Journal of Geophysical Research: Solid Earth

The India–Eurasia continental collision zone (IECCZ) is an ideal setting for studying plate collision processes, plateau uplift mechanisms, and orogenic activities. Several models have attempted to explain the north–south (N‒S) collision and east–west (E‒W) extension based on geological and geophysical observations. Among them, the subduction of the subducted Indian lower crust (SILC) and the deep geometric shape of the southern Tibetan rifts are still controversial. To address these issues, we selected P‐ and S‐wave arrival times from 35,193 earthquakes recorded by 575 permanent and temporary stations and applied an improved double‐difference tomography method to obtain high‐resolution 3‐D P‐ and S‐wave velocity structures of the crust and uppermost mantle and relocated earthquake event locations around the IECCZ. The N‒S velocity profiles display a northward‐subducting high‐velocity layer stretching from the 20–40 km depth range in the Himalayan belt to the 50–60 km depth range beneath the northern Lhasa terrane (NL), suggesting that the eclogitized SILC extended beyond the Indus–Yarlung suture, reaching the NL. Additionally, the E‒W velocity profile at the SILC front reveals a discrete high‐velocity layer at depths of 40–60 km beneath the Longgar, Tingri–Nyima, Xianza‐Dinggye, and Yadong–Gulu rifts, implying that the SILC experienced tearing. Based on a comprehensive analysis of the seismicity, large‐earthquake source mechanisms near the Moho, and tomographic images of the study area, we proposed a new dynamic model of the India–Eurasia collision and N–S‐trending rifts. The significant characteristic of this model is that the rifts cut through the crust obliquely, not vertically.

Utilizing existing telecommunication cables for Distributed Acoustic Sensing (DAS) experiments has eased the collection of seismological data in previously difficult‐to‐access areas such as the ocean bottom. To assess the potential of submarine DAS for monitoring seismic activity, we conducted an experiment from mid‐October to mid‐December 2021 using a 45 km long dark fiber extending from the Greek island of Santorini along the ocean bottom to the neighboring island of Ios. This region is of great geophysical and public interest because of its historical and recent seismic and volcanic activity, especially along the Kolumbo volcanic chain. Besides recording anthropogenic noise and around 1,000 seismic events, we observe the primary and secondary microseisms in the submarine section, the latter inducing Scholte waves in a sediment layer where the cable is well‐coupled. By using the spectral element wave propagation solver Salvus, we compute synthetic strains for earthquakes with varying degrees of model complexity. Despite including topography, a water layer, and a heterogeneous velocity model, we are unable to reproduce the lack of coherence in our observed earthquake waveforms. Backpropagation simulations for four observed earthquakes indicate that clear convergence of the wavefield, and thus the ability to constrain a source region, is only possible when all model complexities are considered. We conclude that, despite the promising emergence of DAS, monitoring capabilities are limited by often unfavorable cable geometries, cable coupling, and the complexity of the medium. Interrogating multiple cables simultaneously or jointly analyzing DAS and seismometer data could help improve future monitoring experiments.

The dynamics of Earth's D″ layer at the base of the mantle plays an essential role in Earth's thermal and chemical evolution. Mantle convection in D″ is thought to result in seismic anisotropy; therefore, observations of anisotropy may be used to infer lowermost mantle flow. However, the connections between mantle flow and seismic anisotropy in D″ remain ambiguous. Here, we calculate the present‐day mantle flow field in D″ using 3D global geodynamic models. We then compute strain, a measure of deformation, outside the two large‐low velocity provinces (LLVPs) and compare the distribution of strain with previous observations of anisotropy. We find that, on a global scale, D″ materials are advected toward the LLVPs. The strains of D″ materials generally increase with time along their paths toward the LLVPs and toward deeper depths, but regions far from LLVPs may develop relative high strain as well. Materials in D″ outside the LLVPs mostly undergo lateral stretching, with the stretching direction often aligning with mantle flow direction, especially in fast flow regions. In most models, the depth‐averaged strain in D″ is >0.5 outside the LLVPs, consistent with widespread observations of seismic anisotropy. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations.

