Abstract

Seismic signals recorded every 0.1 sec by four OhioSeis (state network) seismic stations (in Cincinnati, Portsmouth, Columbus, and Bowling Green, Ohio) for 2 hr before and 30 min after a destructive F4 tornado hit a suburb of Cincinnati (Blue Ash) at approximately 0500 hours Eastern Standard Time on 9 April 1999 were subjected to wavelet-packet transformation analysis and transient-frequency analysis with a Wigner distribution. In the University of Cincinnati station data (12 km from Blue Ash), we found a high-intensity, low-frequency (0.05–0.128 Hz) signal during the reported touchdown and also the loss of all but the lowest-intensity, higher-frequency signals recorded by the seismometer (maximum of 1.5 Hz) as a possible precursor of the tornado approximately 6 min before it hit the ground. This higher-frequency loss continued until 14 min after the tornado touchdown. In the Portsmouth, Columbus, and Bowling Green station signals, we found that the highest correlated matches with the University of Cincinnati station tornado signal occurred at respective arrival times that agreed with an approximately 1.8 km/sec propagation velocity for this low-frequency seismic wave. The seismic record from a second F4 tornado in Xenia, Ohio, which occurred on 20 September 2000, was similar to the Blue Ash tornado record, as recorded by the same University of Cincinnati station, at an 80-km distance from Xenia.

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