LAB2022 is a new temporary array consisting of 273 geophones that was deployed in the Los Angeles basin for one month during the summer of 2022. The array was designed to improve the 3D seismic velocity model of the basin through passive seismic imaging, which is crucial for both earthquake hazard assessment and the understanding of the region’s tectonic evolution. The sensors are 3C 5 Hz Zland and Smart Solo instruments. The data has been archived at the EarthScope SAGE Data Management Center and will be publicly available in summer 2025.

Magma reservoir organization is hidden from view but important to systematic understanding of active volcanoes and intrusive igneous rocks. Seismic anisotropy in active silicic reservoirs suggests horizontally elongated melt sheets or sills, but it remains unclear if shallow silicic reservoirs in low strain rate settings without recent recharge maintain similar organization. Here we present new 3-D seismic tomography of the upper crustal reservoir beneath the tectonically quiescent Valles Caldera using dense nodal arrays. Results reveal >15

In 2018, 32 three-component geophones were deployed in a linear array with ∼90 m average station spacing across the active East Bench fault (EBF) in Salt Lake City, Utah. This west-dipping normal fault is a relatively young section of the Wasatch fault zone which cuts through the northeastern Salt Lake basin (SLB), west of and mostly parallel to the older Foothill fault section at the base of the Wasatch Range. Here, we investigate the variation of bedrock depth across the fault by examining the first peak of teleseismic receiver functions (RFs) along the linear array. The arrival time of this initial phase at stations in the SLB is particularly sensitive to the depth of a velocity discontinuity, referred to as R2, between semi-consolidated sediment and bedrock, which is consolidated sedimentary rock in most of the SLB. We observe that the first peak times are very similar on either side of the EBF but shift abruptly by ∼0.7 s across the fault. Based on two recent 1D velocity models, one for each side of the fault, this time shift indicates a vertical R2 offset of 810 ± 290 m across the EBF. This offset likely reflects the accumulated slip on the fault since the fault’s activation. Combining this offset with a current slip rate of 1.2 ± 0.2 mm/yr, based on paleoseismic studies of the Wasatch fault, and a range of models for how this slip rate may have changed over time, our best estimate for the age of the EBF is 0.7–1.4 Ma. From the R2 offset and the distance between stations over which it is observed, we calculate that the EBF has a minimum average dip of 81° ± 3° in the top 1100 m.

Structural boundaries are often the features of most interest geologically, but imaging them can be difficult due to wavefield scattering and interference caused by the sharp velocity contrasts. One example of this is the apparent Rayleigh-wave anisotropy (1-psi anisotropy) that has been observed near major structural boundaries using seismic arrays. The cause of the apparent anisotropy is the interference between the incident surface wave and waves scattered from velocity discontinuities. In this study, we first investigate the sensitivity of apparent anisotropy measurements to lateral boundary sharpness through 2-D full waveform simulations. We demonstrate that 1-psi anisotropy can vary based on boundary sharpness, station spacing and period of surface waves. We show that a misfit defined using triple-difference traveltimes, that is, the difference in double-difference traveltimes between station pairs with opposite propagation directions, well characterizes the apparent anisotropy. The sensitivity kernel for this triple-difference misfit can be constructed using the adjoint method. We show that triple-difference traveltimes are mainly sensitive to velocity contrasts rather than absolute velocities, in contrast to double-difference traveltimes. With sensitivity kernels constructed, we demonstrate how triple-difference traveltimes can be combined with double-difference traveltimes into a tomography inversion. We show that by including triple-difference traveltimes, seismic inversions converge faster and resolve boundary and average structure better in early iterations, compared to using double-difference traveltimes alone. Recent advancements in dense array experiments could facilitate the application of this method to better delineate tectonic and basin structural boundaries.

Yellowstone's vigorous hydrothermal system has long been recognized, but it remains challenging to quantify its magmatic heat supply. Here, we use two controlled-source seismic reflection transects and 3-D passive-source tomography to guide a simple model of magmatic heat supply coupled to a thermodynamic model of hydrothermal reservoir convection. Key model properties are the surface area of the magmatic-hydrothermal interface, conductive-boundary-layer thickness, and the temperature difference between the top of the magma reservoir and the base of the hydrothermal system. Magmatic temperature is set to an estimated rhyolite solidus, and the base of the hydrothermal system is estimated near the critical point for water. Seismic reflections are consistent with a conductive boundary layer thickness of similar to 120 m or less, and a shear-velocity-tomography contour that encloses the reflections has an area of similar to 1190 km2 (40% of the 0.63 Ma caldera). The magmatic heat supply model predicts at least 7.5 GW, which is consistent with a prior geochemical estimate of 6.6 GW of output to surface water and similar to 12% convective heat loss. The new results support the feasibility of system-scale geothermal budget estimation for onshore magmatic and hydrothermal reservoirs.

