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SEPTEMBER 25, 1998, PYMATUNING EARTHQUAKE
Origin time: 19:52:52 (UTC); 3:52:52 p.m. (EDT)
Latitude: 41.47N
Longitude: 80.48W
Magnitude: 5.0mb; 5.2Lg
Depth: 7.0 km
Geographic location: Between Jamestown, Mercer County, Pennsylvania, and the Ohio border, at the southern end of Pymatuning Reservoir; about 28 miles NE of Youngstown, Ohio.
A moderate earthquake centered near Jamestown, Pennsylvania, on the Mercer County-Crawford County (PA) line, at the southern end of Pymatuning Reservoir and just east of the Ohio border, shook a multistate area from Wisconsin to New Jersey late Friday afternoon, September 25, 1998. Shaking was greatest in Ohio, Pennsylvania, Michigan, New York, and Ontario. The event was recorded on seismographs as far away as Mongolia. Preliminary analysis suggests that the earthquake occurred along a NW-SE-trending fault. This trend is similar to known faults in Ohio located south and southwest of the epicentral area.
Damage reports are incomplete at this time, but preliminary information indicates that damage was light, consisting of broken dishes and a few damaged chimneys in the epicentral area. Dr. John Armbruster, of Lamont-Doherty Earth Observatory, reporting from the epicentral area, observed that the highest intensities were in Jamestown and Greenville, Pennsylvania. Chimney and plaster damage was noted in Greenville and merchandise was knocked off store shelves in Greenville and Jamestown. Maximum peak Modified Mercalli Intensity was VI. The National Earthquake Information Center of the U.S. Geological Survey, with the assistance of the Ohio Geological Survey, is compiling felt reports and will construct an isoseismal map of the event. The Ohio Geological Survey is soliciting reports from individuals who felt the earthquake: mike.hansen@dnr.state.oh.us. Please indicate your location during the earthquake (include zip code), the effects you felt or observed or heard, duration of the event, and damage (describe).
The U.S. Geological Survey, Memphis State University, and Lamont-Doherty Earth Observatory deployed 12 portable seismographs in the epicentral area by noon, Saturday, September 26, in order to record aftershocks: http://groundmotion.cr.usgs.gov/. At least some of these instruments will be deployed for about a month. The current epicentral location is probably accurate to only about 5 to 15 km because of the distance of seismographs from the event. On October 9, at 8:41 GMT, a small (2.0 magnitude) aftershock was recorded near Kennard, Mercer County, Pennsylvania, about 8 miles east of the mainshock epicenter.
The earthquake was recorded by seismic stations at the College of Wooster and at the University of Toledo in Ohio and at a number of out-of-state stations, including the University of Michigan. Analysis of the seismograms by Harvard University indicates that the most likely fault-plane solution is a northwest-southeast-oriented fault (strike, 303 degrees). Movement on the fault, according to analysis by Michigan State University, was a thrust with a small left-lateral component. Although no fault has been mapped in this epicentral area by either the Ohio or the Pennsylvania Geological Survey, this orientation is similar to that of a series of subsurface faults to the south in Columbiana, Mahoning, and Portage Counties, Ohio, and to the north in Crawford County, Pennsylvania. Motion on the fault is still being analyzed.
Three small earthquakes were previously known from the general epicentral area of the September 25, 1998 event. A small earthquake with a Modified Mercalli intensity (MMI) of III occurred on August 17, 1873, and was assigned an epicenter at Sharon, Mercer County, Pennsylvania. Another historic event occurred on August 26, 1936, at Greenville, Mercer County, Pennsylvania, and had an MMI of III. An instrumentally located event, having a magnitude of 3.2 occurred on April 14, 1985, at Conneaut Lake, in Crawford County, Pennsylvania. These minor historic events did not suggest that this area of northwestern Pennsylvania was capable of generating a magnitude-5 earthquake.
The Pymatuning earthquake will be the subject of additional study in the coming weeks as seismograph records are analyzed, felt-area reports are compiled, and basement-rock structure is studied. Each event such as this one provides valuable information on crustal structures beneath Ohio and adjacent areas and further increases our ability to evaluate seismic risk and deep geologic framework.
Links with information about the Pymatuning earthquake
Seismogram of the Pymatuning earthquake recorded at seismic station at the College of Wooster. Seismogram image courtesy of Dr. Robert Varga, Department of Geology, College of Wooster, Wooster, Ohio.
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