A funny thing happened on the way to vacation

Posted by on September 6, 2011 in Geologic Hazards | 0 comments

Even geologists find time to take a break for a short summer vacation. My wife Lou Ann and I travel to Bristol, Tenn., every August to attend NASCAR’s premiere night event at Bristol Motor Speedway.  What a spectacle — watching stock cars go around a track the size of the one at the York Expo Center, but surrounded by 160,000 seats. NASCAR fan or not, everyone at least should see this facility, with or without race cars.

It was Aug. 23, and as we headed south on Interstate 81 in southern Virginia, travel fatigue began to set in. But after a cell phone call at 1:54 p.m., the trip suddenly felt much shorter. The person on the other end of the line — an associate of mine — told me the office building in East York had just shaken pretty hard.  I hung up, wondering what in the world caused that. I had known the caller for many years and very much trusted her testimony.

Only a few minutes passed before another call came through. The caller was from the Hanover and wanted to report a shaking sensation. At that moment I knew that an earthquake had occurred, but I wasn’t sure where. Yes, even my dedicated tremor reporters in the Dillsburg responded, probably thinking that northern York County decided to wake up from several months of peace and quiet.

More calls came in over the course of the next 90 minutes, and Lou Ann became a secretary. Taking voice mail messages, she and I managed to record testimonies from residents and talk to several media representatives who got a head start on the story.  By the time we arrived in Morristown, Tenn., I had learned that the epicenter of the quake was located in Mineral, Va., with a magnitude of 5.8 and a depth of 3.7 miles.

Ironically and tragically, Hurricane Irene struck the same area this past week. Many curious television viewers probably watched the Weather Channel, and brave reporters stood on the beach in the middle of the storm fighting off the blowing sand, horizontal rain and fierce ocean waves. Though dangerous, this is indeed what meteorologists live for. They don’t mind being up for hours with storm gear as energy pumps through their bodies.

Well, it is similar with a geologist. I hear about York County being attacked by an
earthquake, and my adrenalin begins to flow. Knowing I was going to be away for another five days, I actually was relieved when I learned that the epicenter was in Virginia and not in York County.

Wow. We missed the event and I wasn’t around to witness the shake myself.  Although the area we were driving through when the quake occurred reported feeling the shaking, a person in a car will not feel anything unless the tremor was very large. I learned when I got home that Dr. Charles Scharnberger, a very knowledgeable Millersville University seismologist on East Coast quakes, was
also driving at the time of the tremor. What’s up with that?

I find it amazing just how much technology has grown. In my younger days, it took several hours after an earthquake occurred to learn where the epicenter was located. These days, in the age of computers and with the establishment of regional seismic networks, the epicenter of an earthquake can be online within minutes.

I understand that local TV and radio stations were giving the details about the Aug. 23 event on their 2 p.m. news. To add to that, folks who felt quake could go online and report what they experienced. The National Earthquake Information Center is a great resource for up-to-date earthquake reports from around the world. By clicking on the time of any tremor, one can go to a page titled “Did You Feel It?” and register a report. By the evening of the 23rd, site users from as far as Canada had reported feeling tremors from the Mineral quake.

The epicenter of this earthquake is located within what geologists call the Mesozoic Rift Basin. This series of discontinuous “basins” runs south along the East Coast from Connecticut to Alabama. This was an area where the supercontinent Pangaea attempted to pull apart about 220 to 170 million years ago. Although the rifting of this area was not successful, the stress weakened the Earth’s crust.

If you are a regular reader of this blog, the term Mesozoic Rift Basin may sound familiar. It is the same geologic basin where Dillsburg is located. Dillsburg is the site of the earthquake swarm that started in 2008.

Distribution of the Mesozoic rift basins in Pennsylvania and Virginia

Mineral, Va., is also located within the Central Virginia Seismic Zone. Although the area is the location of an earthquake once every year or two, we never hear about them because most of the tremors are local and do not attract regional attention. It’s why people in Mineral probably never hear about all of the earthquake activity in Dillsburg.

The last large earthquake to occur within the Central Virginia Seismic Zone was a 4.8 magnitude quake in 1875. The history of seismic activity within this area dates back to the 1760s.

Although a 5.8 magnitude event is large for the East Coast, this tremor would just hit the bottom of the “we should pay attention to this one” chart in California. And because of the underlying bedrock, according to the U.S. Geological Survey, an earthquake on the East Coast can be felt about ten times farther from an epicenter compared to a small earthquake on the West Coast.
That is why the “rock and roll” sensation was felt in so many states and Canada.

As I watched the daily database on the National Earthquake Information Center on vacation, I was not surprised to see additional small tremors occurring in Mineral. Think of this as the Earth’s crust settling down after a violent shake. Thus far 18 tremors ranging in magnitude between 2.0 and 4.5 have been felt. The 4.5 tremor was felt as far north as Baltimore. What was surprising was the fact that a 5.3 magnitude quake occurred about 12 hours earlier in Colorado; a 2.7 magnitude tremor occurred along the West Virginia – Virginia border northwest of Roanoke on Aug. 25, and a 3.1 tremor was recorded near Marietta, Ohio on Au. 31. Very interesting!

I have enjoyed hearing the stories from many people about shaking houses, rattling pots and pans, a moving computer screen and even a concrete patio that rolled back and forth. A friend who was picnicking on the ground at Western Maryland College actually felt the wave of the tremor go against the ground.  She was frightened, but if I were there, I would have thought it was cool.

I have included several seismograms (recordings produced by a seismograph) to illustrate how vibrations move through the Earth. I always use throwing a stone into a pond as an illustration. The stone would be at the epicenter of an earthquake, and the small wave that travels outwards in all directions would be seismic waves. The first wave to arrive at a seismograph is known as a “P” wave (or primary). This wave moves in a nearly straight line and is also the fastest-traveling wave. The second wave is an “S” wave, or secondary wave, and it moves similarly to a radio wave in an up-and-down motion. The third wave is the “surface wave,” and this was what residents of the eastern half of the United States felt.

Look at the seismograms and see if you can determine the different wave arrivals. The distance to a quake’s epicenter is determined by measuring the time difference between the P and S waves. Today, computers do the calculations, making the task much quicker. At least three seismic stations are required to achieve an accurate location. The closer the stations are to the epicenter, the more accurate the location is.

According to the National Earthquake Information Center, if the primary wave from the Mineral quake could have reached seismic stations around the world, below is a listing of locations and length of time (in minutes and seconds) it would take to reach that station:

Philadelphia, Pa. —   00:48

Brownsville, Texas — 4:38.7

Mexico City, Mexico — 5:35.4

Los Angeles, California — 6:33.3

London, England — 9:28.3

Wellington, New Zealand — 15:39.3

Numerous seismic networks have been established. The advantage of networks is that many stations are hooked into one master computer and, and the network is activated when one of the stations begins recording an event. For example, the Virginia tremor was recorded by the Southeastern Appalachian Cooperative Seismic Network, maintained by Virginia Tech, Memphis State University, Tennessee Valley Authority and University of North Carolina. In our
area, Millersville University’s station is a member of the Lamont-Doherty
Cooperative Seismographic Network monitored by Columbia University in
Palisades, New York.

Besides the National Earthquake Information Center, check out the Millersville University Seismological station at www.milersville.edu/esci/geology/seismograph.php, which includes real-time recording.

The Pennsylvania Geologic Survey (www.dcnr.state.pa.us/topogeo/index.aspx)  has information concerning earthquakes in the state.

The Virginia earthquake also shook our groundwater system.  Recordings of 27 wells in the state are found on the U.S. Geological Survey’s website at http://pa.water.usgs.gov.

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