The central North China Craton (NCC) acts as a transition zone between the stable western and reworked eastern NCC. It is characterized by high seismic activity and experienced volcanic activity with small magma volumes. To assess the dynamic processes of the central NCC, particularly in a zone marked by intense differential tectonic deformation, we have obtained a 3‐D radial anisotropic model of crust and uppermost mantle via joint inversion of Love‐ and Rayleigh‐wave dispersion curves and ellipticity measurements. Compared to models without Rayleigh‐wave ellipticity, our new model shows improved accuracy in crustal radial anisotropy. This refined model reveals two noteworthy geological features: (a) Most of the Shanxi Rift has pronounced positive radial anisotropy in the crust except for Linfen and Xinding Basins, as critical earthquake‐prone areas, which are characterized by weak positive to negative anisotropy with much thinner sediments. This observation suggests that differential rifting processes with uneven sedimentation and crustal deformation occur in these Cenozoic basins due to right‐lateral strike slip motion. (b) The crust in the Lvliang Mountains shows weak positive or negative anisotropy with a lower crustal low‐velocity layer beneath the northern parts, whereas the crust in the Taihang Mountains exhibits positive anisotropy. This implies that the Lvliang Mountains experienced uplift under compressional environments since the Yanshanion Orogeny. Furthermore, the magmatic underplating in the crust accelerated the uplift of the northern Lvliang Mountains. In contrast, the Taihang Mountains underwent relative uplift under extensional environments, along with the subsidence of the Bohai Bay Basin during the Cenozoic.

Submarine calderas with active magma supply have recently been identified as potential sources of volcanic tsunamis due to sudden meter‐scale uplift by trapdoor faulting, occurring every few years to a decade. These trapdoor uplifts are seismically recorded as non‐double‐couple earthquakes with magnitudes M > 5. Kita‐Ioto Caldera, a submarine caldera in the Izu‐Bonin arc, caused such earthquakes every 2–5 years. Our previous study (Sandanbata & Saito, 2024, https://doi.org/10.1029/2023jb027917) analyzed data from a single ocean bottom pressure (OBP) gauge in the Philippine Sea, confirming trapdoor uplifts during the earthquakes in 2008 and 2015. However, high temporal‐resolution data for the earthquakes in 2017 and 2019 were lost, preventing source mechanism investigation. To address this, we examine OBP data of the two recent earthquakes from the array network of Dense Oceanfloor Network system for Earthquakes and Tsunamis, deployed off the southwestern coast of Japan. Despite the poor signal‐to‐noise ratio in each record, we successfully detect clear tsunami signals associated with the earthquakes using a waveform stacking method, with sea‐surface wave amplitudes of only 1–2 mm. By analyzing the data, we propose source models that represent trapdoor uplifts in the submarine caldera and accurately reproduce the detected tsunami waveforms, confirming the recurrence of trapdoor uplifts. Notably, differences in the tsunami waveforms between the 2017 and 2019 earthquakes suggest that different segments of the intra‐caldera fault system were activated. This segmentation likely influences the recurrence characteristics of the inflation cycle in calderas, which would be a key to understanding the magma accumulation process and assessing the sizes and timings of future trapdoor uplifts.

To explore seismic structures beneath the Australian continents and subduction zone geometry around the Australian plate, we introduce a new radially‐anisotropic shear‐wavespeed model, AU21. By employing full‐waveform inversion on data from 248 regional earthquakes and 1,102 seismographic stations, we iteratively refine AU21, resulting in 32,655 body‐wave and 35,897 surface wave measurements. AU21 reveals distinct shear‐wavespeed contrasts between the Phanerozoic eastern continental margin and the Precambrian western and central Australia, with the lithosphere‐asthenosphere boundary estimated at 250–300 km beneath central and western Australia. Notably, a unique weak radial anisotropy layer at 80–150 km is identified beneath the western Australian craton, possibly due to alignments of dipping layers or tilted symmetry axes of anisotropic minerals. Furthermore, slow anomalies extending to the uppermost lower mantle beneath the east of New Guinea, Tasmania, and the Tasman Sea indicate deep thermal activities, likely contributing to the formation of a low wavespeed band along the eastern Australian margin. In addition, our findings demonstrate the stagnant Tonga slab within the mantle transition zone and the Kermadec slab's penetration through the 660‐km discontinuity into the lower mantle.

