Laboratory Experiments and Grain Based Discrete Element Numerical Simulations Investigating the Thermo-Mechanical Behaviour of Sandstone.

Thermo-mechanical loading can occur in numerous engineering geological environments, from both natural and anthropogenic sources. Different minerals and micro-defects in rock cause heterogeneity at a grain scale, affecting the mechanical and thermal properties of the material. Changes in strength and stiffness can occur from exposure to elevated temperatures, with the accumulation of localised stresses resulting in thermally induced micro-cracking within the rock. In this study we investigated thermal micro-cracking at a grain scale through both laboratory experiments and their numerical simulations. We performed laboratory triaxial experiments on specimens of fine-grained sandstone at a confining pressure of 5 MPa and room temperature (20 ∘ C ), as well as heating to 50 ∘ C , 75 ∘ C and 100 ∘ C prior to mechanical loading. The laboratory experiments were then replicated using discrete element method simulations. The geometry and granular structure of the sandstone was replicated using a Voronoi tessellation scheme to produce a grain based model. Strength and stiffness properties of the Voronoi contacts were calibrated to the laboratory specimens. Grain scale thermal properties were applied to the grain based models according to mineral percentages obtained from quantitative X-ray diffraction analysis on laboratory specimens. Thermo-mechanically coupled modelling was then undertaken to reproduce the thermal loading rates used in the laboratory, before applying a mechanical load in the models until failure. Laboratory results show a reduction of up to 15% peak strength with increasing thermal loading between room temperature and 100 ∘ C , and micro-structural analysis shows the development of thermally induced micro-cracking in laboratory specimens. The mechanical numerical simulations calibrate well with the laboratory results, and introducing coupled thermal loading to the simulations shows the development of localised stresses within the models, leading to the formation of thermally induced micro-cracks and strength reduction upon mechanical loading.

A strength inversion origin for non-volcanic tremor.

Non-volcanic tremor is a particularly enigmatic form of seismic activity. In its most studied subduction zone setting, tremor typically occurs within the plate interface at or near the shallow and deep edges of the interseismically locked zone. Detailed seismic observations have shown that tremor is composed of repeating small low-frequency earthquakes, often accompanied by very-low-frequency earthquakes, all involving shear failure and slip. However, low-frequency earthquakes and very-low-frequency earthquakes within each cluster show nearly constant source durations for all observed magnitudes, which implies characteristic tremor sub-event sources of near-constant size. Here we integrate geological observations and geomechanical lab measurements on heterogeneous rock assemblages representative of the shallow tremor region offshore the Middle America Trench with numerical simulations to demonstrate that these tremor events are consistent with the seismic failure of relatively weaker blocks within a stronger matrix. In these subducting rocks, hydrothermalism has led to a strength-inversion from a weak matrix with relatively stronger blocks to a stronger matrix with embedded relatively weaker blocks. Tremor naturally occurs as the now-weaker blocks fail seismically while their surrounding matrix has not yet reached a state of general seismic failure.