Physical constraints on sounds generated by very small earthquakes

Abstract

Vertical vibrations of the ground surface due to elastic waves from an earthquake will generate acoustic pressure waves in the overlying atmosphere. Sufficiently intense vibrations of the ground at frequencies greater than about 20 HZ can produce pressure waves that are within the audible range of many animals, including humans. People in the epicentral region of moderate to large earthquakes, for instance, commonly relate hearing sounds accompanying individual earthquakes (here we exclude sounds associated with mechanical resonances in structures such as houses). Descriptions of these sounds range from the report of distant guns to the rumble of thunder or the rushing of a wind (Davison, 1938; Lanchow Seismological Brigade, Appendix II). In many instances, the sounds are described as starting several seconds before the earthquake is felt. Experimental and theoretical verification that such earthquake sounds are generated by high-frequency ground motion from local earthquakes is based on simultaneous acoustic and seismic recordings recently obtained during a swarm of earthquakes in the Imperial Valley, California (Hill and others, 1976). These results show that for local earthquakes in the magnitude range 2 to-3, audible sounds are generated by the first arriving compressional (P) wave while perceptible shaking begins with the larger but slower shear (S) wave, explaining the reports that "earthquake sounds" are frequently heard several seconds before the earthquake is felt. A commonly proposed explanation for alleged anomalous animal behavior hours to days prior to large earthquakes suggests that animals with acute, high-frequency hearing respond to weak '''ultrasonic" sounds generated by small earthquakes occurring in the epicentral region of the impending event. Such earthquakes are presumably too small to be either felt or heard by humans in the region and too small to be routinely recognized as earthquakes on seismograms recorded on a local seismograph network. Evidence that such "popping and cracking" may occur in the region of an impending earthquake comes from laboratory experiments in which the rate of microfracturing in a rock sample subjected to large stress differences increases dramatically before catastrophic failure of the sample (Scholz, 1968; Stesky, 1975). In this paper we investigate the following question: What are the constraints on earthquake size (fault dimension and displacement) and hypocentral distance (distance from the earthquake focus to a point on the Earth's surface) such that animals noted for their acute hearing might react to the sound generated by the event while the same event would go undetected by humans (either by feeling, hearing, or routine identification on a local seismograph network)? Factors to be considered include: 1) the nature of the displacement spectral amplitudes of elastic waves radiated from an earthquake, 2) the effects of attenuation on the spectral components of elastic (seismic) waves over propagation paths from the focus to the Earth's surface, 3) the appropriate scaling of spectral amplitudes from moderate to very small earthquakes, 4) the coupling of elastic waves in the Earth to acoustic waves in the atmosphere, and 5) the threshold-of-hearing characteristics for various animals, including man. In considering these factors, we will find that, except for extremely small, shallow "earthquakes" (fracture dimension on the order of 10 cm and focal depths on the order of 10 m), acoustic emissions from premonitory fracturing ought to be equally audible to both people and animals in the immediate area of the fracturing.