Seismic Analysis of the 7 January 1998 Chemical Plant Explosion at

An accident at the Sierra Chemical Company Kean Canyon plant, 16 krn east of Reno, Nevada, resulted in two explosions 3.52 sec apart that devastated the facility. An investigation into a possible cause for the accident required the determination of the chronological order of the explosions. We resolved the highprecision relative locations and chronology of the explosions using a cross-correlation method applied to both seismic and air waves. The difference in relative arrival times of air waves between the explosions indicated that the first explosion occurred at the northern site. We then determined two station centroid separations between explosions, which average about 73 m with uncertainties ranging from _ 17 to 41 m depending on the alignment of station pairs. We estimated a centroid separation of 80 m using P waves with a larger uncertainty of + 340 m. We performed a grid search for an optimal separation and the azimuth by combining air-wave arrivals from three station pairs. The best solution for the relative location of the second explosion is 73.2 m $35°E from the first explosion. This estimate is well within the uncertainties of the survey by the U.S. Chemical Safety and Hazard Investigation Board (CSB). The CSB reported a separation of approximately 76.2 m $33°E. The spectral amplitudes of P waves are 3 to 4 times higher for the second explosion relative to the first explosion, but the air waves have similar spectral amplitudes. We suggest that this difference is due to the partitioning of energy between the ground and air caused by downward directivity at the southern explosion, and upward directivity at the northern explosion. This is consistent with the absence of a crater for the first explosion and a 1.8-m-deep crater for the second explosion. Introduction On the morning of January 7, 1998, two explosions occurred 3.52 sec apart at the Sierra Chemical Company facility, 16 km east of Reno, Nevada. Unfortunately, four people lost their lives and six more individuals were injured in the explosions that devastated the facility. The U.S. Chemical Safety and Hazard Investigation Board (CSB), with the cooperation of several state agencies, initiated an investigation into the cause of the accident. The chronological order of the explosions is an important aspect of the investigation because it may help determine the cause of the accident. The CSB requested that the Seismological Laboratory independently estimate the chronology based on airand seismicwave arrival times from seismograms to compare with other results obtained by the CSB. The two explosions were recorded at several stations of the western Great Basin seismic network and by three-component, telemetered, digital broadband stations (Fig. 1). From these records, the estimated origin time of the first explosion is 15 hr 54 min 03.0 sec ___ 0.3 sec UTC (7:54 a.m. local time), with an approximate location of 39°31.8'N, 119°38.1 'W (--1.5 kin). However, based on a survey by the CSB, the location of the northern crater's center is 39°32.5'N, 119°38.1'W (John Piatt, personal communication). When treated as an earthquake, the coda magnitude of the largest explosion is estimated to be Md 2. An examination of the seismograms in Figure 2a show that there are two similar-shaped P-wave arrivals (P1 and P2) at station PAH separated in time by about 3.5 sec. This is followed by two similar-shaped N waves (N1 and N2) also separated in time by about 3.5 sec and arriving approximately 70 sec after the P waves. The N wave is created by the shock wave traveling in air (Kanamori et aL, 1991). The identification of these phases was based on the phase arrivals, which do not exhibit a distance moveout expected for reflected air or seismic waves. The coherency between P and N waves also indicates two explosions at the same location. The P2 phase in Figure 2a appears to be an S wave of the initial explosion but is actually the P-wave arrival from the second explosion, which is nearly coincident with the expected S-wave arrival from the initial explosion at both