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Induced seismicity linked to geothermal resource exploitation, hydraulic fracturing, and wastewater disposal is evolving into a global issue because of the increasing energy demand. Moderate to large induced earthquakes, causing widespread hazards, are often related to fluid injection into deep permeable formations that are hydraulically connected to the underlying crystalline basement. Using injection data combined with a physics-based linear poroelastic model and rate-and state friction law, we compute the changes in crustal stress and seismicity rate in Oklahoma. This model can be used to assess earthquake potential on specific fault segments. The regional magnitude–time distribution of the observed magnitude (M) 3+ earthquakes during 2008–2017 is reproducible and is the same for the 2 optimal, conjugate fault orientations suggested for Oklahoma. At the regional scale, the timing of predicted seismicity rate, as opposed to its pattern and amplitude, is insensitive to hydrogeological and nucleation parameters in Oklahoma. Poroelastic stress changes alone have a small effect on the seismic hazard. However, their addition to pore-pressure changes can increase the seismicity rate by 6-fold and 2-fold for central and western Oklahoma, respectively. The injection-rate reduction in 2016 mitigates the exceedance probability of M5.0 by 22% in western Oklahoma, while that of central Oklahoma remains unchanged. A hypothetical injection shut-in in April 2017 causes the earthquake probability to approach its background level by ∼2025. We conclude that stress perturbation on prestressed faults due to pore-pressure diffusion, enhanced by poroelastic effects, is the primary driver of the induced earthquakes in Oklahoma.

Laumontite is a common and potentially frictionally unstable hydrothermal alteration product present in deep faults of the Gonghe EGS reservoir. We characterize the friction‐stability characteristics of synthetic laumontite gouge under in situ reservoir conditions. The pure laumontite gouge is frictionally strong (μ = 0.73–0.98) and the quartz/laumontite mixture (1:1) is generally less strong (μ = 0.73–0.78) under experimental conditions (Pc = 95 MPa, T = 90–250°C, Pf = 0–90 MPa). The shear velocity was stepped between 6.1, 0.61, then 0.061 μm/s for our experiments. For both gouges, the friction coefficient is independent of temperature and increases with elevated pore pressures. The pure gouge and mixture are strongly velocity‐weakening over a broad range in temperatures (∼90–220°C) and excess pore pressures (0–90 MPa) relevant to the Gonghe stimulation. Microearthquakes (MEQs) observed during stimulation are confined to within the broad depth range of inferred frictional instability—although fluid overpressures are also limited to this region. The observation that laumontite mixtures are frictionally unstable over a broad range of pressures and especially temperatures representative of EGS reservoirs and insensitive to the presence of the coexisting mineral phase (quartz) suggests its presence is a strong indicator of potential seismic hazard. Plain Language Summary Laumontite is a very low‐grade altered mineral that can easily occur in fractures or faults in granite, basalt, or sandstone. Laumontite is widely developed in the Gonghe geothermal reservoir of Western China. Fluid injection into deep geothermal rock mass may reactivate subsurface faults containing altered minerals and cause earthquakes. Hence, we conducted laboratory experimental analysis on the frictional characteristics of simulated laumontite gouge to further understand the impact of fluid injection on the triggering of deep fault earthquakes. These experiments were performed at conditions reflecting the temperature and pressure of the water injection depth of the Gonghe geothermal reservoir. The results showed that the fault's strength and friction stability strongly depend on pore pressure and temperature. Our study emphasizes the significant role of the altered mineral laumontite in controlling fault strength and stability, as well as its potential for inducing earthquakes. A possible implication of this work is that when selecting geothermal resource targets that require fluid‐injection operations, it is best to avoid laumontite‐rich sites or reservoir sections. Key Points We report the first evaluations of frictional stability properties for laumontite gouge and mixtures under hydrothermal conditions Gouges are strongly velocity‐weakening over a broad range of temperatures and insensitive to pressures and relative proportions This instability field is coincident with P‐T conditions typical for EGS and hence an indicator of ubiquitous seismic hazard

Controlling fluid injection is widely considered a key to limiting the size of injection‐induced seismicity, yet whether and how it limits earthquake size remains debated. We perform injection‐reactivation experiments on critically stressed faults to test how different injection strategies shape slip and seismic moment release. We find that injection strategies regulate the cadence of slip events rather than the total seismic moment. Compared to continuous injection, cycled injection triggers frequent and smaller events, reducing maximum moment magnitude and deformation energy of individual events. Injection–extraction cycles actively reduce pore pressure, temporally partition successive slip events, and effectively suppress delayed seismicity. Regardless of constant or cycled injection rates, maximum seismic moment (M0${M}_{0}$ ) scales with cumulative injection volume (ΔV${\\Delta }V$ ) i.e.,M0∝ΔV3/2$\\left(\\mathrm{i}.\\mathrm{e}.,{M}_{0}\\propto {\\Delta }{V}^{3/2}\\right)$ . Our laboratory results suggest that regulating the cadence of moment release promotes effective hazard mitigation.

