I missed the excitement yesterday. An earthquake! My neighbor thought it was a sonic boom or a tree falling by her house. Someone at Roundtop thought it was dynamite going off. The quake was felt but mostly heard up to around 10 miles away.
I wasn’t at the cabin, so the first I knew about it was in text message. When I called my neighbor to see if there’d been any damage, she said, “So that’s what it was.”
My sister works at a nearby school and thought a truck had run into the building. Someone else thought someone had fallen out of bed upstairs. Up until two years ago, this area didn’t get many earthquakes. In the past two years between 2,000 to 3,000 tremors have been felt. This one was a 3.1 or maybe a 2.9, depending on whose instruments are right.
Most of the quakes have been centered just south of Dillsburg, about 5 miles from my cabin. A study about this cluster that was released last year said the quakes were likely caused by “the contrasting strength of igneous and sedimentary rocks” and the fractures caused by the stress between them.
The area around Roundtop is mostly igneous rock, such as granite, that was formed by the cooling and hardening of molten rock. Sedimentary rock, such as limestone, is formed by the accumulation of fine rock particles and is predominant a few miles to the north..
The igneous rock is the source of iron ore deposits that were extensively mined locally in the 18th and 19th centuries. The quakes are said to be too deep for the mines to have a direct role, but the study did not rule out a possible correlation.
Since the quakes started, the tremors have usually come in clusters and those frequently after rain. We had more of that last night, so it would not be surprising if more quakes are felt in the next few days. The geology people still believes it's unlikely the area will experience a major earthquake. I hope they are right.