Charles Scharnberger, retired earth sciences professor with Millersville University [and stalwart member of the LDCSN]:
Wayne Pennington, chairman of the Geological and Mining Engineering and Sciences Department at Michigan Technological University:
Earthquakes in general are caused by stress concentrations within the earth. The rocks break and slide. Faults are not big openings in the earth; they're like cracks in a sidewalk, he said. Rocks on one side won't line up with those on the other. Parts of the earth's crust are shifting, and stress can build up. It can be released in weaker spots. If a fault system is identified, the size of the fault will give experts a good idea of its maximum potential. It takes bigger faults to produce large earthquakes. The San Andreas fault, for example, is hundreds of miles long. If one exists in the Dillsburg area, it's probably not more than a few kilometers long, he said. A kilometer equals 0.62 miles. "It's, in a sense, the earth resettling," he said. Induced seismic activity, such as mines collapsing, is another possibility. It would result from the readjusting of rock near the stress void. The depths of the earthquakes would determine whether the mines are involved. If the mines are collapsing, the seismic activity that the area has been experiencing probably will not get any bigger.
George H. Myer, a professor of earth and environmental science at Temple University:
Earthquakes differ in California and the East Coast. In California, it's a side-to-side motion, known as a strike-slip fault. In Pennsylvania, they're known as normal or vertical faults. Normal faults push part of the rock downward, and vertical faults push the rock upward, he said. Geological maps show countless faults in the Dillsburg area, and it's possible that a reverse fault is at play. That's because of the booms residents are hearing. The primary waves refract off the surface and into the air, creating a vibrating noise that people can hear.