Acoustic Emission/Microseismic Activity at Very Low Strain Levels

Abstract

Laboratory experiments have shown that strain levels as low as one microstrain in competent rock samples produce detectable acoustic emissions (AE) in the frequency range from 100 to 300 kHz. Naturally occurring strain and strain rates have similar levels to those at which laboratory AE was detected. In the buildup of a strain field over the “preparation zone” of an impending earthquake, strain in collapsing pores, grain boundary slippage, and other microscopic ruptures may cause high frequency AE. However, this AE due to tectonic strain must be distinguished from AE from other sources such as air pressure changes, thermal expansion/contraction, earth tides, and cultural activities. Sufficient AE at very low strain levels, and detectable above noise backgrounds, might provide a useful precursor to earthquakes.