Dehydration reactions in subducted slabs have long been correlated with embrittlement and intermediate‐depth earthquakes. However, the physical process of dehydration embrittlement remains unclear due to the complex and poorly constrained interactions between reaction progress, fluid pressure evolution, and deformation. Here we aim to quantify these interactions during antigorite dehydration with 2D hydro‐mechanical‐chemical numerical modeling and explore whether the reaction causes stress perturbations potentially leading to earthquakes. Negative total volume change during the reaction acts toward the relaxation of fluid overpressures, decreasing the chance of embrittlement. The reaction zone is the least likely to fracture due to reaction‐induced weakening and the locally larger increase of total pressure compared to fluid pressure. However, weakening also generates fluid overpressure zones and may induce strain localization/runaway processes potentially leading to brittle failure. Our results also imply that antigorite dehydration could be both the cause and effect of fast deformation in subducted slabs. Plain Language Summary Subducting plates contain water‐bearing minerals that lose their water content (dehydrate) when reaching high enough pressure and temperature. These dehydration reactions may cause the pore fluid pressure in the rocks to increase and can potentially break the rock and induce earthquakes. However, the mechanism of this process is debated. We simulate the dehydration of a key water‐bearing mineral, antigorite, and compute the mechanical effect of the reaction. We find that the dehydration of antigorite can be initiated by the fast deformation of the rock. The reaction then leads to the release of water from the mineral. Despite the rising of pore fluid pressure in the reaction zone, the rock is more likely to break outside of this zone. The reaction weakens the rock, which amplifies stress and pressure anomalies across the reaction zone. These anomalies may fracture the rock or cause highly elevated deformation rates that could lead to earthquakes. Key Points The reaction zone undergoes weakening and does not experience fluid overpressure, meaning that it is unlikely to fracture Brittle deformation may initiate due to fluid overpressure zones along the sides of the reaction zone or strain localization processes Fluid pressure rise doesn't always cause fluid overpressure; greater increase in total rock pressure can lead to fluid underpressure

In the Sichuan Basin, seismic activity has been low historically, but in the past few decades, a series of moderate to strong earthquakes have occurred. Especially since 2015, earthquake activity has seen an unprecedented continuous growth trend, and the magnitude of events is increasing. Following the M 5.7 Xingwen earthquake on 18 Dec. 2018, which was suggested to be induced by shale gas hydraulic fracturing, a swarm of earthquakes with a maximum magnitude up to M6.0 struck Changning and the surrounding counties. Questions arose about the possible involvement of industrial actions in these destructive events. In fact, underground fluid injection in salt mine fields has been occurring in the Sichuan Basin for more than 70 years. Disposal of wastewater in natural gas fields has also continued for about 40 years. Since 2008, injection for shale gas development in the southern Sichuan Basin has increased rapidly. The possible link between the increasing seismicity and increasing injection activity is an important issue. Although surrounded by seismically active zones to the southwest and northwest, the Sichuan Basin is a rather stable region with a wide range of geological settings. First, we present a brief review of earthquakes of magnitude 5 or higher since 1600 to obtain the long-term event rate and explore the possible link between the rapidly increasing trend of seismic activity and industrial injection activities in recent decades. Second, based on a review of previous research results, combined with the latest data, we describe a comprehensive analysis of the characteristics and occurrence conditions of natural and injection-induced major seismic clusters in the Sichuan Basin since 1700. Finally, we list some conclusions and insights, which provide a better understanding of why damaging events occur so that they can either be avoided or mitigated, point out scientific questions that need urgent research, and propose a general framework based on geomechanics for assessment and management of earthquake-related risks.

