Infrasonic observations of bolides on October 4, 1996

During the evening of October 3, 1996, at least 6 bright fireballs were observed over the western US with reports from California to Louisiana. The event over California produced tremendous sonic boom reports in the Los Angeles area. This event was also detected locally by 31 seismometers which are part of a network of seismic stations operated by the California Institute of Technology. Subsequent investigations of the data from the four infrasound arrays used by LANL (Los Alamos National Laboratory) and operated for the DOE (Department of Energy) as a part of the CTBT Program (Comprehensive Test Ban Treaty) Research and Development program showed the presence of an infrasonic signal from the proper direction at the correct time for this bolide from two of the four arrays (Nevada Test Site; NTS and Pinedale, WY; PDL). Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere, having its epicenter near Little Lake, CA for possible sources heights from 40--60 km. The infrasonic arrays are each composed of fur elements, i.e., low frequency pressure sensors that are in near-continuous operation. The nominal spacing between elements is 150--200 m depending on the specific site. The basic sensor is a Globe Universal Sciences Model 100C microphone whose amplitude response is flat from 0.1 to 300 Hz. Each sensor is connected to 12 porous hoses which act to reduce wind noise. The signal characteristics, analyzed from 0.1 to 5.0 Hz, includes a total duration of 5 (NTS) to 20 minutes (PDL) for a source directed toward 230--240 degrees from true North. The signal trace velocities ranged from 300--360 m/sec with a signal velocity of 0.30 {+-} 0.03 km/sec, implying a Stratospheric (S Type) ducted path (with a reflection altitude of from 40--60 km). The dominant signal frequency is from 0.20 to 0.80 Hz, with a peak near 0.2 to 0.25 Hz. These highly correlated signals had a maximum amplitude of 1.0 microbars (0.1 Pa) at PDL and 4.0 microbars (0.4 Pa) at NTS. The analysis indicates that the bolide had a probable, maximum source energy in the range from 150--390 tons (TNT equivalent)