Triggering processes in fracture and compaction experiments: On the importance of large-scale heterogeneities

Natural seismicity is characterized by empirical laws such as the Gutenberg-Richter law and the Omori-Utsu law. The latter is a typical example for the spatio-temporal clustering of seismicity arising from triggering processes related to static and/or dynamic stress changes, yet the microscopic origin of the Omori-Utsu law is not well understood. Here, we study triggering processes in tri-axial compaction and fracture experiments under constant displacement on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. We present strong evidence that triggering of such events by other acoustic emission events plays an important role in the presence of large heterogeneities while such triggering is basically absent if no significant heterogeneities are present. In the former case, we recover all established empirical laws of seismicity including the Gutenberg-Richter law, a modified version of the Omori-Utsu law and the productivity law. For the Gutenberg-Richter law, we find that the b-value is smaller for triggered events compared to background events. Moreover, we show that triggered acoustic emission events have a focal mechanism much more similar to their associated trigger than expected by random chance.