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Abrupt, local frictional fault failure comprises a displacement that is normally accompanied by acoustic emission (AE)—an impulsive elastic wave broadcast with an amplitude proportional to particle velocity. The aggregate of these displacements is the basic fault motion. In laboratory shear experiments, the examination of a sequence of laboratory earthquakes includes continuous measurements of fault motion and the associated AE that is broadcast. From these measurements, connections between the fault motion and cumulative sum of the AE amplitude can be identified. The composition of the AE broadcasts reveals inhomogeneity in the fault mechanical structure from which they arise. This inhomogeneity can be decomposed into a time invariant AE component and an articulated AE component. The articulated AE component serves as a "state of the fault diagnostic" that follows a distinctive pattern to fault failure. Thus, the articulated AE component can be used directly to monitor the state of the fault. [ABSTRACT FROM AUTHOR]

Quantifying the dynamics of sequestered CO2 plumes is critical for safe long-term storage, providing guidance on plume extent, and detecting stratigraphic seal failure. However, existing seismic monitoring methods based on wave reflection or transmission probe a limited rock volume and their sensitivity decreases as CO2 saturation increases, decreasing their utility in quantitative plume mass estimation. Here we show that seismic scattering coda waves, acquired during continuous borehole monitoring, are able to illuminate details of the CO2 plume during a 74-h CO2 injection experiment at the Frio-II well Dayton, TX. Our study reveals a continuous velocity reduction during the dynamic injection of CO2, a result that augments and dramatically improves upon prior analyses based on P-wave arrival times. We show that velocity reduction is nonlinearly correlated with the injected cumulative CO2 mass and attribute this correlation to the fact that coda waves repeatedly sample the heterogeneous distribution of cumulative CO2 in the reservoir zone. Lastly, because our approach does not depend on P-wave arrival times or require well-constrained wave reflections it can be used with many source-receiver geometries including those external to the reservoir, which reduces the risk introduced by in-reservoir monitoring wells. Our results provide an approach for quantitative CO2 monitoring and plume evolution that increases safety and long-term planning for CO2 injection and storage.

Fluid overpressure from the water released by subducted sediments and oceanic crust is an important mechanism for generating earthquakes via brittle failure and frictional instability. If unstable, such fault materials may also host diverse fault reactivation mechanisms from slow slip events to moderate‐sized earthquakes in cold subduction zones. We examine this hypothesis for glaucophane gouge ‐ a key index minerals for blueschist facies ‐ at lower confining stresses where behavior is poorly understood. We measure friction and stability at temperatures of 100°C–500°C and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. The frictional coefficient of glaucophane at representative temperatures and stresses is ∼0.70, insensitive to temperature but with a slight increase at lower effective stresses. Elevating temperature promotes a transition from velocity‐strengthening to weak velocity‐weakening behavior, indicating the destabilizing effect of high temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure further strengthens the velocity‐weakening response and would be manifest as moderate‐sized earthquakes. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and blueschist facies temperatures ‐ with weak to moderate velocity‐weakening responses upon fluid pressurization vital for understanding the abundance of slow slip to moderate‐sized earthquakes apparent in cold subduction zones. Plain Language Summary: Blueschist facies present in subduction zones with lower thermal gradient host abundant slow‐slip and dynamic earthquakes—such events requiring critical stress conditions. Such blueschist layers are characterized by abundant fluids that are released as the slab descends and heats—resulting in large excess fluid pressures that may trigger slip on faults at depth. We measure the frictional properties of simulated faults containing glaucophane powders representative of these blueschist products at temperatures, stresses and especially fluid overpressures representative of the descending slab at the plate boundary. Results indicate a temperature‐independent high frictional strength but a transition to unstable slip at both higher temperatures and increased fluid pressures. In the intermediate‐depth subduction zone, depending on the fluid pressure, it is possible to generate earthquakes in a diverse range of modes such as slow‐slip events through moderate‐sized earthquakes. This can explain documented observations in this region. Our results highlight the importance of fluid pressurization and effective stress reduction in controlling such slow‐slip and moderate‐sized events at higher temperatures and for understanding earthquakes in subduction zones. Key Points: Frictional stability of glaucophane gouge is P‐T dependent, with a change from v‐s to v‐w at ∼300°C with enhanced v‐w at lower stressesMinor v‐w of glaucophane gouge at critical stress conditions is favorable for slow slip events in subduction zonesFrictional stability of glaucophane gouge upon fluid overpressures supports the observation of abundant intermediate‐depth earthquakes [ABSTRACT FROM AUTHOR]

