Small earthquakes rattle area residents

Kaylee Renfrew

Intern Reporter/Dillsburg Banner

Some Dillsburg and Dover residents felt the earth move under their feet last weekend when two small earthquakes hit the Pinchot State Park area. One, a 1.3 magnitude earthquake, hit Saturday at about 2:55 p.m., and the other, of 1.5 magnitude, hit at 4:37 a.m. Sunday morning.

George Defrain wasn’t home Saturday when the first quake occurred, but he received a concerned text message from his neighbors, saying they believed that something had blown up in Defrain’s home, and that they were going to investigate. The sound they heard turned out to be from the earthquake. Early Sunday morning, Defrain woke to a loud noise caused by the second earthquake. “I thought for sure my house had exploded this time,” said Defrain. 

Larry Anderson, fire chief for the Wellsville Fire Company, was called in Sunday morning.  “Two individuals had called in saying they had woken up from a loud noise and their house shaking, thinking something had blown up, and that it had been too loud to have been a car accident,” said Anderson. 

Brian Prosser of Warrington Township, asleep during both earthquakes, was awoken each time by the ruckus. He likened the sound of the earthquake to the sound of someone firing a rifle inside the house. “For literally about two to four seconds, the whole house shook, the windows rattled. The best thing I could say is the ground shook,” said Prosser, stating the earthquakes had taken him by surprise. “You know, I don’t live in L.A., so I’m not really used to them.” 

Earthquakes in Pennsylvania are relatively rare, but have become increasingly more common within the last year, said geologist Jeri Jones of Jones’ Geological Services.  “Excluding the earthquake swarm in 2008, there was only one earthquake historically documented in York County,” he said.  A swarm of more than1,000 reported earthquakes started in October 2008 and spanned into the spring of 2010, however seismographic technology suggests that number could have been well over 3,000 earthquakes, most of which were not reported by residents because of the small size and magnitude. 

“As scientists, we have more questions than answers,” Jones said, noting that often no cause is determined. Jones. Both earthquakes this weekend occurred in an area with a hard, igneous rock base called diabase. When the diabase cracked, it caused the two small tremors that were felt in the area.  “The diabase seemed to have cracked under pressure--and when diabase breaks, it lets everybody know.  If it had been limestone or shale, and the same thing had happened, you would have never known--those are soft rocks,” said Jones.  In addition to earthquakes in the area, Pennsylvania also occasionally experiences “frost quakes,” a phenomenon that occurs usually after a period of warm temperatures when rain or melted snow saturate the ground before temperatures quickly drop. The moisture then freezes and expands in the ground, causing surrounding soil and rocks to crack, creating a “frost quake.”  However, the quakes are typically not large enough to be recorded as seismographic activity.  “There were frost quakes back during Thanksgiving week and another period during the second or third week of January,” said Jones.  Fracking in the northern counties of Pennsylvania sometimes result in “false quakes.” he said. This weekend’s quakes were what Jones referred to as “traditional piedmont earthquakes,” small in size and magnitude. The largest earthquake to occur in south central Pennsylvania was a 4.3 magnitude earthquake on Easter Sunday in 1984. 

For more information see the February 6, 2020 edition.