The origin of highly fragmented, but weakly strained rocks found along major strike‐slip faults has been enigmatic since their first recognition. These so‐called pulverized rocks occur up to 100 m away from the principal slip zone of seismogenic faults around the world. Previous dynamic compression experiments have suggested that rock pulverization occurs at strain rates on the order of 102 s−1, pointing to a coseismic origin; however, strain rates during earthquake rupture 100 m from faults is expected to be 4 orders of magnitude smaller. We present evidence from new modified Split‐Hopkinson Pressure Bar experiments that instead supports a tensile origin for coseismic rock pulverization. In the new experimental configuration, the axial compressive load from the Split‐Hopkinson Pressure Bar induces radially isotropic tension in a Westerly Granite disk bonded between two lead cylinders. The isotropic tensile state of stress results in the formation of polygonal fracture arrays that bound axis‐parallel columnar fragments. The tensile strength of Westerly Granite measured at strain rates between ~5 and 50 s−1 bridges the gap between low strain rate and shock strengths reported previously, supporting an interpretation that highly fragmented rocks may form in a state of isotropic tension. The resulting fragment size is independent of strain rate and instead appears to be controlled by elastic strain energy, a strong function of material strength, and fracture toughness. Our results provide a solution to the strain rate‐distance scaling problem between laboratory experiments and field observations of pulverized rocks and also explain the asymmetric distribution of pulverized fault rocks about strike‐slip faults.

The evolution of shear rupture fronts in laboratory earthquakes is analysed with the corresponding functional networks, constructed over acoustic emission friction-patterns. We show that the mesoscopic characteristics of functional networks carry the characteristic time for each phase of the rupture evolution. The classified rupture fronts in network states–obtained from a saw-cut fault and natural faulted Westerly granite - show a clear separation into three main groups, indicating different states of rupture fronts. With respect to the scaling of local ruptures' durations with the networks' parameters, we show that the gap in the classified fronts could be related to the possibility of a separation between slow and regular fronts.

Nucleation and propagation of a shear fault is known to be the result of interaction and coalescence of many microcracks. Yet the character and rate of the microcracks' interactions, and their dependence on the three-dimensional stress state are poorly understood. Here we investigate formation of microcracks during sandstone faulting under 3D-polyaxial stress fields by analyzing multi-stationary acoustic waveforms. We show that in a true three-dimensional stress state (a) faulting forms in a orthorhombic pattern, and (b) the emitted acoustic waveforms from microcracking carry a shorter rapid slip phase. The later is associated with microcracking that dominantly develops parallel to the minimum stress direction. Our results imply that due to inducing the micro-anticracks, the three-dimensional (3D) stress state can quicken dynamic weakening and rupture propagation by a factor of two relatively to simpler stress states. The results suggest a new nucleation mechanism of 3D-faulting with implications for earthquakes' instabilities, as well as the understanding of avalanches associated with dislocations.

SUMMARY The formation of fragments due to avalanche-like growth of damage under impulsive forces is a process central to numerous studies ranging from shaped charge jet break up and rock blasting to bolide impacts, and, more recently, earthquake rupture. In the latter case, pulverized rocks found millimetres to tens of metres from the principal slip zones of large faults have been associated with fast, even supershear, rupture propagation. It has been postulated that earthquake source characteristics directly affect the degree of fragmentation, and the study of fragment size distribution may shed light on the energy budget of individual earthquakes as well as long-term effects on fault zone properties. The actual fragmentation process, and the partitioning of dissipated energy at fast loading rates, however, is still enigmatic. We use modified Split Hopkinson Pressure Bar experiments, in which we can control stressing rate, amplitude and duration, as a laboratory analogue for the complex natural prototype source processes. In our experiments, we characterize the velocity distribution of ejected fragments from Westerly Granite specimens resulting in a range of fragmentation states, from weakly fragmented to pulverized. Analysis of the velocity distributions (and the related kinetic energy) reveals spatial domains that are free of ejected fragments; these so-called ‘zero kinetic energy modes’ are related to the fragmentation state: increasing fragmentation corresponds to a reduction of zero mode domains. The evolution of these zero modes with strain rate reveals that the transition from low strain rate fracturing to high strain rate pulverization is a smooth, continuous transition, rather than a sharp boundary. Furthermore, our results yield important insights into the process of fragmentation in earthquake process zones, including how dissipated energy is partitioned during fragmentation, and indicate that delocalization of energy is systematically coupled with source parameters.

Abstract. Understanding the physics of acoustic excitations emitted during the cracking of materials is one of the longstanding challenges for material scientists and geophysicists. In this study, we report novel results of applications of functional complex networks on acoustic emission waveforms emitted during the evolution of frictional interfaces. Our results show that laboratory faults at microscopic scales undergo a sequence of generic phases, including strengthening, weakening or fast slip and slow slip, leading to healing. For the first time we develop a formulation on the dissipated energy due to acoustic emission signals in terms of shortterm and long-term features (i.e., networks' characteristics) of events. We illuminate the transition from regular to slow ruptures. We show that this transition can lead to the onset of the critical rupture class similar to the direct observations of this phenomenon in the transparent samples. Furthermore, we demonstrate the detailed submicron evolution of the interface due to the short-term evolution of the rupture tip. As another novel result, we find that the nucleation phase of most amplified events follows a nearly constant timescale, corresponding to the initial strengthening or locking of the interface. This likely indicates that a thermally activated process can play a crucial role near the moving crack tip.