Distributed acoustic sensing (DAS) has emerged as a promising seismic technology for monitoring microearthquakes (MEQs) with high spatial resolution. Efficient algorithms are needed for processing large DAS data volumes. This study introduces a deep learning (DL) model based on a Residual Convolutional Neural Network (ResNet) for detecting MEQs using DAS data, named as DASEventNet. The test data were collected from the Utah FORGE 16A (78)‐32 hydraulic stimulation experiments conducted in April 2022. The DASEventNet model achieves a remarkable accuracy of 100% when discriminating MEQs from noise in the raw test set of 260 examples. Surprisingly, the model identified weak MEQ signatures that have been manually categorized as noise. The decision‐making process with the model is decoded by the classic activation map, which illuminates learning features of the DASEventNet model. These features provide clear illustrations of weak MEQs and varied noise types. Finally, we apply the trained model to the entire period (∼7 days) of continuous DAS recordings and find that it discovers >5,700 new MEQs, previously unregistered in the public Silixa DAS catalog. The DASEventNet model significantly outperforms the traditional seismic method Short‐Term Average/Long‐Term Average (STA/LTA), which detected only 1,307 MEQs. The DASEventNet detection threshold is Mw−1.80 compared to the minimum magnitude of Mw−1.14 detected by STA/LTA. The spatiotemporal distribution of the newly identified MEQs defines an extensive stimulation zone and more accurately characterizes fracture geometry. Our results highlight the potential of DL for long‐term, real‐time microseismic monitoring that can improve enhanced geothermal systems and other activities that include subsurface hydraulic fracturing. Plain Language Summary: We introduce the DASEventNet model, a deep learning approach for detecting microearthquakes (MEQs) using distributed acoustic sensing (DAS) data. The model, based on a Residual Network Convolutional Neural Network (ResNet), is tailored to process large volumes of DAS data, a challenge for traditional methods. The model underwent rigorous validation processes, achieving an unprecedented 100% accuracy rate in distinguishing between 260 MEQs and noise raw test instances. Using a classic activation map, we highlighted the capacity of the model to differentiate between MEQs and varied noise types, even those bearing a resemblance to MEQs. The model could detect weak MEQs initially classified as noise, revealing its sensitivity. When applied to a week‐long data set of DAS recordings, the model identified over 5,700 new MEQs, markedly surpassing the detection capabilities of the standard Short‐Term Average/Long‐Term Average (STA/LTA) approach and the initially cataloged ∼1,307 MEQs. Notably, DASEventNet detects MEQs as low as Mw−1.80, more sensitive than the STA/LTA's Mw−1.14 threshold. The spatiotemporal analysis of these newly detected MEQs revealed a broader stimulation zone, offering a more precise delineation of fracture geometries. These findings underscore significant advantages of DASEventNet for long‐term, real‐time microseismic monitoring in enhanced geothermal systems and other subsurface activities. Key Points: Developed DASEventNet model achieves 100% accuracy rate in distinguishing microearthquakes (MEQs) from noise in the raw test data setCompared to STA/LTA method, DASEventNet model detected ∼5 times more MEQs with minimum detectable magnitude of Mw−1.80The distribution of the newly identified MEQs defines an extensive stimulation zone and more accurately characterizes fracture geometry [ABSTRACT FROM AUTHOR]

