Source Parameters of Laboratory Acoustic Emission Events Estimated From the Coda of Waveforms

Abstract

AbstractWe develop a method to estimate relative seismic moments M0 and corner frequencies fc of acoustic emission events recorded in laboratory experiments from amplitude spectra of signal's coda composed of reverberated and scattered waves. This approach has several advantages with respect to estimations from direct waves that are often clipped and also are difficult to separate in experiments performed on small samples. Also, inversion of the coda spectra does not require information about the source locations and mechanisms. We use the developed method to analyze the data of two experiments: (a) on granite from the Voronezh crystal massif and (b) on Berea sandstone. The range of absolute corner frequencies estimated in both experiments is around 70 − −700 kHz. The range of relative seismic moments covers 103.5. The relation between fc and M0 observed on the first stages of both experiments, consisted of increasing isotropic confining pressure, approximately follow scaling and the b‐value of the Gutenberg‐Richter distribution was found close to 1. This can be interpreted as rupturing of preexisting material defects with a nearly constant stress‐drop and has a similarity with observations of “natural” earthquakes. Deviations from this “earthquake‐like” behavior observed after applying axial loading and initiation of sample damaging can be interpreted as changes in stress‐drop. Lower stress‐drops prevail for sandstone and higher for granite sample respectively that can be related to the strength of corresponding material.