Plain Language Summary The shallow coral reef limestone (CRL), primarily composed of aragonite, originates from the skeletal remains of dead corals and serves as a fundamental material in island reef construction. Understanding its structural properties is essential for ensuring the integrity and stability of island reefs. Due to its unique formation process and the prevalence of aragonite, the CRL generally exhibits an extensive internal pore structure, making it more susceptible to environmental influences. To investigate the effects of dissolution on the structure, nanoindentation tests were conducted to evaluate changes in the micromechanical properties. Due to the rarity of rock samples, nanoindentation enables the quantification of mechanical changes from the microscale to the macroscale through the Mori‐Tanaka method. Simultaneously, CT scanning and image processing techniques were employed to convert real pore structure information into mathematical representations. To achieve more precise modeling of complex micrometer‐scale pore structures, a numerical model based on the level‐set method is employed, which facilitates the study of static dissolution under varying temperature and mineral conditions. Simulations showed that as temperature rises, both the dissolution volume and porosity increase, but the changes in pore radius were more complex. The presence of aragonite suppressed calcite dissolution, thereby altering the dissolution pathway. Furthermore, extracting the structure before and after dissolution and combining it with nanoindentation results enables a visualized analysis of the effects of pore on macromechanical properties. Overall, these findings suggest that dissolution, influenced not only by acidification but also by temperature and mineral composition, drives the evolution of pore structures and ultimately impacts the mechanical properties of CRL.

Seafloor massive sulfide (SMS) deposits in different geological settings can have variable magnetic mineralogy, but the mechanism and implications of their spatiotemporal diversity are poorly understood. Based on seabed shallow drilling and surficial sampling of the Yuhuang hydrothermal field, Southwest Indian Ridge, we investigate here whether ubiquitous oxidative weathering affects the magnetic properties of SMS deposits. Microscopic observation and ferrous iron concentrations reveal that seafloor SMS deposits are extensively oxidized; subseafloor SMS deposits are relatively fresh, but oxidation initiates immediately after sample recovery. Negative frequency dependence of magnetic susceptibility likely due to measurement eddy currents is observed for fresh samples but not for oxidized ones, which suggests that oxidative weathering reduces the electromagnetic detectability of SMS deposits in geophysical investigations. Pyrrhotite (and probably other magnetic iron sulfide minerals), magnetite, and hematite are recognized as dominating magnetic (ferromagnetic, sensu lato) minerals in SMS deposits. Electron and quantum diamond microscope observations reveal pyrrhotite mineralization from high‐temperature reducing hydrothermal fluids, while iron‐oxides are mostly oxidation products of primary sulfides. Oxidative weathering modifies paleomagnetic records of SMS deposits. Bulk magnetic parameters vary systematically with enhanced oxidation degree. Temperature‐dependent magnetic measurements are useful tools for distinguishing the oxidation state of SMS deposits. Overall, these findings explain magnetic mineral variability in SMS deposits, linking mineral magnetic properties with seafloor geophysical investigations. Mineral magnetism can also be a redox state proxy for tracing natural and artificial environmental fluctuations in seafloor hydrothermal fields, inspiring novel interdisciplinary research to understand interactions in the dynamic Earth System.

A shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading.

Kīlauea Volcano on Hawai'i Island is host to a complex volcanic and interwoven fault system. Over the last ∼120 years, a range of seismic events, including large earthquakes such as the 1975 Mw ${M}_{w}$7.7 Kalapana earthquake, creep, and slow slip events, have occurred along the décollement underlying Kilauea's south flank. We explore both the deformation and stress changes of Kīlauea from 1896 to 2018 by collating six geodetic data sets and creating an analytical model to determine the dominant deformation sources (i.e., fault planes, rifts, magma chambers) driving this system at different times. The 1975 Kalapana earthquake significantly altered the region's state of stress and deformation; we find the average slip along the décollement was reduced from 10 cm/yr prior, to 4 cm/yr after the rupture. Prior to 1975 no slip is resolved along the décollement where the earthquake nucleated, suggesting that this portion may have been locked leading up to the rupture. After 1975, décollement slip overall is smaller and more irregular, suggesting increased control by spatial variation of mechanical properties. We find increases in shear stress along the Kīlauea décollement and a decrease in normal compressive stress within the East Rift Zone prior to the Kalapana earthquake, creating favorable conditions for failure of the décollement and subsequent magmatic intrusion.

