Seismic waves emitted by an earthquake can trigger other earthquakes over a variety of spatial and temporal scale, yet extended time delay and rupture behaviors associated with these events are not well understood. Here, we report on the experimental observation of earthquake rupture triggered by explosion-generated stress perturbations at different stress level, showing that the time delay of triggered events on gouge-filled fault is significantly higher than those on bare fault. Furthermore, we identify notable difference in rupture behaviors between the triggered events on gouge-filled and bare faults. For bare fault, the triggered events may be arrested or runaway, depending on the initial stress level at the onset of dynamic triggering. However, for gouge-filled fault, the rupture behaviors is more complex, manifested by scattered micro-ruptures, coalescent micro-ruptures, and runaway rupture. The extended time delay and complex rupture behaviors of triggered events on gouge-filled fault are attributed to the evolution of force chain within the fault gouge. Our results suggest that fault gouge plays an important role in earthquake dynamic triggering, which may provide new insights into the mechanism for extended time delay of dynamically triggered earthquakes on natural fault systems.

Fault reactivations induced by deep excavation can pose significant challenges to underground construction or resource extraction. Laboratory experiments on rock faults demonstrate that unloading-induced fault reactivations obey the Coulomb failure criterion derived from loading-induced events. However, the effect of stress path during unloading on the failure criterion and rupture dynamics of fault reactivations remains poorly understood. Here, we present findings from a series of laboratory experiments aimed at elucidating the effect of the unloading path on the failure criterion and rupture dynamics of fault reactivations. We conducted experiments under various stress conditions, examining two cases of unloading paths. In Case I, we unloaded the minimum principal stress, while in Case II, the maximum principal stress was unloaded. Strain gauges and high-speed photography were employed to capture the transient dynamic rupture process. Our investigations have yielded new insights into the effect of unloading path on the rupture dynamics when the fault is reactivated. In Case I, we observed fault reactivations resembling those loading-induced events characterized by forward sliding. Conversely, in Case II, fault reactivations associated with stress reversal produce mild reversed sliding with lower stress drop and rupture velocity. Furthermore, we find that there is a remarkable reduction in static friction for reversed sliding, indicating that the failure criterion for fault reactivation is influenced by the stress path. We demonstrate that enhanced stress heterogeneity, caused by stress reversal, serves as a mechanism for reduced static friction. These findings contribute to our understanding of the mechanisms underlying fault reactivations, particularly those involving reversed sliding. It is known that underground excavation, accompanied by stress release, can trigger fault reactivation, resulting in induced earthquakes. Understanding how unloading affects the activation and rupture dynamics of these induced events is crucial. To investigate the role of unloading path, we conducted laboratory experiments to simulate unloading-induced fault reactivation on analog material containing a fault. Loadings are applied biaxially at the boundary. We observed that unloading the minimum principal stress (i.e., reducing the minimum loading) could reactivate the fault, generating forward sliding resembling loading-induced rupture events. Conversely, unloading the maximum principal stress (i.e., reducing the maximum loading) induces reversed sliding, featuring smaller stress drop, coseismic slip, and rupture speed compared to forward sliding. Moreover, the static friction coefficient is reduced prior to reversed sliding. We also found that enhanced stress heterogeneity prior to unloading-induced reversed sliding can account for the difference in static friction coefficient. Therefore, it is necessary to incorporate the effect of loading path into the studies of fault reactivation. Rupture process of unloading-induced fault reactivation is dictated by the initial stress state and stress path The static friction of fault is reduced for the unloading-induced reversed sliding Enhanced stress heterogeneity caused by stress reversal contributes to reduced static friction

Investigating mode II (shear) fracture properties in alpine and arctic rock engineering is crucial. However, the effects of sub-zero temperatures on these fracture parameters of rocks remain unexplored. Thus, short core in compression (SCC) specimens are fabricated to investigate dynamic mode II fracture parameters of porous white sandstone (WS) across varying ambient temperatures (-55 degrees C, -40 degrees C, -25 degrees C, -10 degrees C, 0 degrees C, and 20 degrees C). Both dry and fully saturated WS SCC specimens were tested, with dry specimens as the control group. A high-speed camera was used with a dynamic-cryogenic testing setup to perform the dynamic tests on the rock specimens. Additionally, the fracture surface of recovered specimens was quantitatively characterized by laser scanning. The findings illustrate that the dynamic mode II fracture toughness and dynamic fracture energy (DFE) of WS specimens rise with the loading rate at several ambient temperatures. At a specific loading rate, the dynamic mode II fracture toughness of saturated WS demonstrates an initial growth as the temperature drops from 0 degrees C to -10 degrees C, subsequently decreases at -25 degrees C, and finally increases again as temperature continues to decrease below -25 degrees C. Nevertheless, the sub-zero temperatures slightly influence the mode II fracture toughness of dry WS. The fracture surface roughness of saturated and dry WS specimens in terms of the ambient temperature is consistent with the tendency of fracture toughness. However, the DFE of saturated and dry WS specimens remains nearly identical when the temperature is above 0 degrees C. In contrast, the DFE of dry WS specimens is larger than that of the saturated specimens when the temperature is below 0 degrees C. These phenomena are attributed to the water-ice phase transition in pores and the impact melting of ice at microcrack tips. Furthermore, it is found that the fractal dimension of shear fracture surfaces of SCC specimens decreases with rising loading rates, which is attributed to the enhanced occurrence of transgranular fracture during the dynamic failure process.

