Accidental Explosions Observed by Seismometers

A large explosion or shock in the air may excite ground motion that can be observed by seismometers. For example, early U.S. nuclear tests in the atmosphere were detected by seismometers of the USSR Academy of Sciences (Cherepanov, 1996), and a broadband seismometer recorded a terrorist bomb attack in Kenya (Koper et al. , 1999). Ground motions excited by shock waves from a meteorite and space shuttle were recorded in Japan and California, respectively (Nagasawa and Miura, 1987; Ishihara et al. , 2001; Kanamori et al. , 1991). An accidental explosion occurred on the evening of 10 June 2000 JST (Japan Standard Time, GMT+9 hours) at a chemical plant in Ojima, Gunma Prefecture, 80 km north of Tokyo, Japan. The explosion killed four employees and injured 27 people, most of whom were residents around the plant hurt by flying shards of glass ( Japan Times , 2000a). The ground motions excited by the explosion were recorded by a strong-motion instrument installed at the Ojima town office about three hundred meters away from the plant, and also by high-sensitivity seismometers at distances of several tens of kilometers. We can learn important lessons from this disaster if we can make precise quantitative determinations of certain variables. The origin time is one of the most basic unknown variables. The magnitude of the explosion is also required for estimating the scale of resultant disaster. However, neither of these was initially well known. In this study we analyze the seismic records to investigate the characteristics of the explosion including these variables. In addition, we will discuss seismic features of the chemical explosion by comparing them with seismic records of a gunpowder explosion in August 2000. The Nisshin Chemical Co. plant is located near the intersection of National Highway Route 17 from Tokyo and …