Effect of long preexisting fractures on fault nucleation processes

The physics underlying earthquake precursory phenomena remains poorly constrained, particularly in crustal materials containing long preexisting fractures.  We conduct axial compression experiments on pre-heated Fontainebleau sandstone with acoustic emission (AE) monitoring under confining and pore pressures of 30 and 5 MPa, respectively.  Preliminary results show no systematic differences in precursory AE behavior between pre-heated and as-is samples.  All samples exhibit b values between 0 and 1, with a wide range of overall b-value evolution toward failure, including both increasing and decreasing trends.  Despite this variability, many samples show a local decrease in b value immediately before and during failure, preceded by a local increase near peak stress.  These local b-value variations likely reflect distinct microfracturing processes in porous granular rocks, contrasting with the more monotonic b-value decrease commonly reported for crystalline rocks.  Our results suggest that subtle differences in initial granular microstructure promote diverse precursory behavior under otherwise identical experimental conditions.  Such variability may contribute to the range of precursory behavior observed in tectonic earthquakes, where many large events are not preceded by a decrease in b value.  To investigate the role of aseismic deformation in fault nucleation, we are currently quantifying the evolution of microfracture distributions in deformed samples.  In parallel, we are deforming samples containing long preexisting fractures by first pre-deforming them in the semibrittle regime at higher pressure–temperature conditions, followed by deformation to failure at lower pressure–temperature conditions.  These experiments constrain the evolution of seismic and aseismic precursory signals toward large earthquakes in highly fractured crust.