Many people don't see the East coast of the United States to be as geologically active as the West coast,
Figure 1: Dillsburg, Pennsylvania welcome sign
but a recent swarm of earthquakes in Pennsylvania may change that opinion. Dillsburg is a small town of about 2000 located in York County in south-central Pennsylvania. The town was named after one of its first settlers, Matthew Dill, who arrived to the area in 1740 from Ireland. Starting in August 2008, hundreds of small earthquakes have been reported. None of these earthquakes were substantial in magnitude, the largest being a 2.9 in April of 2009, but all have caused the residents of Dillsburg to be on edge.

Background Information

An earthquake swarm can be defined as a series of earthquakes that lack a definitive foreshock, mainshock, or aftershock. The earthquakes that have rattled Dillsburg over the past year are indicative of a swarm; there have been over 1000 small events recorded. While there have been many reported swarms in the past, most usually stop once they've been occurring for about 6 months. The Dillsburg swarm, however, has become the longest-lasting recorded swarm, with events occurring for over 13

Figure 2: Some of the earthquakes associated with the Dillsburg swarm

months. Figure 2 shows a few of the hundreds of earthquakes that have shaken the Dillsburg region. Many residents have reported hearing a "boom" or loud explosions in relation to the earthquakes. This is due to the relative depth of the earthquakes; most are shallow, being less than 3 km deep. Though the earthquakes have been shallow, there has been little damage done to infrastructure or buildings due to the small magnitudes.
Figure 3: Mercalli Intensity Scale of a Dillsburg earthquake
However, according to the Mercalli Intensity Scale (MIS), many residents report a IV, or more specifically, during the day felt indoors by many, outdoors by few. At night some are awakened. Dishes, windows, and doors are disturbed; walls make a creaking sound. Sensation is like a heavy truck striking a building. Standing motor cars are rocked noticeably. It can be noted that the further away from the epicenter of the earthquake, the lower the reported MIS number was reported as can be seen in Figure 3.

Dillsburg Geologic Information

Under the ground of Dillsburg lies magmatic diabase, a material much like basalt but due to the length of cooling time, has a courser texture than basalt. When sedimentary rock comes in contact with a diabase body, it becomes altered due to the high heat. Diabase is much more resistant to erosion than sedimentary rock and therefore tends to be the base of mountains and structures that are of higher elevation than the surrounding area. Due to the high amount of iron based minerals, diabase commonly is the site of great iron ore deposits. One such deposit sat below Dillsburg and was mined extensively during the 1800-1900's. Some first believed that the earthquakes were being caused by collapsing mines.
Figure 4: Dillsburg earthquakes as indicated by the red and green dots. The green is sedimentary rock and the red is diabase.
Upon further research however, this idea was displaced due to the depth of the earthquakes, which while shallow, occurred much deeper than the mines themselves, and the duration of occurrence of the events. Figure 4 depicts some of the earthquakes in the Dillsburg area and the different ground layers.

Causes of the Dillsburg Earthquake Swarm

In the 1940's, the US Geological Survey drilled into the ore deposits of Dillsburg and found that there were two sheets of diabase that were split apart by a layer of sedimentary rock. It was found that the lower sheet of diabase has been downwarped, creating a bowl-type area with sedimentary rock in the middle of the bowl and a small amount of the upper sheet of diabase on top. Much of the top diabase sheet has been eroded away, leaving the sedimentary rock exposed. A theory as to why the swarm happened lies in the understanding of these rock layers. Since they are made up of different substances, the normal compression of the rock is experienced differently. The compression is seen greater in the less rigid local sedimentary rock. When the stress becomes too great for the rock to handle, it fractures, resulting in an earthquake that can be both felt and heard. The Dillsburg earthquakes have yet to be experienced at greater than 1800 feet below the earth's surface which allows for the theory of uneven compression since the diabase layers end at about 1800 feet. It is believed that over the years, the movement of the North American plate and the precipitation amount have contributed to the rock compression. The rising water table and subsequent weight add stress to the already compressed rock layers. Soon, the increased pressure exerted by the water table onto the rock surface will cause the rock to fracture, leading to an earthquake. While it may never be known why the earthquake swarm started exactly when it did, the theory of rock compression and fracture is very plausible as to why a region so far from any plate boundary is experiencing earthquakes.


Costain, John. Hydroseismicity Home Page. Geothermal Data Regional Geophysics Laboratory. Virginia Polytechnic Institute and State University. Web. 8 Mar. 2010. <>.

Delano, Helen. "Pennsylvania Geology." The Great Dillsburg Earthquake Swarm: Cooperative Geology 39.3 (2009): 9-13. Pennsylvania Department of Conservation and Natural Resources. Sept. 2009. Web. 17 Feb. 2010. <>.

Kim, Won-Young, Gold, Mitchell, Scharnberger, Charles, and others, 2009,
The 2008-2009 earthquake swarm near Dillsburg, Pennsylvania: Pennsylvania Geological
Survey, 4th ser., Open-File Report OFMI 09–01.0, 20 p., Portable Document Format (PDF).

Gorsegner, Michael. "Researchers Pinpoint Possible Reasons for Earthquakes in Dillsburg." Tribune Company, 20 October 2009. Web. 17 February 2010. <,0,4058849.story>.

Gross, Greg. "Quaking Dillsburg seeks answers." The Sentinel Online. 12 Nov. 2009. Web. 17 Feb. 2010. <>.

Jones, Jeri. "THE DILLSBURG, PENNSYLVANIA 2008-2009 EARTHQUAKE SWARM." Northeastern Section and Southeastern Section Joint Meeting. Sheraton Baltimore City Center, Baltimore, Maryland. Geological Society of America. Web. 17 Feb. 2010. <>.

Jones, Jeri L. Millersville University. An Update on the Dillsburg Earthquake Swarm. Carroll Township, York County, PA. Carroll Township. Web. 8 Mar. 2010. <>.

Sentinel, Carlisle. "Earthquake Swarm Continues in Dillsburg, PA:Experts try to calm residents." Millenium Ark. 11 Nov. 2009. Web. 17 Feb. 2010. <>.