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Using Fracture Seismic methods to map fluid-conducting fracture zones makes it important to understand fracture connectivity over distances greater 10–20 m in the Earth’s upper crust. The principles required for this understanding are developed here from the observations that (1) the spatial variations in crustal porosity are commonly associated with spatial variations in the magnitude of the natural logarithm of crustal permeability, and (2) many parameters, including permeability have a scale-invariant power law distribution in the crust. The first observation means that crustal permeability has a lognormal distribution that can be described as κ ≈ κ 0 exp ( α ( φ − φ 0 ) ) , where α is the ratio of the standard deviation of ln permeability from its mean to the standard deviation of porosity from its mean. The scale invariance of permeability indicates that αϕο = 3 to 4 and that the natural log of permeability has a 1/k pink noise spatial distribution. Combined, these conclusions mean that channelized flow in the upper crust is expected as the distance traversed by flow increases. Locating the most permeable channels using Seismic Fracture methods, while filling in the less permeable parts of the modeled volume with the correct pink noise spatial distribution of permeability, will produce much more realistic models of subsurface flow.

Geothermal projects utilizing supercritical water (≥400 °C) could boost power output tenfold compared to conventional plants. However, these reservoirs commonly occur in crustal areas where rocks are semi-ductile or ductile, impeding large-scale fractures and cracking, and where hydraulic properties are largely unknown. Here, we explore the complex permeability of rocks under supercritical conditions using mechanical data from a gas-based triaxial apparatus, high-resolution synchrotron post-mortem 3D imagery, and finite element modeling. We report a first order control of strain partitioning on permeability. In the brittle regime, strain localizes on permeable faults without necessarily increasing sample apparent permeability. In the semi-ductile regime, distributed strain increases permeability both in deformation bands and the bulk, leading to a more than tenfold permeability increase. This study challenges the belief that the brittle-ductile transition (BDT) marks a cutoff for fluid circulation in the crust, demonstrating that permeability can develop in deforming semi-ductile rocks. In this study, we reveal that permeability in experimentally deformed ductile granite increases by more than an order of magnitude. We demonstrate that permeability varies spatially and that the distribution of strain across the brittle-ductile transition leads to a corresponding partitioning of permeability.

During the final stages of seafloor spreading in the East Sub‐basin of the South China Sea, spreading transitioned from slow to ultraslow before cessation. However, post‐spreading volcanism has obscured the original fabric along or near the extinct spreading ridge (ESR). We present a detailed VP/VS model along a profile perpendicular to the ESR, constructed using independent tomography of P‐ and S‐wave data. This model provides critical constraints on the crustal lithology and tectonic evolution of the ESR. Our results show significant thinning of the crust (∼3.3–4.3 km) near the ESR. Within the thin crustal regions, anomalously elevated VP/VS ratios (>1.9) may indicate tectonic uplift of serpentinites. These observations indicate a substantial reduction in magma supply during the terminal phases of spreading, resulting in a near absence of lower crust. During this period, tectonic extension dominated, and crustal fracturing facilitated seawater infiltration into the upper mantle, promoting serpentinite formation.

Complex natural fracture networks typically consist of multiple clusters, whose connectivity is rarely quantified. Therefore, for each identified fracture network, we propose a connectivity metric that accounts for individual fracture clusters and their interactions. This metric evaluates contributions from all fracture clusters, considering their relative sizes and interactions among the isolated clusters, which in turn depend on the hydraulic conductance of the interconnecting rock matrix. Furthermore, we investigate how the system connectivity depends on fracture sealing, alterations of central clusters, and cluster linkage. Fracture sealing strongly impacts overall fracture connectivity, with 5 percent of sealed fractures reducing connectivity by 20 percent. The connectivity reduction is small when transitioning the central cluster from the largest to the smallest one. However, the largest cluster significantly contributes to overall connectivity, while the smallest one contributes minimally. Natural fracture networks increase connectivity by linking more clusters, with heterogeneity and anisotropy playing pivotal roles. Plain Language Summary Natural fractures are typical multi‐cluster systems found in many places, not just in crustal rocks but also in construction materials, human bones, and other areas. Multi‐cluster systems are even more widespread, including in biological systems and materials science. In crustal rocks, natural fractures are crucial for assessing rock stability and flow processes because they control the mechanical and hydrological properties of the rocks. Thus, fractures are important in many engineering fields, such as oil and gas production and underground hydrogen or CO2 storage. The connectivity of these multi‐cluster systems is essential as it directly impacts their mechanical and hydrological properties. Here, we propose a new metric to measure the connectivity of complex fracture networks. This method explains how individual fracture clusters contribute and interact based on their sizes and hydraulic conductance. Applying this to outcrop fracture maps shows significant changes in network connectivity due to factors like fracture sealing, changes in central clusters, and cluster linkage. This method can also be applied to 3D fracture systems and other multi‐cluster systems. These findings improve our understanding of how fractured formations evolve and how fluids flow through them, offering practical insights for better engineering practices. Key Points Fracture sealing significantly impacts overall connectivity, with 5% of sealed fractures reducing it by about 20% Altering the central cluster insignificantly affects connectivity but significantly influences production Natural fractures enhance connectivity by linking clusters, with heterogeneity and anisotropy playing a pivotal role

