What's up with the recent earthquake activity in York County, Pa.?
Historically there was once a newspaper article describing an earthquake in York in 1886. Some chimneys were collapsed in the City of York and reports of damage were reported in the newspaper from Wellsville and Wrightsville. Without seismographs on duty, and based on the records, the epicenter of the earthquake was thought to have been in the vicinity of Wrightsville, but this cannot be confirmed.
It wasn’t until August 24, 2000 that the first tremor with its epicenter in York County in recent times occurred near Cly and York Haven (1). The magnitude of this event was 1.5 and its epicenter could have been under the Susquehanna River.
Tremors were felt near Mount Top and Franklintown (2), Carroll Township, in June 2007; the Dillsburg earthquake swarm (3) started in October 2008, and lasted for 16 months; and in August 2017, a tremor occurred in Lower Chanceford Township near Slab Road (4).
The East Berlin Earthquake (5) occurred on September 14, 2019, at 9:11 p.m. The Pennsylvania State Seismic Network placed the epicenter along Pleasant View Drive east of Church Road in Paradise Township. Its magnitude was 2.8 with a depth of 5 km. A total of 1774 people reported this tremor across the region from northern Maryland to Harrisburg and eastward into Lancaster County. I wrote about this earthquake in a column soon after the tremor occurred.
Most recently, the Pinchot State Park area wanted in on the excitement of the seismic game. On February 1, 2020 at 2:55 p.m., a 1.3 magnitude tremor shook the neighborhood near East Campground Road near Alpine Road (6). The quake occurred at a depth of 4 km. The next day, a 1.5 magnitude quake occurred on West Campground Road at 4:37 a.m. with a similar depth (7). Both tremors occurred within dense, hard rock known as diabase. No faults are known in the area.
With the addition of the Mifflintown earthquake in January 2020 (magnitude 3.4 with a depth of 28 km), the media became curious about the sudden rash of earthquakes in Pennsylvania and also in York County. These above tremors occurred in areas where seismic activity was not known. Yes, it is true that Lancaster County and the Sinking Springs, Berks County, area are “hot spots” for earthquakes. These two areas have a historical record of seismicity, with such activity expected from time to time.
We cannot answer the question about why seismic activity has increased in York County the past year or so. Scientists work off of data and observations to develop a theory. The more data we can gather, the better a theory can be developed. These “one in an area” type of tremors cannot provide us with much data.
No local seismograph
So how is data gathered from a tremor? For several decades the first seismic network that monitored our activity was the Lamont-Doherty Seismic Network (LDSN) operated by Lamont-Doherty Earth Observatory (Columbia University) in Palisades, New Jersey. Their closest measuring station was Millersville University. Every time the Millersville University seismograph was activated by a tremor (and probably other stations), the station sent the signal to LDSN, which quickly calculated the location, magnitude and depth of the event. The event was added to the listing of LDSN recent quakes. Recently, money to continue to operate LDSN were discontinued and is no longer monitoring Pennsylvania.
The good news is that a newly installed array of seismographs has been installed across Pennsylvania. This network run by Pennsylvania State University is the Pennsylvania State Seismic Network (PSSN). Unfortunately, funding has not been available yet to install seismographs in Adams, York or Lancaster counties. The word, however, is that the Millersville Station will probably be added to PSSN soon.
Finally, I would like to remind the readers about epicenter locations. If you look at the state map showing the last 25 earthquakes in Pennsylvania, because of the distance from an epicenter to a seismograph in our area there is a +/- factor for the data collected. The further the seismograph is from the epicenter the larger a +/- factor is attached. For example, the two tremors that occurred near Pinchot State Park in early February had +/- factors of 1 km and 0.8 km respectfully. The epicenter could be anywhere within that distance from the plotted location.
The link to the PSSN is paseis.geosc.psu.edu/events.html. The network even differentiates between non-tectonic events (man-made or ones caused by fracking) and tectonic tremors.
Jeri Jones writes regularly about regional geology.