Source parameters of laboratory acoustic emission events estimated from
the coda of waveforms
Abstract
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 ans mechanisms. We use the
developed method to analyze the data of two experiments: (1) on granite
from the Voronezh crystal massif and (2) 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 ~ fc-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.