Dr. David Hardin, Associate Professor of Geography, was sitting in his Chichester Hall office when he felt the room begin to shake. His thoughts raced back to December 2003 when a similar shaking event occurred. Hardin knew the August 23 earthquake wasn't like the December 2003 quake that occurred on the border of Powhatan and Cumberland County. "This quake wasn't like that one," Hardin said. "There would have had to be a convoy of tanks to do that."
The "rumble, rumble, rumble … pause … rumble, rumble, rumble" from 2003 was replaced by a constant shaking with the latest quake. This time around, it was a long rumble without a pause. For some, the shaking lasted about 15 seconds. For others, especially those in the northern United States, it lasted more than 30 seconds.
Hardin and his family went by Mineral, Va., near where the epicenter was located, to look at some of the damage. The actual area is known as Cuckoo, Va., something Hardin said he wished the media had noticed. There were numerous chimneys ripped away from homes and many glass-panned windows in the little town's storefront were busted. Only plywood was covering what used to be a window shopper's paradise.
But why here? Why did Virginia see an earthquake to rival all of those in her past? Hardin explained the reasoning in one word: "settling."
Scattered, fractured pieces of Piedmont rock settled in place all over Central Virginia. The word "Piedmont" can be traced back meaning foot of mountain, thus providing a clue into what makes the area so unique. "This is one of the older segments of North America here," Hardin noted.
At the root of a mountain range, collisions eventually occurred millions of years ago causing granite and metamorphic rocks to be left behind. This rock wore down and when Africa and North America collided, the Blue Ridge Mountains were formed. Fracturing occurred and the Atlantic Ocean opened up afterwards. Hardin provided the analogy of a "bendy straw," showing that the rocks, thanks to tension, slipped through the faults that were created after separation.
The faults can be accredited to ancient tectonic processes that compress and separate these Piedmont rocks. When slipping occurs, Triassic Basins are created.
According to Hardin, the basins formed in pre-history also cause trouble. There are three main Triassic basins that require a close watch: one near the Farmville Municipal Airport, one west of Richmond, Va., and one in Culpepper, Va. All of these basins run northeast to southwest.
Hardin said it is often easy to pinpoint these basins. Some are recognized as half grabens. This is defined as a topographic depression which occurs due to the movement of a normal fault. The rock above the fault moves down and the top layer moves up. Sediment layers then fill the uplifted layer, usually creating a flat and sometimes swampy area. Even lakes can develop under proper conditions
The faults are slipping and have been for the last 200 million years. "Earthquakes happen all the time. We usually don't feel them." He said the quake in August was special because it was such a sensitive feeling for so many.
You can thank the type of sediment in this area for that feeling. This sediment material helps those shake waves travel a much larger distance. When an earthquake occurs, the United States Geological Survey (USGS) offers a reporting option on their website where people can report what they felt from a quake.
The USGS then generate a shake map, detailing where people felt the earthquake and the intensity it was felt. The 5.8 magnitude earthquake from August had a large shake map because of the fault line and the hardness of the rocks. On the west coast, shake maps are usually more compact since shock waves travel less distance in softer sediment, Hardin said.
Hardin said in order to feel anything from a quake, it should be 3.0 or above. It was felt more in the north, along the areas built on the same rock structure. Hardin also said it is no real surprise that people more than 600 miles away could feel the quake.
As far as aftershocks, Hardin said do not be surprised to see more in the upcoming weeks. "It's normal. They happen all the time. This was just unusual for us."
