Seismic cycle and deformation processes in quartz gouges under hydrothermal conditions

My master’s thesis project is focused on the study of the mechanical behavior of a quartz gouge during the seismic cicle (and the related deformation mechanisms) under hydrothermal conditions and in presence of hot and pressurized water. To achieve this purpose, I am exploiting the rotary shear apparatus “RoSA” equipped with the pressure vessel “Hydros” which is installed in the Rock Mechanics Laboratory of the Department of Geosciences. This machine allows to simulate the seismic cycle, applying an effective stress (σn’) to the analyzed samples and making the samples shear with temperatures (T) up to 500 °C and especially in the presence of pressurized fluids. Specifically, I performed 10 Slide-Hold-Slide (SHS) tests alternating slip phases (slide) with stationary contact phases (hold), to measure how a fault regains strength by increasing the stationary contact time (healing). This parameter is fundamental to describe the recurrence of seismic events within a fault. Five experiments were carried out with an F110 heterometric angular quartz, varying effective stress, fluid pressure (Pp) and temperature. The remaining 5 experiments instead were conducted using microquartz, replicating the same experimental conditions used before, to evaluate the different mechanical behavior as a function of the experimental parameters, such as σn’, Pp, T and grain size. The used experimental procedure consists in: 1. Sample preparation (usually 1.2-1.5 g of gouge) and assembly of the apparatus. 2. Achievement of the experimental parameters (σn’, Pp, T). 3. Wait for chemical equilibrium to be reached between fluids and sample (H2O and SiO2, usually 2 hours). 4. Initial shear, usually between 40 and 50 mm, at a velocity of 10 μm/s, to reach steady state in terms of shear torque and compaction. 5. SHS test, which includes a series of 1 mm slides alternating with increasing hold time (usually from 3 to about 50000 s), 6. New 10-15 mm slide. 7. Repeat the previous SHS series, in order to investigate the effect of strain on the mechanical response of the gouge. 8. Extraction of the sample from the apparatus. 9. Drying of the sample, ready for a microstructural/microanalytical analysis. The samples, which are initially powder, at the end of the experiments are slightly cohesive, allowing their correct extraction from the apparatus to analyze the produced microstructures. I have developed different Matlab codes to analyze the obtained mechanical data. Precisely, after some data cleaning steps including removal of outliers and smoothing, I am converting the measurements of the machine (shear torque, vertical displacement, rpm, etc.) into data with geological meaning such as friction, slip, compaction, etc. Once these values have been calculated, plotting them against each other is the way to visualize the different trends.