Earthquake infrasonic-microphone signals can be generated by several mechanisms. The first is the response of the sensors to the ground motion induced by seismic waves. In this case vertical motions of the sensor in a stratified atmosphere can produce signals from the microphones as the surface Rayleigh waves propagate across an infrasonic array. This is not an actual infrasonic signal. The second is the initiation of infrasonic waves by the ground motion near an earthquake’s epicenter.  Studies of infrasonic waves associated with the great Alaskan earthquakes of March 1964 and November 2002 have found that acoustic waves could be launched by movement of the ground as seismic waves propagate through a region. Strong evidence that infrasonic waves could be generated by secondary sources associated with motion of the seismic rupture along an earthquake fault has been found at I53US from a 7.9 magnitude earthquake on Nov. 3, 2002 in the Alaska Range.   

The great Sumatra earthquake at 00:58:50 UT on December 26, 2004 produced a powerful set of seismic Rayleigh waves that were observed on the infrasonic arrays at both I53US in Fairbanks and I55US in Antarctica. The epicenter of this earthquake was 9940 km from I55US at a bearing of 289 degrees and 10,087 km from I53 at a bearing of 295 degrees. The Rayleigh waves detected at I53US and I55US from the great Sumatra earthquake epicenter on December 27, 2004 are shown below in Figures 1 and 2 below.

The seismic Rayleigh waves sweep across the infrasonic microphone array at I53US at ten times the speed of sound. Because of the high speed of Rayleigh waves  there will be  very large uncertainties in the estimates of trace-velocity and azimuth of arrival for an infrasonic array . For a velocity of 3.0 km/sec, as associated with Rayleigh waves propagating through rock, the uncertainty in trace-velocity would be +/- 0.93 km/sec and the uncertainty in azimuth would be +/- 16 degrees. The celerity for the Sumatra Rayleigh waves, as determined by the great circle distance and their arrival time on the USGS seismometers at Fairbanks, was 3.00 km/sec. The trace -velocity shown in Figure 1 for I53US of 3.53 km/sec and the azimuth of arrival of 322 degrees are both within the calculated uncertainties.

The Rayleigh wave speed in ice at I55US on the Ross Ice Shelf is much lower at 1.70 km/sec than that for rock. Thus the trace-velocity shown in Figure 2 for the Rayleigh wave at I55US is understandingly much lower at 1.48 km/sec. The celerity for the Sumatra Rayleigh wave signal, as determined by its arrival time on the infrasonic microphones at I55US, was calculated to be 3.51 km sec. For a wave speed across the I55US array of 1.70 km/sec the uncertainties are: for trace-velocity +/- 0.420km/sec and for azimuth +/- 14 degrees.

A good example can be seen in Figure 3 below of infrasound observed at I53US that was generated by the ground motion in the epicenter region of a 6.7 magnitude earthquake in the Alaska Range that occurred on October 23, 2002. The earthquake infrasonic waves arrived about 9 minutes after the seismic Rayleigh waves swept across the array at 11:28 UT. The trace-velocity of the earthquake infrasound was 0.340 km/sec from an azimuth of 190 degrees which is the direction of the epicenter as seen from I53US. The peak-to-peak amplitude of this earthquake infrasound was about 1.0 Pascal.

The great Alaskan earthquake  of magnitude 7.9 that occurred on the Denali-Totschunda fault on November 3, 2002 produced the largest infrasonic signal from a natural source that has ever been observed at I53US during its four years  course  of operation. According to the USGS report: “The M7.9 shock, one of the largest ever recorded on U.S. soil, occurred on the Denali-Totschunda fault system, which is one of the longest strike-slip fault systems in the world. The fault rupture of the 7.9 shock initiated about 25 km east of  the M6.7 Oct. 23 foreshock and continued eastward and southeastward for about 300 k. The north side of the Denali fault moved to the east and vertically up relative to the south side. Preliminary measurements of fault displacements in the field by geologists range from under a meter in some locations in the west to nearly 9 meters near Mentasta Lake.” The sudden displacement by many meters of high mountains on the north side of the Denali fault eastward during the 7.9 earthquake produced a  very large amplitude infrasound wave train lasting tens of minutes that was observed at I53US on November 3, 2002 as shown in Figure 4 below.  

The map of interior Alaska showing the location of Fairbanks relative to the Denali fault (red line) in the Alaska Range to the south is shown in Figure 5. The epicenter of the earthquake is shown by the red star at the west end of the fault line. The green lines show the progressive change in the infrasound wave train azimuth from 170 deg to 127 deg. over the time period 22:27 to 22:35 UT as the fault rupture propagated eastward along the fault. This eastward progressive change in the azimuth indicates that the source of the infrasound is the motion of the mountains at the location of the fault rupture as it moves eastward along the fault.

A plot is given in Figure 6 of the azimuth of arrival of the earthquake infrasound as a function of time. The solid line in the plot is the least-squares fit to the infrasonic arrivals between 22:27 and 22:35 UT. The rate of change of the azimuth is approximately 2 degrees /minute. The azimuth estimates were based on a six minute sliding window. The association of the direction of arrival of infrasonic waves with particular regions along the Denali fault comes through the agreement between azimuth and time-of-flight calculations as well as that of good a correlation between the amplitudes of the infrasonic waves and the measured amplitudes of the ground motion at the source regions along the fault.

Alaska is one of the most seismically active regions in the world and therefore Fairbanks is an excellent location for I53US for the detection and study of earthquake infrasound. The Alaska Earthquake Information Center is located at UAF in the Geophysical Institute building wherein the I53US array operation hub also resides.