SUMMARY Fault rupture dynamics is expected to be significantly affected by the geometry of fault system, especially for orthogonal faults. However, the rupture behaviours of orthogonal faults, especially the coseismic interactions, are far from fully understood. Here, we present experimental results from a series of laboratory earthquakes to elucidate the effect of the stress state and initiation location on the rupture behaviours of orthogonal faults. Our results reveal a phase diagram of rupture behaviours of orthogonal faults, which is collectively controlled by stress state and rupture initiation location. For events initiating from the main fault, the rupture cannot jump to the branch, which may be due to the clamping effect or the inhibited shear stress accumulation on the branch. On the contrary, events initiating from the branch can persistently trigger ruptures of the main fault. This difference highlights the directional effect associated with the rupture of orthogonal faults. Further, the rupture length of triggered ruptures on the main fault is controlled by the stress state of the fault system. With the increase of the ratio between the shear stress and normal stress, the rupture length of the main fault increases. Our results reproduce the rupture behaviours of orthogonal faults, which may provide insights into the rupture characteristics of natural earthquakes.

To explore the effect of dynamic triggering on the meta-instable stage, a series of highly controlled experiments are conducted to simulate the dynamic triggering of earthquakes under different stress states based on the servo-controlled biaxial apparatus and digital image correlation (DIC) method combined with high-speed photography technology in the laboratory. The results show that the triggered rupture is controlled by the stress state (tau/sigma(n), shear stress/normal stress) of the fault. Two kinds of dynamically triggered earthquakes are identified. If the fault is far from instability (tau/sigma(n) < 0. 42), the triggered rupture always assumes low rupture velocity and quenches before sweeping the entire fault (i. e., self-arrested rupture). On the contrary, if the fault is sufficiently stressed (tau/sigma(n) > 0. 42), the triggered rupture would run away at a velocity approaching Rayleigh wave speed or even exceeding the shear wave speed. Overall, the increase in tau/sigma(n) facilitates the rupture of the entire fault. Specifically, when the fault enters the meta-instable stage, the dynamic disturbance persistently triggers the rupture of the entire fault. In addition, the stress drop and rupture velocity of the triggered events in the meta-instable stage are significantly higher than those before the meta-instable stage. Moreover, the relative stress drop (Delta tau/sigma(n)) increases with the initial stress state. The results indicate that when the dynamic disturbance is fixed, there is a clear correlation between the triggered ruptures and the static stress state of the fault. This study systematically reveals the rupture characteristics of dynamically triggered earthquakes, which enriches the study of the meta-instable stage of faults. Meanwhile, our results provide an experimental basis for measuring the stress state of the fault by dynamically triggered earthquakes.

The monitoring of acoustic emissions (AEs) is one of the most effective means to study the source mechanisms of laboratory earthquakes. However, conventional AE monitoring methods are usually suffered from aperture effect and weak coupling problems. Hence in this study, we propose a new method to monitor AEs using a kind of ultrasonic sensors made of polyvinylidene fluoride piezoelectric film. These ultrasonic sensors are embedded in the sample. Elaborately designed calibration experiments are performed to obtain the instrument response. The calibration results show that the sensor has a broad frequency band (1.2-400 kHz), and there is a nearly flat response spectrum with respect to velocity. This means that the ultrasonic sensor can be regarded as a velocity sensor. In addition, our experiments also verify that the ultrasonic sensors can capture the source characteristics of the laboratory earthquakes. This method may facilitate the quantitative analysis of the source mechanism of AEs in laboratory investigations of earthquakes.

Huge earthquakes are frequently preceded by slow slip events (SSEs) that are speculated as the precursor to regular earthquakes (REs). However, the way in which earthquakes initiate, as well as the interactions between SSEs and REs remain poorly understood, adding more mysteries to the initiation of earthquakes. Here, we perform systematic numerical simulations to explore the relationships between SSEs and REs on faults including locally increased fluid pressure. We identify four types of fault slip behaviors distinguished by SSE and earthquake initiation mode. The observed interactions between SSEs and REs share similar features with those reported for natural earthquakes. Our results show that the occurrence of SSEs may temporarily hasten fault decoupling, leading to the clock advance of mainshocks. Furthermore, the interactions between SSEs and REs are more complicated than previously thought. On the one hand, since SSEs with extremely high peak slip rates tend to directly transform into huge earthquakes, the possibility of huge earthquakes may increase when SSEs happen. On the other hand, there is no threshold in peak slip rate for SSEs to trigger the nucleation of REs. Therefore, it is difficult to distinguish the SSEs that could trigger a huge earthquake from regular ones only with the knowledge of the peak slip rate. We also verify that the spatial extent of SSEs is related to the occurrence of earthquakes to some extent. These findings may have major implications for understanding the interactions between SSEs and REs, and the mechanism of earthquake initiation.