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Earthquakes shake Dillsburg second time
Thursday November 05 2009
This is a seismograph showing the Oct. 25 tremor that shook Dillsburg in York County. Some residents thought they were hearing explosions. The earthquake is not the first one of its kind in the area; was another documented Oct. 5.
Seismologists who studied these tremors report that the first quake reached a magnitude of 2.8, the second 1.8 and the third 2.6 on the Richter scale. The first earthquake hit the small town in York County at approximately 7:20 a.m. Sunday.
Residents of Dillsburg are almost used to the earthquakes in their area — in fact, they have experienced about 600 small earthquakes over the last year, and researchers recently released a 20-page report examining why earthquakes are so common in that particular area.
The 2008-2009 Earthquake Swarm Near Dillsburg, Pennsylvania report, compiled by the Commonwealth of Pennsylvania, the Department of Conservation and Natural Resources, and the Bureau of Topographic and Geological Survey takes an in-depth look at the history of earthquake occurrences in Dillsburg in the hopes that some logic might be drawn from the information collected.
The report gathered information from Oct. 2008 through April 2009 during which a micro swarm of over 600 earthquakes occurred in the Dillsburg area.
The swarm itself included 23 larger shocks with magnitudes ranging from 0.8 to 2.9 that were detected by seismograph stations in the mid-Atlantic region. Oct. 24, the Lamont-Doherty Earth Observatory of Columbia University installed a network of portable seismographs around the area of Old York Road and Mandy Lane, which, according to reports from residents, seemed to be the center of the earthquake activity.
The portable seismographs operated until Dec. 11 and recorded 49 other small earthquakes in the area.
However, only one of the 49 earthquakes detected was large enough to be graphed by the regional network of permanent stations. The 49 micro earthquakes were distributed along an approximately 1.5 km (or 0.93 miles) line trending northwest-southeast along Old York Road.
The hypocenters for these shocks measured less than 1 km (0.62 miles), with an average depth of about 0.5 km (0.3 miles), causing a booming sound and noticeable shaking despite their small magnitudes. Even with their frequent occurrences, no significant damage from these earthquakes has been reported; the quakes themselves seem to be coming from a small amount of rock as opposed to a well-defined fault plane.
According to the report, their depth corresponds roughly to the base of a thick sheet of igneous rock that underlies the area and is thought to be the source of the iron ore deposits that were formerly mined there. The focal depths of the earthquakes are smaller than what the researchers would have expected if the old mines had a direct role in the origin of the earthquakes, but at this point researchers cannot rule out a possible correlation.
The contrast in strength between the diabase and underlying sedimentary rocks, according to the report, may have resulted in a concentration of stress along the contact between the two rock types.
The seismic activity also may be localized along this contact due to the presence of pre-existing zones of weakness, or fractures, in the rock.
However, researchers are unclear as to the exact cause of the earthquake swarm or the reason why it began when it did.
Often, Mother Nature acts in rather mysterious ways, and in this case Dillsburg is witnessing real evidence of that enigma.
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