Bolides

There was a bolide event on April 23, 2001 in the North Pacific that produced infrasonic waves that were observed at six separate infrasonic arrays located in Hawaii, Alaska and the continental US. The bolide event time and location were determined by LANL optical sensors on satellites to be: Latitude 27.9 N, Longitude 133.89 W, and 2001/04/23, 06:12:35 UT. An infrasonic signal from this event was observed in Fairbanks on the four microphone DOE infrasonic array at the University of Alaska. The DOE array at Fairbanks was modeled on the 4 sensor CTBT prototype triangular array with a fourth sensor in the middle with an approximate aperture of one kilometer.  The range and bearing from the DOE array to the bolide event are 4224 kilometers and 159.7 degrees east of north. Assuming celerity of 0.29 km/sec for propagation of the infrasound signal to Alaska the corresponding travel time of 4.046 hours would give an arrival time at the DOE array of 10:15:20 UT for the bolide signal. The bolide signal received at Fairbanks is shown in Figure 1 with the four individual microphone pressure traces (Ball, Cigo, Deer, and Apex) and the phase-aligned overlay of all four traces at the bottom.

The predicted time of arrival at 10:15:20 UT coincides with the maximum amplitude of the bolide signal wave train in Figure 1. The trace-velocity of the signal was estimated at 0.309 km/sec and the estimated azimuth of arrival was 151 deg. The average value of the cross-correlation for the six microphone pairs was 0.847. The data were Pure-State and bandpass filtered from (0.10 to 5.0 Hz). The tentative epicenter of the April 23rd bolide is shown in the map in Figure 2. The intersection of the six great-circle azimuth paths in the North Pacific gives the approximate location of the bolide epicenter using the infrasonic signal azimuths from six arrays.

There was an earlier bolide event on August 25, 2000 from which an infrasonic signal had been detected in Fairbanks by the DOE array.  Infrasound was also observed from the August 25th event by three other infrasonic arrays namely IS08, IS25 in South America and IS59 in Hawaii.  The bolide event time and location were determined by LANL optical sensors on satellites to be: 2000/08/25, 01:12:25 UT and Latitude 14.45 N, Longitude 106.13 W. The range and bearing from the DOE array to the bolide event are 6425 kilometers and 130 degrees east of north. Assuming celerity of 0.30 km/sec for propagation of the infrasound signal to Alaska the arrival time of the bolide signal would be 07:09:22 on Aug. 25th. The signal shown in Figure 3 is a portion of the bolide infrasound wave train from 07: 10:30 to 07:16:00 UT. The microphone pressure data were bandpass filtered from 0.3 to 0.8 Hz to eliminate the microbarom background. The data were additionally pure-state filtered to further enhance the bolide signal. The trace-velocity estimate was 0.358 km/sec, the azimuth estimate was 137.8 deg. and the average cross-correlation for all 6 microphone pairs was 0.837 for the bolide signal. Aug. 25th event.

In Figure 4 a bolide infrasonic signal is shown that was observed at the Windless Bight, Antarctica array I55US on September 3, 2004. The signal had very large peak-to peak amplitude of almost one Pa. The trace-velocity of the signal was 0.323 km/sec. The azimuth of arrival changed with time from 204 to 214 degrees during a period of 13 minutes starting at 15:54 UT. The maximum correlation was 0.897 over the I55US array.

Avalanches

Infrasound that is created by snow and ice avalanches on the glacier-hung peaks in the Alaska Range is frequently observed at I53US in Fairbanks. Mt. McKinley rises to an elevation of 20,300 feet to the southwest of Fairbanks at distance of 236 km at a bearing of 222 degrees. Avalanches on the great north face of McKinley drop 19,000 feet from the summit to the Peters glacier at base of the mountain. A very large infrasonic signal from an azimuth of 221 degrees that came from a McKinley avalanche at 10:18 UT on April 8, 2005 is shown in Figure 5. The peak-to-peak amplitude of the avalanche infrasound wave train was 0.9 Pa. The trace-velocity of the signal was 0.339 km/sec. A second signal was observed at I53US at 10:22 from the same azimuth as that of the 10:18 UT event.

A large rock and ice avalanche occurred 540 km southeast of Fairbanks, Alaska on September 14, 2005. According to the Alaska Earthquake Information Center in Fairbanks, approximately 50 million cubic meters of rock and ice plummeted 2.6 km from the south face to the base of 3100 meter Mt Steller. It is estimated that the debris run-out extended 9.5 km from the base of the mountain. This event is described by Kenneth M. Arnoult Jr. et al, “Infrasound Associated with Mt. Steller Avalanche”, in the on-line journal Inframatics, Number 12, December 2005 pages 4 to 7. The I53US infrasound signal from the Mt. Steller avalanche is shown for the time interval 20:30:06 to 20:42:42 UT in Figure 6. The trace-velocity for the signal is 0.354 km/sec, the azimuth of arrival is 150.4 deg and the MCCM correlation value is 0.914.

The mountains on Ross Island, to the north of the I55US array at Windless Bight, Antarctica, are a frequent source of avalanche infrasound. In particular there is an ice fall named Vee Cliffs on Mt. Terror 15 kilometers to the north-northeast of I55US that produces ice avalanches. In Figure 7 an example is shownof a large amplitude avalanche signal was observed at I55US from Vee Cliffs on April 6, 2004. The trace-velocity was 0.321 km/sec, the azimuth was 28.3 deg. and the correlation MCCM was