Since 1900, there have been about 45 earthquakes in the Commonwealth of Pennsylvania1. Records from the 1700s are rather sketchy and deal primarily with the parts of the state settled by Europeans, which place an inordinate number of earthquakes in the area around Philadelphia. In the 19th Century there were another 30 or more quakes.
Compared to tectonically-active areas in the USA like California, Pennsylvania is relatively quiet - and Pennsylvania's record of earthquakes is mild even when comparing it with other eastern states in the USA. However, Pennsylvanian earthquakes do still occur and given their relative infrequency, it is always a shock when one happens.
First, a little background information about earthquakes. Earthquakes occur when there is a sudden release of stored energy along a fault within the rocky crust of the earth (known as the lithosphere). When a quake occurs, that stored energy is released and dissipates outward in waves through the earth.
The most common way to express the amount of energy released by a quake is to measure the magnitude of the event using the Richter Scale.The Richter Scale expresses the magnitude of an earthquake by powers of 10 so that a 4.0 earthquake is 100 times more powerful than a 2.0 earthquake. There is no upper or lower limit to the Richter Scale, but no quake has ever been recorded with a magnitude of more than 9.5. Earthquakes in eastern North America generally have magnitudes lower than 5.0 on the Richter Scale.
In December 1811, the most powerful earthquake recorded in eastern North America occurred in New Madrid, Missouri, which exceeded magnitude 8 on the Richter Scale. The quake caused significant damage in and around Missouri, but while it was felt in western Pennsylvania it caused no damage there.
The largest recorded earthquake in Pennsylvania was the 25 September, 1998 quake that was centred around Jamestown in Mercer County. At magnitude 5.2 on the Richter Scale, it was followed by many aftershocks in the following months.
So what causes earthquakes in Pennsylvania?
You should expect earthquakes in areas where boundaries between tectonic plates meet, such as in California where the San Andreas fault is formed by the North American and Pacific plates rubbing against each other. However, in Pennsylvania, no such plate boundary exists. In fact, the nearest plate boundary is in the middle of the Atlantic Ocean, some 2,000 miles to the east.
Earthquakes in Pennsylvania can be traced back to events 200 million years ago when the supercontinent Pangea broke apart, forming the Atlantic Ocean. This caused faults and weaknesses throughout the underlying rocks of eastern North America. When the North American tectonic plate experiences stresses, sometimes that pressure can be released in areas of weakness far away from a plate boundary.
The vast majority of earthquakes in Pennsylvania occur in the south-eastern part of the state - in Berks, Lancaster, Lebanon and York counties. Geologists have no way of knowing if or when the next quake will occur in this area, where almost 40 have occurred during the last 200 years.
Generally, quakes in Pennsylvania are minor events. However, geologists cannot guarantee that a quake with a magnitude of five or six on the Richter Scale won't happen tomorrow.
Kind of gives a whole new take on the Pennsylvania's nickname 'The Quaker State', eh?