Studying the subsurface structure of volcanoes is crucial for understanding volcanic mechanisms, current status, and potential risks. However, the intricate physical and chemical processes occurring over geological timescales make it challenging to characterize subsurface features such as volcanic structures and hydrothermal systems. Given the highly attenuating nature of magma, 3‐D scattering and intrinsic attenuation tomography are critical methods for advancing our understanding of tectonic, magmatic, and hydrothermal processes and their interactions. Previous imaging techniques, however, required substantial memory usage and long computational times, limiting their application to only 1‐D velocity models. This paper proposes a novel sensitivity‐kernel calculation method for imaging shear wave seismic scattering and intrinsic attenuation. This method has the advantages of dramatically reducing memory and computational costs, as well as incorporating a 3‐D seismic velocity model. We apply this approach to illustrate the 3‐D scattering and intrinsic attenuation structures beneath the Toba volcano region in Northern Sumatra down to 20 km depth. Our results show high‐intrinsic attenuation anomalies around the Toba caldera, revealing the magma chambers feeding the volcanoes. A conspicuous high‐scattering attenuation anomaly is identified along the Great Sumatran Fault, possibly caused by the fault zone structure. Magmatism also likely contributes to the seismic activity south of the Toba caldera, as evidenced by the overlap of scattering and intrinsic attenuation anomalies.

Volcano deformation models contribute to hazard assessment by simulating magma system dynamics. Traditional magma reservoir pressure source shape assumptions often fail to replicate irregular, geophysically identified geometries. Uncertainties regarding the influence of reservoir geometry can limit the effectiveness of using deformation models to decipher unrest signals. Here, we aim to determine the feasibility of using a magma reservoir geometry directly derived from a seismic tomography survey in a volcano deformation model for Soufrière Hills Volcano, Montserrat. Three‐dimensional deformation models are created to simulate displacement using a pressure source geometry constrained from a low seismic velocity anomaly, inferred to be a region of partial melt, and contrasted against a traditional ellipsoid reservoir geometry. We also test a “hybrid” model combining a seismically inferred reservoir upper geometry and ellipsoidal base. Results of each model are evaluated against ground displacement observed on Montserrat from 2010 to 2022. Our results show that different reservoir geometries change the horizontal and vertical displacement fields across the island: the ellipsoid reservoir best reproduces vertical displacement magnitude, while the hybrid reservoirs best simulate horizontal displacement vectors and the region of maximum uplift. Overall, the ellipsoid‐shaped reservoir provides our best‐fit to the observed data, but we note this result could be biased due to prior years of optimization helping constrain the ellipsoid shape, size, and location. Our results show the potential for further use of geophysically constrained reservoir geometries in deformation modeling, and our methods could be applied to other deforming volcanoes worldwide.

The role of solid Earth tide in fault reactivation has significant implications for understanding earthquake triggering, carbon sequestration, and the global carbon budget. Despite extensive research on this topic, the relationship between Earth tide and fault reactivation remains unclear. In this study, we investigate the influence of solid Earth tide on the reactivation of sub‐seabed faults, which may lead to the release of methane. We monitored the sub‐seabed temperature and pore‐fluid pressure at two sites on a fault system located in the Black Sea. Two sets of data obtained from distinct periods revealed inconsistent results. For the first set of data and despite the distance between the two sites (∼790 m), the responses in terms of temperature and pore pressure changes were synchronous (September 2021). We showed from these data that the presence of over‐pressured fluid promotes fault reactivation under Earth tide cycles, resulting in synchronized degassing events. For the second set of data recorded from the same two sites (September 2021–May 2023), we did not identify any concomitance between Earth tides and the monitored parameters. Our analyses show that discrepancies in observations could be related to the fluid discharge/recharge process. The fluid discharge observed during the first period resulted in a decrease in excess pore‐pressure, making the fault insensitive to Earth tides during the subsequent recharge period. Our data also sheds light on conflicting literature results, suggesting that the interaction between faults and Earth tides primarily depends on fluid pore pressure, a parameter rarely measured.

