AbstractPore fluids play a key role in how crustal rocks deform, particularly in a volcanic environment where fluids span a wide range of types, and exist across a wide spectrum of temperature, pressure, and phase, influenced by the presence of the magmatic system at depth. Not only do pressurized pore fluids affect the mechanical properties and the elastic velocities of the host rock mass (volcanic edifice), but they are also responsible in the generation of a range of seismic signals, characterized by Low Frequency and long coda as compared to the seismicity generated by simple shear, resulting in Volcano-Tectonic events.
While great progress has been made in understanding Volcano-Tectonic events, fluid-induced signals resulting in Low Frequency seismicity are not fully understood, and how these signals evolve from other signal types in time and space. To investigate, this study presents a series of rock triaxial deformation (in both wet and dry conditions) and fluid depressurization experiments, using a servo-controlled triaxial testing machine and state-of the-art acoustic emission (AE) instrumentation. AE signals are the laboratory analogue of field-scale earthquakes, representing the key to understand the physics of the macro-scale events.
Considering shallow volcanic conditions (up to 1.6 km deep), this thesis shows that the presence of pore fluid delays the fracturing and the onset of microseismic activity, likely explaining sudden increase of precursory seismic activity before volcanic eruptions. Fluids also homogenize the rock material, decreasing the elastic wave anisotropy as they flow inside the newly formed cracks. In addition, the depressurization of fluids reveals how different fluid phases contributes to form different spectral peaks, characterizing the fluid-induced signals. Finally a fundamental microseismic event, (which presents a remarkable similarity with a natural volcanic earthquake, Tornillo), has been generated during gas depressurization, representing a new key link between earthquake features (such as amplitude modulation) and a physical properties (such as pressure drop).
|Date of Award||Jan 2017|
|Supervisor||Philip Benson (Supervisor), Sergio C. Vinciguerra (Supervisor) & Philip G. Meredith (Supervisor)|