Continuous Excitation of Earth’s Free Oscillations
 

By
Mustafa Yilmaz
Paroscientific, Inc.

Scientists who study earthquakes thought that Earth’s free oscillations were a transient phenomenon occurring after large earthquakes.  It has been discovered that the Earth freely oscillates at an observable level even in seismically inactive periods. Observed features of these oscillations suggest that their most likely source is atmospheric disturbances.  Scientists are studying this phenomenon as a coupled Earth-atmosphere system.

These atmospheric acoustic free oscillations must be excited persistently and there is evidence of resonance between the seismic free oscillations and atmospheric acoustic drive at around 0.0037 and 0.0044 Hz.  Although the resonant amplitudes of the seismic records suggest atmospheric excitation of the acoustic free oscillations, there had been no direct observations.

To detect these long-period acoustic waves, scientists from the Earthquake Research Institute of the University of Tokyo installed an instrumentation array that included Digiquartz® Broadband Barometers and Digiquartz® High Performance Pressure Ports to make these sensitive measurements.
 

These pressure signals are in the infrasound region of the acoustic frequency spectrum.  Digiquartz® Broadband barometers are capable of detecting pressure fluctuations in the atmosphere smaller than 1 microbar.

To reduce errors due to wind, the systems used Paroscientific’s High Performance Pressure port. The recording system featured long-term battery operation and 0.1 ppm clock timing over one year period.
 

The system is deployed as an array of 20 instruments over a distance of 10km north to south and 7 instruments over a distance of 4km east to west in a forest where no AC power supply is available.  The resolution of the measurements is 0.01, 0.1 and 1 ppm for a sampling rate of 10, 1 and 0.1 sec, respectively. The pressure data are recorded on a compact-flash of 256 MB that can store the data for more than 80 days. A pipe array system was developed to reduce the wind noise, which was very efficient at periods less than 100 seconds.

Acoustic waves traveling from the northwest (possibly from the mountainous region) were detected in the frequency range from 0.01 Hz to 0.1 Hz. A larger array size is necessary to detect the expected acoustic free oscillations at frequencies around 0.0037 and 0.0044 Hz.