Talc is expected to be an important water carrier in Earth's upper mantle, and understanding its electrical and seismic properties under high pressure and temperature conditions is required to detect possible talc‐rich regions in subduction zones imaged using geophysical observations. We conducted acoustic and electrical experiments on natural talc aggregates at relevant pressure‐temperature conditions. Compressional wave velocity (Vp) was measured using ultrasonic interferometry in a Paris‐Edinburgh press at pressures up to 3.4 GPa and temperatures up to 873 K. Similar Vp values are obtained regardless of the initial crystallographic preferred orientation of the samples, which can be explained by talc grain reorientation during the experiment, with the (001) plane becoming perpendicular to the uniaxial compression axis. Electrical conductivity of the same starting material was determined using impedance spectroscopy in a multi‐anvil press up to 6 GPa and 1263 K. Two conductivity jumps are observed, at ∼860–1025 K and ∼940–1080 K, depending on pressure, and interpreted as talc dehydroxylation and decomposition, respectively. Electrical anisotropy is observed at low temperature and decreases with increasing pressure (∼10 at 1.5 GPa and ∼2 at 3.5 GPa). Comparison of acoustic and electrical results with geophysical observations in central Mexico supports the presence of a talc‐bearing layer atop the subducted Cocos plate.

Plain Language Summary Volcano seismology is one of the main geophysical methods used to study volcanic processes and to forecast the eruptions. It is based on analysis of ground motion recorded by seismographs installed in the vicinity of volcanoes. Different seismic signals such as impulsive volcanic earthquakes and nearly continuous volcanic tremors are recorded during periods corresponding to preparation of eruptions. Some of them originate from depths of a few tens of kilometers, that is, from the roots of the system that feeds the magma supply to volcanoes and their eruptions. Therefore, such deep seismic sources are particularly interesting because they may represent early eruption precursors. While we still lack physical understanding of the processes leading to this deep volcanic seismicity, there are several reasons to consider that it is not caused by a sudden slip on faults responsible for the majority of “regular tectonic” earthquakes. In this paper, we use numerical simulations to test another possible mechanism of generation of deep volcanic earthquakes. Namely, we assume that they can be caused by rapid growth of bubbles from the gas that was initially dissolved in the magma. We use numerical simulations to demonstrate that this model predicts main properties of the observed seismic signals.

Offset geomorphic markers are commonly used to interpret slip history of strike‐slip faults and have played an important role in forming earthquake recurrence models. These data sets are typically analyzed using cumulative probability methods to interpret average amounts of slip in past earthquakes. However, interpretation of the geomorphic record to infer surface slip history is complicated by slip variability, measurement uncertainty, and modification of offset features in the landscape. To investigate how well geomorphic data record surface slip, we use offset measurements from recent strike‐slip surface ruptures (n = 39), faults with geomorphic evidence of multiple strike‐slip earthquakes (n = 29), and synthetic slip distributions with added noise (n> ${ >}$10,000) to examine the constraints of the geomorphic record and the underlying assumptions of the cumulative offset probability distribution analysis method. We find that the geomorphic record is unlikely to resolve more than two paleo‐slip distributions, except in specific cases with low slip variability, high slip‐per‐event, and semiarid climate. In cases where site‐specific conditions allow for interpretation of more than two earthquakes, lateral extrapolation along a fault is not straightforward because on‐fault displacement and distributed deformation may be spatially variable in each earthquake. We also find that average slip in modern earthquakes is adequately recovered by probability methods, but the reported prevalence of strike‐slip faults with characteristic slip history is not supported by geomorphic data. We also propose updated methods to interpret slip history and construct uncertainty bounds for paleo‐slip distributions.