Volcanic seismicity provides essential insights into the behavior of an active volcano across multiple time scales. However, to understand how magma moves as the eruption cycle develops, better knowledge of the geometry and physical properties of the magma plumbing system is required. In this study, using full‐wave ambient noise tomography, we image the three‐dimensional (3‐D) crustal shear‐wave velocity structure below Great Sitkin Volcano in the central Aleutian Arc. The velocity model reveals two low‐velocity anomalies correlating with the migration of volcanic seismicity. With a bulk melt fraction of about 2.5%–9%, these low‐velocity anomalies are interpreted as mushy magma reservoirs. We propose a six‐stage eruption cycle to explain the migration of seismicity and the alternating eruption of the two reservoirs with different recharging histories. These findings have broad implications for the dynamics of magma plumbing systems and the structural control of eruption behaviors. Plain Language Summary Understanding magma accumulation and transport systems below active volcanoes is essential for predicting eruption behavior and assessing the potential hazards. The distribution of earthquakes can partly be used to infer the development of magmatic activity at different times. However, to understand how magma moves at different stages of an eruption cycle, better knowledge of what the magma plumbing system looks like is necessary. In this study, we use an advanced seismic imaging method to construct the 3‐D crustal shear‐wave velocity structure below Great Sitkin Volcano in the central Aleutian Arc. The velocity model reveals two crustal magma reservoirs, which correlate with the migration of seismicity. We propose a six‐stage eruption cycle to explain the evolution of seismicity in space and time across the island and the alternating eruption of two reservoirs. The findings in this study help to understand better the control of eruption behaviors by the underlying magma plumbing system at active volcanoes. Key Points The pre‐ and co‐eruptive seismicity below Great Sitkin Volcano, Alaska, shows a spatiotemporal migration A new 3‐D shear‐wave velocity model reveals two crustal low‐velocity anomalies that correlate with the migrating seismicity We propose a six‐stage eruption cycle involving two magma reservoirs to explain the long‐term and short‐term seismicity patterns

The InSight mission expects to operate a geophysical observatory on Mars for at least two Earth years from late 2018. InSight includes a seismometer package, SEIS. The Marsquake Service (MQS) is created to provide a first manual review of the seismic data returned from Mars. The MQS will detect, locate, quantify and classify seismic events, whether tectonic or impact in origin. A suite of new and adapted methodologies have been developed to allow location and quantification of seismic events at the global scale using a single station, and a software framework has been developed that supports these methods. This paper describes the expected signals that will be recorded by SEIS, the methods used for their identification and interpretation, and reviews the planned MQS operational procedures. For each seismic event, the MQS will locate events using all available body and surface phases, using the best estimates of the Martian structure, which will become more accurate as more Martian marsquakes are identified and located. The MQS will curate the Mars seismicity catalogue, with all events being relocated to use revised suites of structure models as they are introduced.

Given the recent developments in machine-learning technology, its application has rapidly progressed in various fields of earthquake seismology, achieving great success. Here, we review the recent advances, focusing on catalog development, seismicity analysis, ground-motion prediction, and crustal deformation analysis. First, we explore studies on the development of earthquake catalogs, including their elemental processes such as event detection/classification, arrival time picking, similar waveform searching, focal mechanism analysis, and paleoseismic record analysis. We then introduce studies related to earthquake risk evaluation and seismicity analysis. Additionally, we review studies on ground-motion prediction, which are categorized into four groups depending on whether the output is ground-motion intensity or ground-motion time series and the input is features (individual measurable properties) or time series. We discuss the effect of imbalanced ground-motion data on machine-learning models and the approaches taken to address the problem. Finally, we summarize the analysis of geodetic data related to crustal deformation, focusing on clustering analysis and detection of geodetic signals caused by seismic/aseismic phenomena. Graphical Abstract

Naturally fractured rock mass is highly inhomogeneous and contains geological discontinuities at various length scales. Hydraulic fracture stimulation in such a medium could result in complex fracture systems instead of simple planar fractures. In this study, we carried out fully coupled multiscale numerical analysis to investigate some key coupled processes of fluid-driven fracture propagation in naturally fractured rock mass. The numerical analysis follows the concept of the synthetic rock mass (SRM) method initially developed in the discrete element method (DEM). We introduce a total of five case study examples, including fracture initiation and near wellbore tortuosity, hydraulic fracture interaction with natural fractures, multi-stage hydraulic fracturing with discrete fracture network (DFN), in-fill well fracturing and frac hits after depletion-induced stress change, and induced seismicity associated with fault reactivation. Through those case studies, we demonstrate that with an advanced numerical modeling tool, the complex fracturing associated with hydraulic fracturing in naturally fractured rock mass can be qualitatively analyzed and the extent of various uncertainties can be assessed.

A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (M w) of 6.4 followed by a M w 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.