Be thankful Tuesday's quake wasn't closer

Be glad it didn't originate here.

If an earthquake of magnitude 5.8 had hit Philadelphia, the city's older housing stock could have been in serious trouble, said Joseph Martin, professor of civil engineering at Drexel University.

Philadelphia has adopted all the latest building codes appropriate to the region's seismic risk, but codes were less stringent before the mid-20th century, when many of the city's signature redbrick homes were built, Martin said. He predicted that newer rowhouses in the Northeast would be OK. But the older ones elsewhere in the city?

"The fronts of thousands of rowhouses could literally have peeled off," Martin said.

Luckily, nothing close to Tuesday's quake has hit Southeastern Pennsylvania since records have been kept. But as many can attest, the impact from Virginia was easily felt here and on up to New England.

That's largely because the East Coast's rocky crust is unlike that in the West, said Jonathan Nyquist, chairman of Temple University's department of earth and environmental sciences.

"The rock's pretty solid, not as much busted up and broken and faulted as in some places out West," Nyquist said. "We sort of ring a little bit like a bell when we get an earthquake."

Preliminary data gathered by the U.S. Geological Survey suggest that in Pennsylvania and New Jersey, the Tuesday quake felt like one with a magnitude of 4.

The data also indicate that the quake was caused by compression forces between subterranean rocks, as opposed to the slippage between plates that occurs along California's San Andreas fault, said Charles Ammon, a professor of geoscience at Pennsylvania State University.

The Tuesday earthquake was roughly 30 times less powerful than the strongest recorded quake to strike the East Coast, said Temple's Nyquist. That was the temblor that struck Charleston, S.C., in 1886, measuring an estimated 6.6 to 7.3.

Still, Tuesday's 5.8-magnitude event was unusual, Nyquist said.

"Any way you look at it, it's a pretty good size for an East Coast earthquake," Nyquist said.

The last earthquake in Pennsylvania was recent, on July 6, about six miles west of Bristol in Bucks County, according to a preliminary earthquake report by the U.S. Geological Survey's Earthquake Hazards Program. But its magnitude was a mere 0.4.

Pennsylvania's largest quake was magnitude 5.2, in 1998. The epicenter was in the northern part of the state near the Pennsylvania-Ohio border, where it produced minor damage. It was felt throughout most of Pennsylvania and northern Ohio, as well as in parts of New Jersey, Indiana, Michigan, New York, and Illinois.

New Jersey's largest known quake was magnitude 5.3, occurring in 1783. Although the USGS does not specify the epicenter, the quake was felt from New Hampshire to Pennsylvania.