Analysis of laboratory simulations of volcanic hybrid earthquakes using empirical Green's functions

Abstract

[1] Here we present a new analysis of experimental simulations of the seismic signals characteristically observed in volcanic environments. We examine the waveforms of laboratory microseismic events generated during two rock deformation experiments performed on samples of Mt. Etna basalt to determine their source characteristics and establish evidence for a mode of failure. Events were recorded during deformation under (a), unsaturated (dry) conditions, and (b), samples saturated with water. We employ an empirical Green's function approach to isolate the acoustic emission event source spectra from attenuation and travel path effects, and estimate the spectral corner frequency using a least squares fit to a Brune spectral model. Spectral fits indicate that the acoustic emission events occurring under dry conditions follow the expected scaling of moment and corner frequency for standard brittle-failure in an elastic medium with constant stress drop, namely M0 ∝ fc−3. However, the events occurring during the fluid decompression phase of the saturated experiment have estimated corner frequencies which are not easily described by any simple scaling relationship. The implication of the observed scaling is that the events occurring under dry conditions must result from a standard stick-slip (i.e., brittle-failure) source. The observed moment-corner frequency scaling also suggests that event durations change in a predictable way with increasing moment for the events occurring under dry conditions. Conversely, events occurring under wet conditions do not show any distinctive relationship between duration and event size. The specific dependence of duration on event size exhibited by the events in the dry experiment must consequently rule out fluid-flow as a source, as there is no plausible reason for the driving pressure for fluid-flow to be dependent on duration in such a specific way. We compare laboratory observations of brittle-failure scaling (M0 ∝ fc−3) to previous observations of volcanic hybrid events in a field environment. Scaling dissimilarities between field observations and the wet laboratory events suggest that hybrid seismic signals observed in a volcanic environment do not always require fluid-flow to explain their signal.