Faulting and folding of basement rocks together accommodate convergence within continental orogens, forming complex zones of intraplate deformation shaped by the fault interaction. Here we use the river terraces along the Dongda river to examine the tectonic deformation patterns of the hinterland and the foreland of the eastern North Qilian Shan, a zone of crustal shortening located at the northeast margin of the Tibetan Plateau. Five Late Pleistocene–Holocene terraces of Dongda river are displaced by three major reverse faults: Minle‐Damaying fault, Huangcheng‐Ta'erzhuang fault, and Fengle fault, from south to north. Based on displaced terrace treads, we estimated vertical slip rates along the Minle‐Damaying fault as 0.7–0.8 mm/a, and along the Fengle fault as 0.5–0.7 mm/a. Deformed terraces suggest an additional uplift of ∼0.2 mm/a through the folding of the Dahuang Shan anticline. Inhomogeneous uplift of the intermontane basins between the Minle‐Damaying fault and the Dahuang Shan anticline indicates a 0.9 ± 0.2 mm/a uplift rate along the Huangcheng‐Ta'erzhuang fault. Kinematic modeling of this thrust system shows that deformation propagated northward toward the foreland along a south‐dipping 10° décollement rooted into the Haiyuan fault at the depth of ∼20 km. This system accommodates 2.7–3.4 mm/a total crustal shortening rate. We suggest this broad thrust belt and the relatively high rate of shortening within this part of the eastern Qilian Shan is a result of the oblique convergence along a restraining bend of Haiyuan fault system. The elevated shortening rate within this area indicates high potential seismic hazard.

In basaltic eruptions, bubbles move freely and collide within a volcanic conduit, leading to frequent bubble coalescence. Understanding the dynamics of buoyancy‐driven coalescence of bubbles is crucial for predicting the explosivity of basaltic eruptions. We examine the evolution of the bubble volume distribution while considering buoyancy‐driven coalescence and expansion due to decompression. We find that, at lower decompression rates, the bubble volume distribution n(v,t) n(v,t) rapidly evolves into a power‐law distribution with an exponent of approximately −2 ${-}2$ as n(v,t)∝v−2 $n(v,t)\propto {v}^{-2}$. This suggests that, in basaltic magma, the repeated coalescence of bubbles rapidly forms large bubbles within 45 min to 3 days. We then examine the occurrence of eruption styles, specifically Strombolian or Hawaiian, under the assumption that the bursts of slugs, produced from bubble coalescence within the conduit, trigger Strombolian eruptions. Consequently, we identify a critical condition for the transition between eruption styles in terms of the ascent velocity of magma. This critical ascent velocity is consistent with the observed transitions between Strombolian and Hawaiian eruptions at Izu‐Oshima and Kilauea.

We performed a series of hydraulic stimulations at 1.1 km depth in the Bedretto underground laboratory, Switzerland, as part of an overall research strategy attempting to understand induced seismicity on different scales. Using an ultra‐high frequency seismic network we detect seismic events as small as Mw < −4, revealing intricate details of a complex fracture network extending over 100 m from the injection sites. Here, we outline the experimental approach and present seismic catalogs as well as a comparative analysis of event number per injection, magnitudes, b‐values, seismogenic index and reactivation pressures. In our first‐order seismicity analysis, we could make the following observations: The rock volume impacted by the stimulations in different intervals differs significantly with a lateral extent from a few meters to more than 150 m. In most intervals multiple fractures were reactivated. The seismicity typically propagates upwards toward shallower depth on parallel oriented planes that are consistent with the stress field and seem to a large extent associated with preexisting open fractures. This experiment confirms the diversity in seismic behavior independent from the injection protocol. The overall seismicity patterns demonstrate that multi‐stage stimulations using zonal isolation allow developing an extended fracture network in a 3D rock volume, which is necessary for enhanced geothermal systems. Our stimulations covering two orders of magnitude in terms of injected volume will give insights into upscaling of induced seismicity from underground laboratory scale to field scale.

