TY - JOUR A1 - Davidsen, Joern A1 - Kwiatek, Grzegorz A1 - Charalampidou, Elli-Maria A1 - Goebel, Thomas A1 - Stanchits, Sergei A1 - Rueck, Marc A1 - Dresen, Georg T1 - Triggering Processes in Rock Fracture JF - Physical review letters N2 - We study triggering processes in triaxial compression experiments under a constant displacement rate on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. We present strong evidence that event-event triggering plays an important role in the presence of large-scale or macrocopic imperfections, while such triggering is basically absent if no significant imperfections are present. In the former case, we recover all established empirical relations of aftershock seismicity including the Gutenberg-Richter relation, a modified version of the Omori-Utsu relation and the productivity relation-despite the fact that the activity is dominated by compaction-type events and triggering cascades have a swarmlike topology. For the Gutenberg-Richter relations, 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 chance. Y1 - 2017 U6 - https://doi.org/10.1103/PhysRevLett.119.068501 SN - 0031-9007 SN - 1079-7114 VL - 119 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Dresen, Georg A1 - Kwiatek, Grzegorz A1 - Goebel, Thomas A1 - Ben-Zion, Yehuda T1 - Seismic and aseismic preparatory processes before large stick-slip failure JF - Pure and applied geophysics N2 - Natural earthquakes often have very few observable foreshocks which significantly complicates tracking potential preparatory processes. To better characterize expected preparatory processes before failures, we study stick-slip events in a series of triaxial compression tests on faulted Westerly granite samples. We focus on the influence of fault roughness on the duration and magnitude of recordable precursors before large stick-slip failure. Rupture preparation in the experiments is detectable over long time scales and involves acoustic emission (AE) and aseismic deformation events. Preparatory fault slip is found to be accelerating during the entire pre-failure loading period, and is accompanied by increasing AE rates punctuated by distinct activity spikes associated with large slip events. Damage evolution across the fault zones and surrounding wall rocks is manifested by precursory decrease of seismic b-values and spatial correlation dimensions. Peaks in spatial event correlation suggest that large slip initiation occurs by failure of multiple asperities. Shear strain estimated from AE data represents only a small fraction (< 1%) of total shear strain accumulated during the preparation phase, implying that most precursory deformation is aseismic. The relative contribution of aseismic deformation is amplified by larger fault roughness. Similarly, seismic coupling is larger for smooth saw-cut faults compared to rough faults. The laboratory observations point towards a long-lasting and continuous preparation process leading to failure and large seismic events. The strain partitioning between aseismic and observable seismic signatures depends on fault structure and instrument resolution. KW - Earthquakes KW - rupture KW - stick–slip tests KW - seismic KW - aseismic Y1 - 2020 U6 - https://doi.org/10.1007/s00024-020-02605-x SN - 0033-4553 SN - 1420-9136 VL - 177 IS - 12 SP - 5741 EP - 5760 PB - Springer CY - Basel ER -