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Delaware Geological Survey geologist, Stefanie Baxter shows the seismic recordings from detectors Wednesday.
July 2, 2009

When the earthquake hit a little before 10 a.m., everyone who heard the crack and felt the ground rumble thought something had exploded or crashed. Chimneys fell over, windows shattered, people panicked -- on Oct. 9, 1871.

The magnitude 4.1 earthquake that shook Wilmington that morning -- the most intense temblor ever recorded in Delaware -- easily outshook the magnitude 2.8 quake that hit the state Wednesday morning.

The 1871 quake released about 150 times more energy than Wednesday's tremor, according to the U.S. Geological Survey's National Earthquake Information Center.

Shifting bedrock two miles east of Pennsville, N.J., and three miles below the surface caused the quake at 9:44 a.m., said John Talley, director of the Delaware Geological Survey.

The USGS took reports from residents who felt it from Elkton, Md., to Glassboro, N.J., from Dover to Malvern, Pa. Authorities said the quake caused no significant damage.

Most described the quake as a crashing or explosive sound.

"It sounded like two tractor-trailers colliding outside my window," said Cpl. Joseph DiStefano, spokesman for the Delaware River and Bay Authority Police, which covers the Delaware Memorial Bridge.

"Or I thought it was a tanker exploding in the river," DiStefano said.

Cpl. Trinidad Navarro, spokesman for New Castle County Police, said emergency dispatchers thought a vehicle had crashed into the building. Moments later, they began taking calls from nervous people.

Joe Negron, who lives on Lovering Avenue in Wilmington, said he felt everything shake.

"I thought it was just a dump truck going down the street," he said.

The quake was a reminder that Delaware and the entire East Coast sit atop bedrock laced with small faults that shift and shrug every few years, said Stefanie Baxter, a geologist with the Delaware Geological Survey.

"You usually don't feel the small ones, but you can hear it," Baxter said. "They can be quite loud."

Before Wednesday's quake, the most recent one centered in Delaware hit April 9, 2005. The magnitude 1.2 tremor was centered near the intersection of Foulk and Naamans roads.

For a quake bigger than Wednesday's tremor, it's necessary to go back to Nov. 17, 1983. That magnitude 2.9 quake hit the Trolley Square area.

One of the biggest quakes felt in Delaware actually was centered in northwestern Salem County, N.J. The magnitude 3.8 quake shook people out of their beds at 3:21 a.m. on Feb. 28, 1973.

"It was like someone shaking your bed to wake you up," said Hank Sundermeyer, of Newark, who was living in Rumson, N.J., at the time.

The state's biggest quake, though, hit on a Monday morning in 1871.

"People immediately rushed to the highest points of observation, many to their housetops," reported the Every Evening, a predecessor to The News Journal, "expecting to see the usual black cloud rising in the northwest which always follows an explosion at DuPont's Powder Mills."

A rescue party raced to the gunpowder plant. The newspaper telegraphed questions to the company, to other cities, to the gunpowder magazine at Fort Delaware. None reported an explosion.

The event "caused no little alarm," the paper reported.

About twice a century, an earthquake causes moderate damage somewhere in the urban corridor covering New York, Philadelphia and Wilmington, according to the U.S. Geological Survey. Smaller earthquakes are felt about every two or three years.

Earthquakes are measured on a logarithmic scale, which means that an increase of one point translates to a 10-fold increase in ground motion. As a magnitude 3.8 quake, the 1973 tremor was 10 times more powerful than Wednesday morning's magnitude 2.8 quake.

The Richter scale, developed by Charles Richter in the 1930s, is the most famous scale, but several others have been developed in recent decades. Most are similar, so scientists usually report quakes in magnitude numbers without specifying a scale.

Scientists also use the Modified Mercalli Intensity Scale, which measures from one to 12 how strong an earthquake feels and the damage it causes.

The 1973 quake hit a five or six, while the 1871 quake reached a seven. Reports to the USGS about Wednesday's quake put the intensity between a one and a three.

Scientists say there are about 3 million earthquakes worldwide every year, but 98 percent of them fall below magnitude 3.

Delaware is far from the nearest tectonic plate boundaries, which are in the center of the Atlantic Ocean and Caribbean Sea.

But many smaller faults run through the bedrock under the East Coast. They're called blind or buried faults because they're hidden below the surface, Baxter said. Some are known, but most are undetected.

Baxter said it's impossible to identify the fault that caused Wednesday's quake. The region doesn't get enough quakes for scientists to analyze their exact location.

Because the faults are buried, it's also impossible to know if the bedrock is moving horizontally, called a strike-slip fault, or vertically, called a normal or reverse fault, Baxter said.

Most of the bedrock under Delaware formed as continents collided to form a supercontinent 500 million to 300 million years ago, the USGS says. The collision also raised the Appalachian Mountains.

The rest of the bedrock formed as the supercontinent drifted apart about 200 million years ago, forming what is now the northeastern United States, Europe and the Atlantic Ocean.

Devastating earthquakes are possible along the East Coast. A magnitude 7.3 quake hit Charleston, S.C., in 1886, toppling buildings and killing 60 people. Its tremors could be felt in Delaware, reaching from Boston to Cuba.

Such a quake could happen here, Baxter said.

"Is it possible? Yes," she said. "But it is not very probable."


Previously published by Mike Chalmers and Terri Sanginiti
The News Journal, July 2009
