization in the Earth Sciences (KeckCAVES). “Because there is high stress in the plates, there are occasional earthquakes in the eastern United States.”

Faults, or breaks in the Earth’s crust, form when the tectonic plates that make up the crust are mechanically stressed and form cracks.

Sometimes, these faults mark the places where the major plates meet. This is the case for the San Andreas Fault, which marks the boundary between the Pacific Plate and the North American Plate.

“In California there are many other faults, but they’re normally associated with the plate boundary,” Turcotte said.

The geology in Virginia is somewhat different. Rather than the major plate boundary of the San Andreas Fault, Virginia has the less dramatic Central Virginia Seismic Zone. The area is no stranger to small and moderate magnitude earthquakes, but the tremblers can’t all be linked to a single causative fault. Instead, the earthquakes in this zone are caused by various small faults moving in different ways.

If earthquakes in this area are caused by such small faults, why are they felt so much further away than similar earthquakes along the San Andreas Fault? The answer is in the temperature of the rock.

“What happened on the eastern U. S. is that the rocks are colder so that the waves propagate further,” Turcotte said. “So the shaking from the earthquake of a certain size can go further on the eastern U. S. than on the western U. S.”

Since the molecules of rock on the East Coast are cold, they are clustered tighter than are the molecules of rock on the West Coast; this tight arrangement allows the energy from the earthquake to travel more easily between molecules and thus travel further.

Despite the rarity of such a large earthquake in this area, researchers on the East Coast have been working to design better structures to withstand the shaking. The Earthquake Engineering and Structures Laboratory (EESL) at George Washington University in Virginia has been using a 100 square foot shake table since June 2001 to simulate earthquakes.

“What we do is simulate various types of earthquakes that have been recorded around the world, and see what type of effect they would have on buildings, bridges and other civil infrastructure,” said Majid Manzari, director of EESL and UC Davis alumnus.

Two nuclear reactors in the North Anna Power Station, in the same county as the epicenter, were taken offline automatically in the wake of the earthquake. For them, the event was not entirely unexpected.

“The nuclear power plant we have in Virginia had actually been designed for an earthquake very close to this earthquake,” Manzari said.

People and businesses on the East Coast are unlikely to have to deal with another trembler of high magnitude anytime soon.

“Of course we can always be proven wrong, but we engineers deal with probabilities of events,” Manzari said. “Our best estimate currently is that the probability of another large event is not very high.”

AMY STEWART can be reached at science@theaggie.org.

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