Progress towards a net zero carbon economy involves subsurface activities, such as geothermal energy production and geological storage of carbon dioxide and hydrogen, that disturb tectonic stresses in the Earth's crust. Seismicity induced by such stress perturbations is associated with risk from damage and potential loss of public confidence. Safe operation of these activities therefore requires effective management to minimise induced seismicity.
Failure in brittle, porous materials initiates when structural damage localises along an emergent failure plane in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are not fully understood and difficult to capture. However, these mechanisms are crucial drivers for earthquakes, including induced seismicity.
Here we observe these micro-mechanisms directly by controlling the rate of micro-seismic events to slow down the transition in a unique triaxial deformation apparatus that combines acoustic monitoring with contemporaneous in-situ x-ray imaging of the microstructure. We compare the seismic signatures between this experiment and a sister experiment carried out under constant strain rate loading. The results [Cartwright-Taylor et al., 2022; Mangriotis et al., submitted] provide the first integrated picture of how damage and associated micro-seismic events evolve together during sample weakening, allowing us to directly constrain the partition between seismic and aseismic deformation at the micro-scale.
The evolving damage in the 3D x-ray volumes and local strain fields undergoes a breakdown sequence involving self-organised exploration of candidate shear zones, spontaneous tensile failure and rotation of individual grains within a localised shear zone, and formation of a proto-cataclasite, highlighting the importance of aseismic mechanisms such as grain rotation in accommodating bulk shear failure. Dilation and shear strain remain strongly correlated throughout failure confirming the existence of a cohesive zone but with crack damage distributed rather than concentrated solely at the propagating front of a discontinuity. Seismic amplitude is not correlated with local imaged strain intensity, and the seismic strain partition coefficient is very low overall.
Compared with loading under a constant strain rate, reactive loading to maintain a constant micro-seismic event rate increases the seismic b-value, decreases the maximum event magnitude, suppresses the number of events of all sizes, and reduces the seismic strain partition coefficient. Adding event rate control to that of maximum recorded magnitude may therefore be more effective than the current 'traffic light' system for managing induced seismicity.
