Carroll Township and the Earthquake Swarm
Jeri L. Jones, Jones Geological Services
Dr. Charles K. Scharnberger, Professor Emeritus, Millersville University

The title sounds like a fairy tale and to some residents of Carroll Township, they wished it was. Beginning on Friday, October 3rd and still continuing at the writing of this articlein mid December, the area has been “rocked” or “boomed” by numerous tremors. Over 150 tremors have been felt or heard by area residents. Of the large number of tremors, 18 of these events were recorded by seismic stations at Millersville University, Frankliln and Marshall College, Soldier’s Delight State Park, and several other seismograph stations in the region. Several dates on which recordable tremors were noted were Sunday, October 5th, Sunday; October 19th; Monday, October 20th; Thursday, October 23rd; Sunday, October 26th and Thursday, November 6th. The largest of these tremors had a magnitude of 2.1 on October 19th. The same day also included 11 other tremors recorded at the above stations.

Much information from area residents was reported to these investigators. From the township website and television channel, personal and telephone interviews and constant record keeping of residents, we have been able to construct intensity maps for the October 5th and October 19th events as well as a listing of all of the tremors felt. The quadrant between Mandy Lane, Old York Road, Warrington Road and Stoney Run Road has the highest intensity for these tremors. Along with the booms and rumbles, during the 2.1 tremor of October 19th, items fell off of shelves, the feeling that the house floor rose up two feet and a old window pane cracking were among the responses from residents.

On Friday, October 24th, personnel from Lamont-Doherty Earth Observatory of Columbia University installed four portable seismographs in the Dillsburg area. These portable units will record all of the tremors, many of which are not strong enough to be recorded at the permanent stations. From data retrieved from these stations, the epicenters and depths of these tremors can be precisely located. This information is not expected back to us until late December.

A community meeting was held on October 28th at the township building. During the meeting, the local geology was explained, history of earthquakes in the eastern United States and a lengthy question and answer period eased the minds of some of the area residents. Based on the history of earthquakes in the eastern section of the United States, there is a low probability that a severe earthquake will occur in the Dillsburg area.

What makes this swarm of shallow earthquakes interesting to investigate?
1). The number of tremors felt over the last ten weeks
2). The regular “boom” that accompanies many of these tremors
3). A sulfur smell associated with the larger tremors and sometimes within the groundwater

These issues are being investigated by us. We hope to schedule another informational meeting early in 2009 at a meeting place to be announced. There will be information also placed on the Carroll Township website and here at this website.

For additional information, check out these websites:
Millersville University Seismic Station: www.millersville,edu/esci/geology/seismograph.php
Lamott-Doherty Earth Observatory –
National Earthquake Information Center –
Free downloadable Earthquake Hazards in Pennsylvania booklet

To view a larger map, click on the map image.

Intensity map of the October 5, 2008 2.0 earthquake.  Zone III is the epicenter area

Intensity map of the October 19, 2008 2.1 earthquake - Zone IV in in the epicenter area and was a more widely felt event than the October 5th tremor

Open the PowerPoint to view what days have had seismic activity.
May take longer to load on dial-up. (file size: 3.5M)

Measurable Earthquakes from the Lamott-Doherty
Cooperative Seismographic Network from the Dillsburg
Pennsylvania area (PDF Available)

Date Local Time Magnitude
October 5, 2008 6:36 2.0
October 19, 2008 4:21 1.9
October 19, 2008 4:22 2.1
October 19, 2008 4:26 1.2
October 19, 2008 4:58 1.8
October 19, 2008 5:06 1.0
October 19, 2008 5:08 0.8
October 19, 2008 5:08 1.0
October 19, 2008 5:17 1.7
October 19, 2008 5:45 1.5
October 19, 2008 5:49 1.5
October 19, 2008 5:50 1.6
October 20, 2008 10:16 1.5
October 20, 2008 20:08 1.2
October 20, 2008 20:14 1.1
October 23, 2008 11:55 1.2
November 6, 2008 23:07 1.4
December 31, 2008 00:34 2.1
April 22, 2009 9:21 1.1
April 23, 2009 6:26 2.4
April 24, 2009 1:36 2.9
April 30, 2009 18:36 2.0
May 11, 2009 1:18 1.3
May 11, 2009 1:34 1.2
October 25, 2009 7:16 2.6
October 25, 2009 7:18 1.8
October 25, 2009 7:21 2.8
June 3, 1020 ** 8:25 2.9


The report on the portable seismographs from Columbia University - Lamont-Doherty Earth Observatory has been published into an open-file report by the Pennsylvania Geologic Survey.  Click here to read report which includes epicenter, foci and some thinking on what are causing the tremors.  

** 2.9 magnitude earthquake rattles region on June 3, 2010

At 8:25 a.m. local time, a 2.9 magnitude earthquake rattled a large region in York, Adams, Cumberland and Dauphin counties, Pennsylvania. Reports came in from Derry Township and Highspire, Dauphin County, the Capital building in Harrisburg, Boiling Springs area, Dover, Mechanicsburg and York Springs. As explained in the Lamont-Doherty Observatory report on the other Dillsburg tremors, the closest seismograph to the Dillsburg area are the seismographs at the Pennsylvania Geologic Survey in Middletown and Millersville University. No portable stations are deployed in the Dillsburg region at this time. The further a station is from the epicenter, the more inaccuracy you get on locating the origin of the tremor. A calculation of ±5 km accuracy was derived from the previous Dillsburg tremors.

After several days of interviews, an epicenter of just south and west of Mt. Pleasant, Monaghan Township was established. Some folks living in this area reported the tremor as “thought a Mack truck was coming through the wall and my floor felt like it lifted up a foot.” This epicenter is about 4 miles northeast of the epicenter area of the many other Dillsburg tremors.

As mentioned above, the magnitude of this tremor was established as a 2.9. Initial reports listed the event as a 3.1, but was down-graded by the Lamont-Doherty seismologists to a 2.9. Based on the regional extent of this tremor compared to another 2.9 tremor in the Dillsburg area, the June 3rd tremor had a much larger reach out into the community. Both Dr. Charles Scharnberger of Millersville University and myself like the 3.1 magnitude better.

Below is an isoseismal map for the June 3rd event. The small circle in the middle represents the epicenter with a Modified Mercalli 4 ranking.


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