Geomechanical Simulation of Dynamic Triggering of Microseismicity Using a Bonded Particle Model

Abstract:

In seismology, numerous small aftershocks can be triggered locally and at a considerable distance by a moderately-sized mainshock. Aftershocks may be triggered by both the static and dynamic stress perturbation, i.e., rock deformation and transient wave motion respectively. This is true for microseismicity at the micro-scale too in that both the static displacement and seismic waves generated from acoustic emissions will induce additional cracks if the nearby bonds are close to the failure state. It is however unknown what the relative contributions are of respectively dynamic and static triggering. Does one play a dominant role or do both contribute equally? We investigate this question using a bonded-particle method simulating a biaxial compression test on an intact rock sample.

The geomechanical simulations were run both dynamically and statically. In the static runs, all wave propagation is eliminated, leaving only static deformations in the sample. On the other hand, in the dynamic runs, a low damping coefficient is used such that both dynamic (transient waves) and static deformation occurs. In both cases, the sample is compressed and we monitor the resulting stress-strain curves, the temporal and spatial distribution of cracks, released kinetic energy, and the orientation and magnitude of the internal forces.

We find in both simulations clear evidence of the development of strong- and weak-force networks, similar to those seen by Van der Baan and Chorney (JGR, 2019). Conversely, the sample behaved differently if all dynamic waves are eliminated in that it became stronger, reaching higher peak stresses, the microseismicity dropped by 70% but some fracture patterns are maintained. We interpret this as evidence that dynamic failure plays an important role in brittle rocks in that wave propagation accelerates rock failure, thus strongly influencing aftershock patterns. Additional simulations where the sample was first deformed under a constant loading rate, followed by a second test, where a series of external sinusoidal vibrations are applied to the bottom platen during loading, confirmed the importance of dynamic stresses on failure patterns.