Understanding the coupling between rock permeability, pore pressure, and fluid flow is crucial, as fluids play an important role in the Earth's crustal dynamics. We measured the distribution of fluid pressure during fluid‐flow experiments on two typical crustal lithologies, granite and basalt. Our results demonstrate that the pore‐pressure distribution transitions from a linear to a non‐linear profile as the imposed pore‐pressure gradient is increased (from 2.5 to 60 MPa) across the specimen. This non‐linearity results from the effective pressure dependence of permeability, for which two analytical formulations were considered: an empirical exponential and a new micromechanics‐based model. In both cases, the non‐linearity of pore pressure distribution is predicted. Using a compilation of permeability versus Terzaghi's effective pressure data for granites and basalts, we show that our micromechanics‐based model has the potential to predict the pore pressure distribution over the range of effective pressures expected within the brittle crust. Plain Language Summary Fluids distributions and fluid migrations play an important role in the Earth's crustal dynamics and how fluids migrate through a rock will depend primarily on permeability. However, the permeability of crustal rocks may exhibit important pressure dependence, because cracks and fractures will increasingly close with increasing tectonic pressure. In this experimental study, we show that the couplings between increasing pressure, crack closure, and permeability reduction may result in non‐linear pore pressure distributions on a rock specimen at the laboratory scale, which confirms for the first time pioneering theoretical and experimental works. Two simple analytical expressions of the pressure dependence of permeability predict this non‐linearity. One empirical expression, most commonly used in the literature, takes the form of an exponential. The second one, a new model, based on crack micromechanics, was developed within this work and shown to outperform the exponential formulation at low Terzaghi's effective pressure. Key Points Pore pressure was measured locally in rocks exhibiting pressure‐dependent permeability We observed a transition from linear to nonlinear pore pressure distribution with increasing fluid pressure gradients A new, micromechanics‐based, analytical model was developed for the pressure dependence of permeability in microcracked rocks

To investigate the dominant deformational patterns and stress conditions in the upper crustal structure of the Kinki region, southern-central Japan, we constructed a high-resolution 3D azimuthal anisotropy model to a depth of ~ 11 km. We used 6-month-long ambient noise data recorded by the densely distributed permanent and temporary stations. From this dataset, cross-correlations were retrieved. We then obtained a 3D isotropic velocity model by inverting Rayleigh wave dispersion data, followed by a direct joint inversion for both 3D azimuthal anisotropy and additional isotropic velocity perturbation. The resolved 3D azimuthal anisotropy reveals significant contrasts of anisotropy across the Kinki region. The predominant fast axes observed in the northwestern Kinki region align with the direction of the maximum horizontal compressional stress and the principal strain rate axes, suggesting that the observed anisotropy is mainly stress-induced. In the southern part of the study area, furthermore, the predominant fast axes trend NE–SW and near E–W, also indicating the presence of stress-induced anisotropy. On the depth profile of the anisotropy, we found depth-dependent variation of azimuthal anisotropy. There exists a significant consistency between the anisotropy observed beneath 3 km depth and the dense distribution of earthquake hypocenters. This interrelationship between anisotropy and seismicity demonstrates that the observed anisotropy could be linked to local crustal stress or fractures relevant to earthquake ruptures. Our 3D anisotropy model therefore contributes towards understanding the locations and features of the seismicity region.

This study focuses on samples that underwent rapid hydration (∼10 hr) and evolved in permeability (∼10−9 to 10−8 m2) as a result of crustal fracturing. A coupled reactive transport model and thermodynamic analyses, focusing on Si alteration processes within reaction zones, are used to estimate the fluid volume required to induce fluid‐driven seismic activity. Estimated fluid volumes (101–104 m3) are used to approximate the moment magnitudes of potential seismic events. The resulting moment magnitudes (−0.6 to 3.8) and short timescales of fluid infiltration (∼10 hr) are comparable to some slow‐slip events, such as tremors and low‐frequency earthquakes. This indicates that the voluminous fluid flow in a single fracture could be a key control on the generation of crustal fracturing and the induction of seismic activity above the tremor and slow slip events source regions in the lower–middle crust. Plain Language Summary Short‐lived fluid flow in the crust modifies the hydrological properties of rocks and controls the earthquakes triggering. However, there are limited numerical constraints on the fluid volumes that can be rapidly transported. This study focuses on fluid flow through a single fracture in metamorphic rocks. We discuss the relationship between estimated fluid volumes and a series of low‐magnitude fracturing events, such as tremors and other types of slow slip events in the lower‐middle crust. Specifically, we analyze unique geological and geochemical evidence preserved in fluid‐rock reaction zones to approximate the duration of fluid infiltration and the volume of fluids transported. We use two independent methods for constraining generated seismic moment and magnitude based on fluid volumes and single fracture geometry. The transportation of fluid volumes through a fracture (101–104 m3) may be related to short seismic events, as suggested by duration (∼10 hr) and cumulative magnitude, representing the maximum values as 2.0–3.8. We observed a dramatic change in hydrological properties: from low permeable rocks to high‐permeable fractures, which are not dead‐end and can effectively transport a large volume of fluids in a short time. Such fluid infiltration can possibly trigger seismic activity above the earthquake source regions. Key Points Fluid volumes estimated via reactive‐transport modeling and thermodynamic analyses are used to approximate the moment magnitudes Moment magnitudes (−0.6 to 3.8) and short timescales of fluid infiltration (∼10 hr) are comparable to slow‐slip events Voluminous fluid flow in a single fracture may be related to the generation of crustal fracturing and the induction of short seismic events

