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

We 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 M0 similar to fc-3 ${M}_{0}\sim {f}_{c}<^>{-3}$ 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. Earthquakes generation mechanisms and conditions favoring their occurrence are still debated. Inability to observe these processes in-situ and long lasting earthquake preparation period favor using laboratory experiments to verify quickly the adequacy of proposed hypotheses. Fracturing of small rock samples with high pressures and recording acoustic waves from their micro-fractures is among them. In most cases, the laboratory acoustic emission (AE) is analyzed and compared to natural seismicity statistically, demonstrating similar Gutenberg-Richter power-law magnitude distribution. More advanced analyses can include source characteristics (corner frequencies, seismic moments, and stress-drops), responsible for the source size, forces acting there and stress changes. Ensembles of these characteristics can give ideas on the common generation mechanisms. In laboratory, several technical limitations slow down the implementation of such analyses, widely used in Earth's seismology. We propose a method that use coda waves (signal's decaying part) to estimate source parameters of the laboratory AE. We tested it on two similar experiments conducted on different rock types. Source analyses revealed the high similarity of well-studied tectonic earthquakes and fracturing of pre-existing inhomogeneties in the rock samples by applying equally distributed external pressure to it. The active production of new fractures under high one-directional pressure in contrary deviated significantly. Coda of acoustic emission (AE) waveforms can be used for the source characterization Scaling between the corner frequency fc and seismic moment M0 varies in function of loading regime and rock type The M0 - fc scaling of AE events was found similar to that of tectonic earthquakes when isotropic confining pressure applied to intact rocks