Distributed acoustic sensing (DAS) is an innovative technology with great potential for acquiring seismic data sets in urban areas. In this work, we check the suitability of a DAS data set acquired in Granada (Spain) for retrieving subsurface reflectivity from ambient noise. The fiber‐optic is a pre‐existing underground telecommunication cable that crosses the city from Northwest to Southeast. We use a 10 hr recording of strain rate from a 2020 experiment to obtain seismic reflections using the autocorrelation method. We compare the DAS results with reflections obtained from seismic ambient noise recorded in nine seismometers deployed close to the fiber‐cable for 7 days in November 2022. The novel approach proposed in this study for the identification of the reflections is to use autocorrelations after bandpass filtering for specific central frequencies and to check the stability of the signals over a broad frequency band. Microtremor Horizontal to Vertical Spectral Ratio (MHVSR) measurements at a total of 14 stations, five of them outside the city, help to constrain the reflection interpretation. These include one station at the borehole that reaches the basement in the Granada Basin crossing all the Cenozoic units. We use the legacy sonic log to obtain a relationship between frequencies of MHVSR peaks and depth. Autocorrelation and MHVSR methods give consistent results delineating bedrock depth deeper than 1,000 m in Granada. These results confirm that DAS can provide valuable subsurface information in urban areas.

Rare bedrock exposures of the eastern Denali fault zone in southwestern Yukon allow for the measurement, sampling, and analyses of brittle regime fault slip data and deformation mechanisms to explore relations to far field, oblique plate motions. Host rock lithologies and associated slip surfaces show episodic damage zone‐related deformation and calcite ± hematite ± chlorite related hydrothermal fluid flow. This regional scale network of asymmetric fault damage is spatially and kinematically linked to a discrete and narrow fault core. Fault network observations, orientations, slip data, and strain inversions document a slip partitioned strike‐slip fault system with locally and mutually overprinting strike‐, oblique‐, and dip‐slip components. Microstructural analyses reveal crystal plastic and co‐seismic brittle deformation mechanisms active in a narrow range of upper crustal temperature, pressure, fluid, and chemical conditions. The net damage related slip is not exclusively formed by a single kinematic system, but rather a fully partitioned, time integrated system likely operative for much of the fault's brittle regime evolution temporally constrained by previously published thermochronometric data. Although the fault slip data was collected from outcrop‐scale exposures at sites tens of kilometers apart, results show remarkable correlation between fault kinematics and plate motions along the ∼580 km long eastern Denali fault segment. End member, subhorizontal, northeast directed reverse and north directed dextral strike slip fault strain axes closely reflect relative plate motion interactions over at least the last 30 m.y. and act as a proxy for far‐field stresses compatible with the kinematics of the damage zone network.

Full waveform inversion (FWI) creates high resolution models of the Earth's subsurface structures from seismic waveform data. Due to the non‐linearity and non‐uniqueness of FWI problems, finding globally best‐fitting model solutions is not necessarily desirable since they fit noise as well as the desired signal in data. Bayesian FWI calculates a so‐called posterior probability distribution function, which describes all possible model solutions and their uncertainties. In this paper, we solve Bayesian FWI using variational inference, and propose a new methodology called physically structured variational inference, in which a physics‐based structure is imposed on the variational distribution. In a simple example motivated by prior information from imaging inverse problems, we include parameter correlations between pairs of spatial locations within a dominant wavelength of each other, and set other correlations to zero. This makes the method far more efficient compared to other variational methods in terms of both memory requirements and computation, at the cost of some loss of generality in the solution found. We demonstrate the proposed method with a 2D acoustic FWI scenario, and compare the results with those obtained using other methods. This verifies that the method can produce accurate statistical information about the posterior distribution with hugely improved efficiency (in our FWI example, 1 order of magnitude reduction in computation). We further demonstrate that despite the possible reduction in generality of the solution, the posterior uncertainties can be used to solve post‐inversion interrogation problems connected to estimating volumes of subsurface reservoirs and of stored CO2 ${\text{CO}}_{2}$, with minimal bias, creating a highly efficient FWI‐based decision‐making workflow.

