A shakeup in Dillsburg

Posted by on February 1, 2011 in Geologic Hazards | 0 comments

The epicenter area with Blair Mountain in the background

If I were asked what one of the most exciting geological projects I did in York County was, I would not take long to answer. The Dillsburg earthquake swarm has been interesting in two respects: the human interest story, and the scientific end.

Why do we call the tremors a swarm?  A swarm is a series of tremors that doesn’t have a pattern, such as foreshocks, a main quake followed by aftershocks.  Swarms contain many tremors, mostly low in magnitude on the Richter scale.  A swarm disappears as mysteriously as they appeared.

After research was done concerning East Coast earthquake swarms, most appear to only have duration of six to nine months.  To date, the Dillsburg swarm continues today, a total of 27 months.

After the initial 2.0 magnitude tremor on October 5, 2008, I went to the Dillsburg area for interviews with residents.  With no seismic equipment in Dillsburg (the closest seismograph is about 25 miles away), the accuracy of pinpointing the epicenter was about five to six kilometers.  Collecting as much information as possible from residents might give the research team an idea of the location of epicenter.  It actually is a pretty basic process — the residents who offer the bigger reactions are closer to the origin of the earthquake.

The location of the Dillsburg magnetite mines one mile east of Dillsburg

Because I had done considerable research on the 19th century iron mines east of Dillsburg in the 1980’s, I was familiar with the “neighborhood.”  Also, on my way to Dillsburg for the first tremor investigation, I was thinking the event could have been related to a mine collapse. I would be talking to these residents one time and that would be it. Little did I know that these interviews would lead to a new network of friends.  Once the information was gathered from the residents, a map was compiled showing the zone of greatest reactions, thus marking the possible epicenter — not very scientific, but logical.

Isoseismal map showing the believed epicenter area of the Oct 5, 2008 2.0 tremor

An example of how residents supplied information to the research team.

Many of the larger tremors in the fall of 2008 occurred in the early morning or late evening.  Reactions from residents ranged from scared to “this is pretty cool.” Many wondered if the “big one” was coming and asked what was next.  Because many of the tremors were accompanied by a “boom” sound, folks began to think these were explosions of some sort, which added to the insecure feeling. Many of the telephone calls to the York County 911 Center were “outside disturbance” calls.  Thanks to the police log records, this information also contributed to our research.

Reported tremors were plotted in an attempt to detect a pattern,

With the unrest of some of the citizens in Carroll Township and Dillsburg, the township Board of Supervisors contacted the research team to hold a “Community Meeting.” The purpose of the meeting was to relay to citizens what we knew at this point (not much at this point, only one month into the swarm). Dr. Charles Scharnberger, a close friend of mine and Professor Emeritus at Millersville University and Director of the Seismograph at the school was invited to attend the meeting to give a history of East Coast earthquakes.

With the media presence already intense in Dillsburg due to the rarity of an earthquake swarm occurring, nearly 150 residents and many media outlets attended the meeting. We believe the information we provided “smoothed the waters” for many residents. After more data was gathered from scientific equipment and the situation became a little more clear, another community meeting was held in September 2009. About 200 people gathered at the Northern High School for that presentation.

Without the glut of residents, government officials and the media, we would not have been able to gather as much information as we have. Residents continue to either call or e-mail me about tremors. To date, 957 events have been reported to me.  Carroll Township placed our contact information on their website so the resident database could grow.  The more information, the better!

What made Dillsburg unique in one way was that some of the tremors occurred in “bunches.”  That is, a number of tremors within a short period of time.  For example, early in the morning of December 31, 2008, 24 tremors were reported within three hours.  Booms followed booms, sometimes only seconds apart.  The Earth’s clock was off by 24 hours, thinking it was the New Year’s.

During the winter of 2009, while an ice-covered snow pack was on the ground in the Dillsburg area, a tremor occurred, cracking the surface ice. Residents heard and even recorded the cracking of the ice. One of the mine shafts within the mine district opened up, either as a result of the tremors or the heavy rainfall, or a combination.

There are some really innovative people in Carroll and Monaghan townships.  One resident constructed his own sand-pit-plumb bob seismograph, another resident is known the “Human Seismograph,” and then we had Seismic Sam.  Seismic Sam was a horse whose mate would chew on his mane, often prior to a tremor. Seismic Sam passed away in 2010. A number of animal stories have been told to the research team.

Cracks in ice from a winter 2009 tremor.

A mine shaft uncovered during the winter of 2009.

Dr. Bill Kregier of York College and his son Jason conducted their own interview process and joined forces with the research team.

One of the portable seismographs installed in the Dillsburg area.

In the fall of 2008, Columbia University brought and installed a four-seismograph portable network in the area. A similar network was placed in the area during the summer of 2009 to gather additional data — again thanks to the residents who allowed these instruments to be installed on their property.  A resident of Carroll Township also sponsored the second network of seismographs.

These networks very accurately plotted the epicenters, but that information will appear in a future post.

CRacks in ice from a winter, 2009 tremor

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