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We present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities. Plain Language Summary: Can we determine the size of a future earthquake based on the size of past earthquakes? This fundamental question has been controversially debated over the years, without an agreed‐upon answer. Here, we tackle this question under controlled conditions in a lab setting by studying the frictional stick‐slip dynamic of rough granite faults, which gives rise to mm‐scale seismic events. We find that the sizes of these seismic events are not independent during periods containing a macroscopic slip but instead are clustered such that larger seismic events tend to directly follow other large seismic events. We show that this can be explained by temporal changes in the frequency of occurrence of seismic events associated with the sliding motion of the fault. Our findings link the properties of mm‐scale seismic events with macroscopic slip on lab faults. Key Points: Magnitude clustering of acoustic emissions (AEs) is strongest during major slip events in fluid‐saturated samples of Westerly graniteVariations in the frequency‐magnitude distribution of AEs during slip events explain the strong magnitude clusteringThese variations indicate a prevalence of larger AEs right after the onset of slip events [ABSTRACT FROM AUTHOR]

The state of Oklahoma has experienced an unprecedented increase in earthquake activity since 2009, likely driven by large-scale wastewater injection operations. Statewide injection rates peaked in early 2015 and steadily decreased thereafter, approximately coinciding with collapsing oil prices and regulatory action. If seismic activity is primarily driven by fluid injection, a noticeable seismogenic response to the decrease in injection rates is expected. Langenbruch and Zoback suggest that "the probability of potentially damaging larger events, should significantly decrease by the end of 2016 and approach historic levels within a few years." We agree that the rate of small earthquakes has decreased toward the second half of 2016. However, their specific predictions about seismic hazard require reexamination. We test the influence of the model parameters of Langenbruch and Zoback based on fits to observed seismicity distributions. The results suggest that a range of realistic aftershock decay rates and b values can lead to an increase in moderate earthquake probabilities from 37 to 80% in 2017 without any further alteration to the model. In addition, the observation that all four M ≥ 5 earthquakes to date occurred when injection rates were below the triggering threshold of Langenbruch and Zoback challenges the applicability of the model for the most societally significant events.

We discuss data of three laboratory stick‐slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough fracture surfaces. The experiments produced complex slip patterns including fast and slow ruptures with large and small fault slips, as well as failure events on the fault surface producing acoustic emission bursts without externally‐detectable stress drop. Preparatory processes leading to large slips were tracked with an ensemble of ten seismo‐mechanical and statistical parameters characterizing local and global damage and stress evolution, localization and clustering processes, as well as event interactions. We decompose complex spatio‐temporal trends in the lab‐quake characteristics and identify persistent effects of evolving fault roughness and damage at different length scales, and local stress evolution approaching large events. The observed trends highlight labquake localization processes on different spatial and temporal scales. The preparatory process of large slip events includes smaller events marked by confined bursts of acoustic emission activity that collectively prepare the fault surface for a system‐wide failure by conditioning the large‐scale stress field. Our results are consistent overall with an evolving process of intermittent criticality leading to large failure events, and may contribute to improved forecasting of large natural earthquakes. Plain Language Summary: We discuss failure events in laboratory experiments on a rough fault performed at pressures existing in the Earth's crust. The laboratory faults were subjected to constant displacement resulting in short‐lasting slips of their fault surface. We observe complex slip patterns including fast/slow ruptures with large/small fault slips. Very small slips on the fault surface were observed only with acoustic emission activity, representing tiny earthquakes of sub‐mm size that produce elastic waveforms that can be recorded with piezo sensors. Using parameters derived from acoustic emission data, we analyzed physical processes leading to large slip events of the lab fault surface, an equivalent of a large earthquake in nature. Our parameters characterize local and global damage, stress, as well as interactions of small fractures before the labquake. We identify evolving fault roughness at different length scales, and find that the preparatory processes preceding lab quakes are facilitated by small earthquakes marked with bursts of acoustic emission activity. These bursts indicate ruptures of individual fault patches, which then interact and collectively prepare the fault surface for the labquake. Our results provide a set of physics‐based parameters describing complex processes leading to lab slip events that may allow to improve earthquake forecasting along natural faults. Key Points: We study preparatory processes preceding large slip events on rough laboratory faults using seismo‐mechanical features derived from AE dataThe analysis highlights multi‐scale rapidly evolving damage, roughness and stress changes along the fault surfaceIntermittent criticality marked by evolving stress correlations on different length scales can explain the observed patterns leading to large labquakes [ABSTRACT FROM AUTHOR]

Hypersonic boundary-layer stability has significant importance in vehicle design and successful operation. This paper investigates the stabilization effects of local wall cooling on the hypersonic boundary layers over a 5 deg half-angle blunt cone with a nose radius of 0.0254 mm. We employed a high-order-accurate flow solver to calculate the steady flow for a freestream Mach number of 6.0 and a unit Reynolds number of 25.59×106/m. In simulations, we considered partial wall cooling, entire wall cooling, and adiabatic wall scenarios. Furthermore, we examined partial cooling parameters such as strip location, length, and temperature profiles. We calculated the growth rates, phase speed, and N-factor diagrams using a linear stability analysis. The results showed that complete wall cooling destabilizes the boundary layer. However, the cooling strip upstream of the synchronization point stabilized the boundary layer by damping the disturbances. The longer cooling strip further stabilized the boundary layer. The cooling strip placed downstream of the synchronization point destabilized the boundary layer. [ABSTRACT FROM AUTHOR]