In subduction zones, along‐strike and downdip variations in megathrust slip behavior are linked to changes in properties of the subducting and overriding plates. Although marine geophysical methods provide insights into subduction zone structures, most surveys consist of sparse 2D profiles, limiting our understanding of first‐order controls. Here, we use active‐source seismic data to derive a 3D crustal‐scale P‐wave velocity model of the Alaska Peninsula subduction zone that encompasses both plates and spans the Semidi segment and SW Kodiak asperity. Our results reveal modest variations within the incoming plate, attributed to a series of fracture zones, seamounts and their associated basement swell, collectively contributing to plate hydration. Basement swell appears to modulate the distribution and type of sediment entering the trench, likely impacting observed variations in slip behavior. The overriding plate exhibits significant heterogeneity. The updip limit and width of the dynamic backstop are similar between the SW Kodiak asperity and eastern Semidi segment, but differ significantly from the Western Semidi segment. These distinctions likely account for differences in earthquake rupture patterns and interseismic coupling among these segments. Additionally, high‐velocities in the mid‐lower forearc crust coincide with the location of megathrust slip during the Mw 8.2 2021 Chignik event. We interpret these velocities as intracrustal intrusions that contributed to the deep rupture of the 2021 event. Our findings suggest that the contrasting structural and material properties of both the incoming and overriding plates influence the spatially complex and semi‐persistent segmentation of the megathrust offshore the Alaska Peninsula.

It remains controversial whether the interaction between a mantle plume and a craton destabilizes or reinforces the craton. The Tarim basin, with a craton core, a Permian Large Igneous Province, and internal deformation, is an ideal place to investigate this interaction. Here, we construct high‐resolution S‐wave velocity structures down to 15 km in depth using multi‐frequency receiver functions from two temporary seismic arrays that largely cover the Tarim Basin. Our results reveal a strong velocity‐increasing discontinuity across the basin and several large‐scale high‐Vs anomalies. The discontinuity is flat at about 3.5 km depth in the majority of eastern Basin but is uplifted and folded to ∼3 km depth around the Bachu Uplift in the central‐western basin and depressed to more than 6 km depth in the northwestern and southwestern basin. The high‐Vs anomalies, with an average Vs of ∼3.4 km/s, are concentrated under this discontinuity around the Bachu Uplift. Analysis with drilling data, experimental rock‐physics data and previous geophysical observations indicates that the discontinuity corresponds to the top of early Permian strata, and the high‐Vs anomalies are the magmatic intrusions from the early Permian mantle plume. There is strong deformation around the Bachu Uplift formed during Cenozoic Indian‐Eurasian collision, exhibiting a strong spatial correlation with the Permian magmatic intrusions. This suggests that the western Tarim Craton, compared to the east, may be weakened in strength by the Permian mantle plume and exhibits more localized Cenozoic deformation.

The Lhasa terrane in southern Tibet occupies a central position in Asian tectonics, yet its pre‐Mesozoic petrologic and tectonic evolution is poorly constrained, especially the Southern Lhasa subterrane (SLS). Here, new zircon U–Pb ages, zircon trace element and Hf isotopic compositions, and whole‐rock geochemical data for mafic meta‐igneous rocks from the SLS distinguish three tectono‐thermal events at ∼290 Ma, ∼126 Ma and ∼49 Ma. Whole‐rocks and zircons with ages of the two older events have arc magma geochemistry, but Hf isotopes are distinct from Mesozoic Gangdese arc magmas. Zircon cores and, arguably, whole rocks instead derive from ∼290 Ma magma formed during southward subduction of Paleo‐Tethys beneath the SLS. These rocks later underwent Early Cretaceous (∼126 Ma) remelting and early Cenozoic (∼49 Ma) metamorphism with P–T conditions of ~800°C and 15.3 kbar, and record a retrograde P–T path characterized by exhumation with cooling, consistent with a collisional origin. These data suggest Permian igneous rocks underwent reworking during both the Early Cretaceous and the early Cenozoic, reaching crustal thicknesses of at least ~50 km during the early Eocene. Combined with regional data, Paleo‐Tethys evidently experienced early Permian double‐sided subduction within the Lhasa terrane, with back arc basins forming between the SLS and the Indian margin to the south. These back arc basins ultimately widened to form the Neo‐Tethys Ocean, which then subducted during the Mesozoic, leading to Cretaceous arc magmatism and overprinting, followed by early Cenozoic metamorphism and final reworking during collision with India.