We exploit nonlinear elastodynamic properties of fractured rock to probe the micro‐scale mechanics of fractures and understand the relation between fluid transport and fracture aperture under dynamic stressing. Experiments were conducted on rough, tensile‐fractured Westerly granite subject to triaxial stresses. We measure fracture permeability for steady‐state fluid flow with deionized water. Pore pressure oscillations are applied at amplitudes ranging from 0.2 to 1 MPa at 1 Hz frequency. During dynamic stressing we transmit ultrasonic signals through the fracture using an array of piezoelectric transducers (PZTs) to monitor evolution of interface properties. We examine the influence of fracture aperture and contact area by conducting measurements at effective normal stresses of 10–20 MPa. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film. These experiments are conducted separately with the same fracture and map contact area at stresses from 9 to 21 MPa. The measurements are a proxy for "true" contact area for the fracture surface and we relate them to elastic properties using the calculated PZT sensor footprints via numerical modeling of Fresnel zones. We compare the elastodynamic response of the fracture using the stress‐induced changes in ultrasonic wave velocities for transmitter‐receiver pairs to image spatial variations in contact properties. We show that nonlinear elasticity and permeability enhancement decrease with increasing normal stress. Additionally, post‐oscillation wave velocity and permeability exhibit quick recoveries toward pre‐oscillation values. Estimates of fracture contact area (global and local) demonstrate that the elastodynamic and permeability responses are dominated by fracture topology. Plain Language Summary: We perform laboratory experiments with fractured rock to understand the relation between fluid flow, fracture openness, and elastic properties under oscillating stresses. These experiments are conducted on rough, pre‐fractured granite specimens under normal stress conditions similar to those found in the shallow earth, a few kilometers in depth. Fluid pressure in the fracture is oscillated at various amplitudes at a fixed frequency. During this dynamic stressing, we use an ultrasonic device to monitor the evolution of the fracture interface. We examine the influence of fracture aperture and contact area by conducting measurements at increasing normal stress state. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film, showing an image of where the two halves of the fracture are in contact. The ultrasonic monitoring reveals spatial variations in contact properties, which is informed by fracture contact area measurements. These measurements are also related to the fluid‐flow in response to dynamic stressing and similar comparisons are made for how the fracture interface evolves, recovers, following the stress perturbations. Key Points: Lab experiments to simulate fracture processes under dynamic stressingSimultaneous measurements of fluid flow and elastic wave properties to quantify elastodynamic and hydraulic responses to dynamic stressingLocal fracture aperture, not necessarily the stress state, dominates elastic and hydraulic responses to dynamic stressing [ABSTRACT FROM AUTHOR]

The seismic cycle of repeated earthquake failure requires that faults regain frictional strength during the interseismic phase, when the fault is locked or undergoing quasi-static creep. Fault healing plays a central role in determining earthquake stress drop, recurrence interval, elastic radiation frequency and other source parameters. In particular, the longer a fault remains quasi-stationary, the stronger it becomes and the larger the potential stress drop can be for the next event. Here, we address the role of shear strain and strain localization on fault healing and healing rate. We performed slide-hold-slide friction experiments on quartz gouge in the double-direct shear configuration for shear strain up to 25 and hold times from 10 to 1000 s. The results show that both healing and healing rate increase nonlinearly with increasing shear strain. Frictional healing scales with volumetric strain within the laboratory fault zone. Using the volumetric strain upon reshear as a proxy for strain localization, we demonstrate that the capacity of a fault to heal is directly proportional to shear bandwidth and degree of strain localization. The more the deformation is localized, the higher are the healing and healing rate, and thus, the fault strength. Our data provide a framework for understanding variations in fault strength over the seismic cycle and the role of brecciation and strain localization on spatiotemporal variations in fault strength. [ABSTRACT FROM AUTHOR]