August 31, 2011

West Coast or East Coast, earthquakes pack a punch

Vince Stricherz

Vince Stricherz

Ive lived in Seattle nearly 24 years and, as part of my duties in the Office of News and Information, Ive been covering UW earthquake research and the seismology laboratory for more than 13 years.

But the first earthquake I ever felt was in Indianapolis in 1987, months before moving to Seattle. I also just happened to be in “the other Washington” when the earthquake hit last week (Aug. 23), so I have experienced quakes on both sides of the country.

Neither of those quakes measured up to the Nisqually earthquake that hit Western Washington in 2001, but my lasting impression is that earthquakes in the East have a distinctly different flavor from what we are used to, and West Coast residents laughing up their sleeves at the reaction in the East last week might want to temper their disdain.

John Vidale, a UW professor of Earth and space sciences who heads the Pacific Northwest Seismic Network, gives credence to that idea.

In the East, the ground is older and denser and has fewer faults. As a result earthquakes tend to have a sharper edge.

“The ground soaks up less of the energy as the wave goes by,” Vidale said. “The ground is more solid so it can deform and spring back without losing energy.”

This National Park Service photograph shows a crack in the face of the Washington Monument in Washington, D.C., shortly after a magnitude 5.8 earthquake in Virginia on Aug. 23.

National Park Service

This National Park Service photograph shows a crack in the face of the Washington Monument in Washington, D.C., shortly after a magnitude 5.8 earthquake in Virginia on Aug. 23.

My first earthquake, in June 1987, was a magnitude 5.1 temblor in southeastern Illinois. It was felt in 17 states and one Canadian province and set off alarms at several nuclear power plants. Damage was minimal, but public concern was widespread.

About 100 miles from the epicenter, I felt a jolt, as if a truck had rammed the newspaper building where I worked. I had a fleeting sensation that I tipped on my side and then suddenly was upright again, which Vidale attributed to acceleration coming from the side.

“If the ground moves, its going to change the apparent direction of gravity,” he said.

The Aug. 23 quake in south-central Virginia was magnitude 5.8, but it was felt in 22 states and at least three Canadian provinces. Like the 1987 quake, it set off alarms at some nuclear power plants. The damage was notable, and again public concern was widespread.

I was at Washington National Airport preparing for a flight back to Seattle when the terminal suddenly jolted and people near me on the ticketing level began dodging small metal and plastic objects falling from the ceiling. The shaking didnt last long, but it got peoples attention.

Vidale said that earthquake reinforces the idea that scientists have limited ability to forecast specific locations with significant seismic danger.

“A magnitude 5 earthquake where nobody lives doesnt do much,” he said. “But no one was forecasting that as the next place for an earthquake. And from what we know, it could just as easily have been a 6.8 instead of a 5.8.”

The difference between those numbers is stark – a 6.8 earthquake is 10 times larger than a 5.8, but it releases more than 30 times more energy.  That probably would have meant substantially more significant damage than was recorded at, for example, the National Cathedral and the Washington Monument.

The U.S. Treasury also suffered damage to its building in the earthquake.

U.S. Treasury

The U.S. Treasury also suffered damage to its building in the earthquake.

Both the eastern earthquakes I experienced were relatively shallow. The Illinois quake was a little less than 6 miles deep and the one last week was less than 4 miles deep.

By comparison, the biggest recent earthquake in western Washington, in 2001, was more than 32 miles beneath the Nisqually River delta near Olympia. Even though it registered magnitude 6.8 and did substantial damage, the two eastern quakes were felt over a much larger area.

But transplanting those two smaller earthquakes to western Washington could cause significant damage and disruption, depending on where they happened, Vidale said. The shaking would take place over a shorter time than during the Nisqually quake, so less soil would be liquefied, but other damage could be greater than what occurred in 2001.

“It all comes down to how close to fragile facilities it is. If it were that shallow … right under downtown, Id expect there to be considerable damage,” he said. “Only an engineer could tell you what would happen to the Viaduct, but I wouldnt want to be on it.”

Oh, did I mention that my carpool uses the Viaduct every day? I didnt have to worry about that in Indianapolis.