By Margie Wuebker
Some residents of northern Mercer County felt the earth shake and heard a loud boom Saturday night -- telltale signs that have been attributed to the second small earthquake to strike the Rockford area in less than 14 months.
Michael Hanson, coordinator of the Ohio Seismic Network, said calculations indicate the 2.1 magnitude activity came from an area three miles southwest of Rockford.
"We are still awaiting data from Indiana stations in hopes of determining the exact location," Hanson told The Daily Standard this morning. "The epicenter is believed to be south of state Route 707 and east of Wabash Road."
An earthquake registering 2.5 on the Richter Scale struck Jan. 30, 2004, at the northwest corner of state Route 118 and Purdy Road, approximately one mile south of Rockford.
Rockford Police Chief Paul May was relaxing in an easy chair at home when he heard a loud boom at 11:02 p.m. followed by a brief tremor that rattled some belongings. "I thought a semi had blown up," May said. "I immediately went to the front door expecting to smell smoke. Fortunately, there was nothing to see."
The police department received one call from a concerned citizen, and Mercer County Central Dispatch recorded three calls from people reporting a loud noise and/or rumbling that shook their homes.
There was no damage reported anywhere. Damage generally does not show up until an earthquake reaches a magnitude of 4.5.
Daily Standard reporter Nancy Allen heard the loud boom and then felt the slight tremor that was sufficient to rattle knick knacks in her Rockford-area home. She dismissed initial thoughts of an earthquake, attributing the noise and movement to a strong clap of thunder. She had heard a rumble reminiscent of thunder while outdoors earlier in the evening.
Duane Hamrick, who lives near the suspected epicenter, listened to several reports on the police scanner. He experienced none of the things the callers did.
The seismic activity showed up most notably at the nearest recording station -- Wright State University-Lake Campus along state Route 703, east of Celina. It also registered on equipment at stations in Botkins (Shelby County) and Bowling Green (Wood County). Telltale waves also registered at the Ohio Geological Survey headquarters at Alum Creek State Park near Delaware.
Hansen received a telephone call from May on Monday morning and labeled the reported phenomena an earthquake within minutes upon checking the seismic equipment. His office also received two e-mail reports from area residents.
Hansen believes amplification of the earthquake's P and S caused people to notice what was taking place. Fast-moving P waves cause booming noises as bedrock moves beneath the earth's surface. Secondary S waves cause the shaking.
Hanson explained earthquakes have occurred more than 40 times since 1875 along a fault known as the Anna Seismic Zone, which includes Champaign, Shelby, Auglaize and Mercer counties before extending toward Fort Wayne, Ind.
An earthquake registering 4.5 seismic activity caused damage in the St. Marys area in 1986. Prior to 2004, the last earthquake in Mercer County measuring a 3.3 magnitude caused sidewalks to crack in the Celina area in 1977.
Anyone wishing to record what they heard or felt during the latest earthquake should log onto the Ohio Seismic Network at www.ohiodnr.com/OhioSeis.