Earthquakes in Pennsylvania?

When most north-easterners hear the word earthquake, they would almost immediately think of California. Our western friends have always lived on considerably shaky ground thanks to the infamous San Andreas Fault. Luckily in Pennsylvania and most of the northeast we are not so prone to earth shaking. In the state of Pennsylvania we live on fairly steady ground but even this passive state has felt earthquakes of it's own.

The closest plate boundary to the East Coast is the Mid-Atlantic Ridge, which is approximately 2,000 miles to the east of Pennsylvania. However, 200 million years ago when the continent Pangaead rifted apart forming the Atlantic Ocean, the north eastern coast of America was at a plate boundary. Being at the plate boundary many faults were formed in the region. Although these faults are geologically old and are contained in a passive margin, they act as pre-existing planes of weakness and concentrated strain. When a strain exceeds the strength of the ancient fault, it ruptures causing an earthquake. Earthquakes can also be induced through human activities such as mining and injection or pumping of groundwater. The 1994 earthquake in Reading is just a reminder that there is still a possibility of an earthquake in Pennsylvania. Along with this piece are some articles and descriptions of earthquakes in Pennsylvania occurring in the 18th, 19th, and 20th centuries.

Here are some descriptions of an earthquake felt in Harrisburg and other Pennsylvania cities on March 8, 1889. Although some people were aware of what an earthquake was in the year 1889, the majority of people were naïve to this natural disaster. The center of this particular earthquake was in York, Pennsylvania. The vibrations extended to Philadelphia to the east, Reading to the north, Frederick to the west, and Baltimore to the south.

A York dispatch reported that the most severe shock of this particular earthquake was felt at 6:40 p.m. 2 "The whole city and it's suburbs were shaken, houses tumbled, and heavy articles swayed while dishes were shaken off the supper tables, stoves rattled, and in fact so great was the disturbance that many people were nearly crazed with fear. Women ran shrieking from their houses with their infants in their arms and strong men blanched with terror" It was only when the people went out of their houses to find everybody also outside that they found it was an earthquake. Many people in the 19th century were not as fortunate with information on natural disasters as we are today.
2 Earthquakes Felt in the State of Pennsylvania. Gladees Abdypoor and Richard E. Bischke,Temple University, Department of Geology, 1982

New York Times article of the earthqauke that occured in Reading, Pennsylvania

Paper Summary of Reading Earthquake(JGR)

Largest recorded Pennsylvania earthquake

Pennsylvania Earthquake Links

Link to local earthquake history

Link to USGS earthquake history 1737-1973

Earthquakes that occur far from plate boundaries are considered intraplate earthquakes. Pennsylvania earthquakes are intraplate earthquakes because the closest plate boundary, the Mid-Atlantic Ridge, is over 2000 miles away. The spreading of the Mid-Atlantic Ridge causes stress and strain. This stress and strain is also put on the older fault such as the Paleozoic collisional thrust faults and the mesozoic extensional faults. During the Jurassic, the main trend of the faults formed were North-South, parallel to the opening of the Atlantic Ocean. Since then, the faults are more or less perpendicular to the current state of stress. This may be the cause of the intraplate seismicity. These faults are still subject to stress and strain which may reactivate older faults causing seismicity.

Maps of Pennsylvania seismicity from 1798 to present

In Pennsylvania there are two seismically active zones. One is located in Lancaster while another zone is in the northwestern part of the state by Lake Erie. The Lancaster zone is the most active of the two due to the Paleozoic thrust sheets and the Newark extensional basin. The largest earthquake that occured in the Lancaster zone registered 4.6 on the Richter Scale. Little is known or understood about these zones; our seismometer will help us understand more. The Lancaster seismic zone includes areas around Lancaster, Reading, and Allentown.

Intraplate Seismicity

An intraplate seismic zone lies on the inner part of the tectonic plate. Most earthquakes occur where two plates meet each other, the plate boundary. However, earthquakes can and do occur at faults within the plate itself. In eastern North America there are three large intraplate areas that have been seismically active; the Mississippi Valley near the town of New Madrid, Missouri; the St. Lawrence Valley in eastern Canada; and Charelston, South Carolina. All of these places were once an mid-continental rift zone creating extensional faults. These regions have experience earthquakes of magnitude 7 on the Richter scale. Hazard rates for intraplate earthquakes are significantly less to none when compared to the more active plate boundary areas. However, an earthquake in an intraplate area can cause considerable damage, particularly in the Eastern United States. Many of the rocks in Pennsylvania are old, crystalline, and fractured. When a magnitude 4 or 5 earthquake strikes Pennsylvania, the energy from the earthquake is transferred much quicker and farther than in the Western United States. An earthquake that causes little damage in California, Oregon, or Washington, may cause considerably more damage in the Eastern U.S.