Water infiltration into fractures is ubiquitous in crustal rocks. However, little is known about how such a progressive wetting process affects fracture stiffness and seismic wave propagation, which are highly relevant for characterizing fracture systems in situ. We study the acousto‐mechanical behavior of a free‐standing fractured granite subjected to gradual water infiltration with a downward‐moving wetting front over 12 days. We observe significant differences (i.e., by an order of magnitude) in wave amplitudes across the fractured granite compared to an intact granite, with both cases showing a strong correlation between wave amplitudes and wetting front movement. Effects of water infiltration into the fracture and surrounding matrix on seismic attenuation are captured by a numerical model with parameters constrained by experimental data. Back‐calculated fracture stiffness decreases exponentially with the wetting front migration along the fracture. We propose that moisture‐induced matrix expansion around the fracture increases asperity mismatch, leading to reduced fracture stiffness. Plain Language Summary In the shallow layers of the Earth, hydrological cycles such as snowmelt, fog, dew, and rain have been shown to change the moisture content of crustal rocks, which can alter the elastic properties of natural fractures and affect the propagation of seismic waves. Understanding how seismic waves propagate in the near‐surface environment is crucial for the assessment of earthquake hazards and the characterization of geologic heterogeneities. In this work, we perform well‐controlled laboratory experiments to study the acousto‐mechanical behavior of a single fracture in granitic rock subjected to progressive wetting over 12 days. We report that the fracture stiffness decreases exponentially as the wetting front advances along the fracture. Our research sheds light on an important question in fracture characterization: how elastic waves propagate across a fracture undergoing moisture‐induced expansion. Key Points A laboratory study establishes a relationship among water imbibition, seismic attenuation, and stiffness evolution in a wetted fracture Wave amplitudes across a fracture correlate strongly with the wetting front movement of infiltrated water within the fracture Fracture stiffness exponentially decreases with the advance of the wetting front along the fracture

The metavolcanics of Chitradurga region host numerous shallow crustal veins and fractures and faults of multiple orientations. Several high and low Pf cycles have been recorded in the region, leading to the reactivation of most of the pre-existing fractures for high Pf and selective reactivation of some well-oriented fractures under low Pf conditions. The pre-existing anisotropy (magnetic fabric) in the metavolcanics acted as the most prominent planar fabric for fracture propagation and vein emplacement under both conditions, thereby attaining maximum vein thickness. In this study, we emphasize the reactivation propensity of these pre-existing fracture planes under conditions of fluid pressure variation, related to the high and low Pf cycles. Multiple cycles of fluid-induced fracture reactivation make it difficult to quantify the maximum/minimum fluid pressure magnitudes. However, in this study we use the most appropriate fluid pressure magnitudes mathematically feasible for a shallow crustal depth of ∼2.4 km. We determine the changes in the reactivation potential with states of stress for the respective fracture orientations under both high and low Pf conditions. Dependence of fluid pressure variation on the opening angle of the fractures is also monitored. Finally, we comment on the failure mode and deformation behaviour of the fractures within the prevailing stress field inducing volumetric changes at the time of deformation. We find that deformation behaviour is directly related to the dip of the fracture planes.

The relationship between groundwater and lakes in Qaidam Basin is often overlooked. Therefore, we employed Landsat satellite images and meteorological data to investigate the causes of lake expansion through model calculation and statistical analysis and then determine groundwater sources through isotope analysis (2H, 3H, and 18O). In the two study periods of 2003–2011 and 2011–present, temperature, precipitation, and runoff increased at a steady rate, whereas the expansion rate of Tuosu Lake increased from 1.22 km2/yearr to 3.38 km2/yearr. This significant increase in the rate of lake expansion reflects the substantial contribution of groundwater to lake expansion. The groundwater contribution to the lake includes not only the glacial meltwater that infiltrates the piedmont plain but also other, more isotopically deleted water sources from other basins. It is speculated that the 2003 Ms 6.4 earthquake in the northwest of the Delingha region was a possible mechanism for lake expansion. Earthquakes can enhance crustal permeability and keep fractures open, which promotes groundwater contribution to lakes and in turn causes rapid lake expansion and an increased groundwater level. This study is important for understanding the sources, circulation, and evolution of groundwater in Qaidam Basin.