Earthquakes having magnitudes greater than a 5, the size of the one centered just northwest of Richmond, Va. Tuesday that rocked much of the East Coast, can occur in Pennsylvania.
This was demonstrated by the 5.2 magnitude earthquake that was centered in Crawford County on Sept. 25, 1998. Although it is not known to have experienced an earthquake with magnitude greater than 4.7, southeastern Pennsylvania is the state's most seismically active region. But historical records go back only about 200 years, so the area could have been affected by a quake before that time period.
Lancaster County has been an active region for quakes, with nine tremors with a magnitude between 2.6 and 4.1 occurring between 1972 and 2000.
In his book, Earthquake Hazard in Pennsylvania, written as part of a Pennsylvania Geological Survey series, Charles K. Scharnberger notes that the areas that have generated the largest historical earthquakes in eastern North America New Madrid, Mo., and Charleston, S. C. are too far away for earthquakes having epicenters there to cause damage in Pennsylvania, but that a magnitude 7 earthquake occurring in those areas would be felt in the Keystone state.
On Nov. 1, 1935, a magnitude 6.4 earthquake with an epicenter near Timiskaming, Ontario, rated a IV on the intensity scale in northwestern Pennsylvania and, at lower intensities throughout the commonwealth. The IV rating means a quake can be felt indoors by many, outdoors by few. At night people are awakened. Dishes, windows, and doors are disturbed; walls make a creaking sound. The sensation of such an event is like a heavy truck striking a building. Standing motor cars are rocked noticeably.
A magnitude 7 earthquake having an epicenter near New York City cannot be completely discounted, according to Scharnberger. Such an event could produce intensity VIII scale damage in eastern Pennsylvania. Under this rating, "damage is slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls are thrown out of frame structures.
"Chimneys, factory stacks, columns, walls, and monuments fall; heavy furniture is overturned. Sand and mud are ejected from the ground in small amounts. Changes occur in well water. Persons driving motor cars are disturbed."
Scharnberger notes in his conclusion that Pennsylvanians will likely continue to feel small earthquakes generated on local faults, although the exact identity of those faults is likely to remain elusive.
"A large local earthquake, one with magnitude greater than 6, though unlikely, is not impossible," he said.