|The USGS geologists are compiling their information on a new website: http://groundmotion.cr.usgs.gov/pym/pym.htm The site includes a map of the area that shows the locations of all 15 seismographs.|
By Erin Remai
Herald Staff Writer
The aftershocks have not hit yet, but if they do, scientists will be ready.
Geologists from Columbia University, the University of Memphis, and the United States Geological Survey in Golden, Co., have placed 12 portable seismographs within 15 miles of the earthquake's epicenter near Jamestown. If an aftershock hits, geologists will be able to pinpoint the quake's origin within a couple hundred feet.
Geophysicist Edward Cranswick said the nearest station recording Friday's quake is about 150 miles away, making it difficult to locate an exact epicenter.
The geologists think the tremor's source was 5 kilometers below the surface.
"We want to understand the main shock and find the location and causes," Cranswick said Wednesday. "We're also studying why some areas had more damage than others."
Geologist Jonathan Cox explained: Valleys filled with glacial till, or sediments, make up much of Mercer County. He said the areas made up of sediments were much more likely to feel the shaking than areas located over bedrock.
"Geologists call it site response," Cox said. "This explains why you would feel the shaking at your house, but your neighbor half a mile away doesn't."
Mark Meremonte, a geophysicist with the United States Geological Survey, checks the seismograph in Jamestown to see if it recorded any aftershocks. The USGS, the University of Memphis, and Columbia University have placed 12 seismographs around the area believed to be the earthquake's epicenter in West Salem township. (Jean Neice/Herald)
"We want to have some idea of the main shock location, and we want to record the aftershocks really well," Cranswick said. "We need a good distance all the way around where the earthquakes might occur."
All of the sites the scientists chose for the seismographs are quiet and set back from main roads. They also looked for areas without trees so the seismographs' antennas can communicate with a satellite in geosynchronous orbit.
Geophysicist Mark Meremonte has been visiting each site once a day to see if the seismometers recorded any aftershocks. The computer downloads data collected over 20 hours, then Meremonte takes the data back to the motel to process it.
So far, the only activities the seismographs recorded have been traffic and people walking.
"Having none to few aftershocks is characteristic of earthquakes in the East."
If an aftershock does occur, Meremonte said, the scientists would reconfigure the network to study how much the ground moves in different places. Studies like these are used to design buildings in earthquake-prone areas.
Meremonte said Mercer County has about a 2 percent to 4 percent chance of having a catastrophic earthquake in the next 50 years.
"In the past history of earthquakes here, none were damaging," Meremonte said.
Some faults extend through this area beneath about 1.5 miles of sediment deposited by water erosion, Meremonte said. A possible cause of the earthquake could have been stresses from the Mid Atlantic plate moving west and reactivating existing faults. The Mid Atlantic plate meets the Eurasian plate at the Mid Atlantic ridge. Meremonte explained the ocean floor constantly grows at the Mid Atlantic ridge, where magma comes out of the earth between the two plates and pushes them outward.
Another cause could be the earth rebounding after being compressed by glaciers, Meremonte said. He compared that to pushing on a sponge and watching it rise again.