Analysis of Micro-Seismic Signals and Source Parameters of Eruptions Generated by Rapid Decompression of Volcanic Rocks

Seismic evaluation of well-controlled experimental volcanic simulations offers the hope of a better understanding of source mechanisms in natural volcanic seismicity. Here, we have performed the first investigation of the dynamics of explosive volcanic eruption of magma under controlled laboratory conditions. Specifically, we analyzed the micro-seismicity generated by the rapid depressurization of volcanic rocks in a shock tube apparatus, which represents the seismic mechanism. The source parameters and the force system have been analyzed considering the relationship F=πr2 Po. Our well-constrained physical mechanism consists of the slow pressurization of the system (using Argon gas) followed by rapid depressurization of natural volcanic samples (ash, pumice and fragmented particles of pumice) contained in a steel pipe-like conduit of radius r and height ~2r. Several experiments with samples with different porosities were performed under controlled pressure conditions (ranging from 4 to 20 MPa), at room temperature. We calculated the magnitude of the vertical and downward forces and forces at the walls of the reservoir, and the kinetic energy involved during decompression and fragmentation processes from the micro-seismic signals detected at several points in the apparatus using highly dynamic piezo-film transducers. We first characterized the frequencies of the apparatus in order to distinguish in the signals between the waves produced by the natural resonance of the system due to the pressure shock and the waves generated by the rapid depressurization of the samples. In the micro-seismic records the inflation-deflation states of the pipe-like conduit and the fragmentation process after the rapid removal of the diaphragm can be recognized clearly. The decompression time is directly measured from the pressure drop curves of the system recorded by dynamic pressure transducers and correlates well with the duration of maximum amplitudes of micro-seismic waves, therefore the source time function of the system can be determined. We investigated the relationships between pressure and decompression time versus maximum amplitudes, the source duration, the decay of seismic waves and the counter-force, considering different combinations of gas and solids in the reservoir. From these relationships, the pre-eruption conduit state can be estimated in volcanic systems, and thus the ejection velocity can be calculated in order to evaluate the implications for hazard analysis. In addition, we discuss important considerations regarding the deduction of parametric scaling laws for volcanic explosions using field seismic data. This experimental approach and the high quality of seismic records allow us to obtain a direct measure of the source parameters of the physical mechanism and evaluate the viability of the theoretical single force model to quantify real volcanic eruptions.