Ongoing climate change leads to an increase in prolonged drought and severe weather events in the United States, particularly pronounced in semi-arid regions such as the western United States. It could have lasting social and environmental impacts. Continuous monitoring of near-surface hydrological processes and groundwater resources will provide helpful information for effective water resource management. The seismological signature of groundwater fluctuations is clear in the temporal variations in seismic velocities, dv/v. To this end, developing a proxy for groundwater level using dv/v is an opportunity but requires further understanding of the relation between dv/v and subsurface hydrology. In this study, we apply single-station cross-component correlation analysis to 28 broadband seismic stations in Utah between January 2006 and March 2023 and analyze the dv/v in the 2-4 Hz frequency band. To explain dv/v, we linearly superimpose thermoelastic stresses, soil moisture estimated from remote sensing data products, and a long-term deep water table pore pressure. We find that the relative contributions of each depend on the location, but adding a long-term water table decline, which is not systematically observed in soil moisture, better fits our data. We conclude that soil moisture alone does not explain the variations in total water storage when subsurface moisture is decoupled from the deep water table. We also conclude that dv/v can be used as a proxy for water storage.

Abstract We employ passive seismic interferometry to study seismic hydrological responses in a snowmelt‐driven, semi‐arid catchment. Seismic velocity changes (dv/v) at 2–4 Hz and 4–8 Hz are analyzed from six stations along a hillslope transect between May and November 2023 and compared to precipitation, soil moisture, and groundwater level timeseries. Variation in dv/v correlates with groundwater level and mirrors seasonal catchment wetting and evapotranspiration. Correlations between dv/v and groundwater are stronger at stations on the cooler, wetter, north‐facing hillslope than on the warmer, drier, south‐facing hillslope, consistent with expected recharge and evapotranspiration patterns. Different dv/v behaviors across two bands suggest depth‐dependent pore‐pressure changes, outlining different hydrologic zones. These findings highlight the utility of dv/v measurements in detecting laterally supplied water inputs, providing insight into local vertical water losses, effectively tracking groundwater–vegetation interactions, and supporting ecohydrological research and water management in semi‐arid snowmelt systems.

Abstract We apply ambient noise tomography to a seismic array from the Trans‐Haiti project to obtain a 2‐D shear wave velocity (Vs) across Haiti. We perform multi‐component noise cross‐correlation, measure Rayleigh wave phase velocity and its horizontal‐to‐vertical amplitude ratio (H/V) between periods of 3–18 s, and jointly invert both measurements into Vs for the crustal structures of Haiti. Both H/V and phase velocity measurements exhibit consistent patterns related to the geologic units. Sedimentary basins—CSE and Plateau Central basins—show higher H/V values, while mountain areas—Massif de la Selle, Chaine des Matheux, Montagnes Noires and Massif de Nord—exhibit lower H/V. Regarding phase velocity, higher velocities are observed in northern and southern Haiti, likely reflecting the thinner crust compared to the thicker crust showing lower velocities in the central part. While our Vs model is consistent with previous model that suggested thinner crustal thickness in the northern and southern Haiti, with thickening in the center, the Moho interface in the central domain might be shallower than previously thought.

This study presents a new velocity model for the Salt Lake basin (SLB) in Utah, determined using data from permanent and temporary seismic stations located on top of the basin in the Salt Lake Valley (SLV) and nearby. A three‐dimensional (3D) velocity model for the SLB is needed for accurate predictions of future damaging earthquake ground shaking in the heavily urbanized SLV, including Salt Lake City. The SLB part of the Wasatch Front community velocity model (WFCVM) currently serves this purpose. However, the current WFCVM is based primarily on gravity and borehole data with relatively few seismic constraints below depths of 100 m. In this study we use the first peak of SLV receiver functions (RFs), which is sensitive to a strong impedance contrast at the base of a semi‐consolidated sediment layer. We jointly invert the RF waveform with Rayleigh wave ellipticity (H/V) and phase velocity measurements using the Markov chain Monte Carlo approach. Our new velocity model shows a greater combined thickness of unconsolidated and semi‐consolidated sediments, compared to the WFCVM, in the northeastern SLB between the west‐dipping East Bench fault section of the Wasatch fault and the antithetic West Valley fault zone to the west. We show that the new seismic velocity model explains the gravity patterns in the valley. The new velocity model from this study provides a basis for revising the SLB model in the WFCVM.

Numerical simulations of seismic wave propagation are crucial for investigating velocity structures and improving seismic hazard assessment. However, standard methods such as finite difference or finite element are computationally expensive. Recent studies have shown that a new class of machine learning models, called neural operators, can solve the elastodynamic wave equation orders of magnitude faster than conventional methods. Full waveform inversion is a prime beneficiary of the accelerated simulations. Neural operators, as end-to-end differentiable operators, combined with automatic differentiation, provide an alternative approach to the adjoint-state method. State-of-the-art optimization techniques built into PyTorch provide neural operators with greater flexibility to improve the optimization dynamics of full waveform inversion, thereby mitigating cycle-skipping problems. In this study, we demonstrate the first application of neural operators for full waveform inversion on a real seismic dataset, which consists of several nodal transects collected across the San Gabriel, Chino, and San Bernardino basins in the Los Angeles metropolitan area.

Earthquakes in continental regions overwhelmingly occur in the crust where low pressure and temperature promote brittle failure in response to tectonic stress. In rare cases, primarily in the thickened lithosphere near the Himalayas and Tibet, continental earthquakes occur in the uppermost mantle, perhaps implying an abnormally deep brittle‐ductile transition zone created by relatively low temperatures (≲600°C) and the increased strength of olivine‐rich mantle rocks. Here we present evidence for nine mantle earthquakes—only four of which were previously recognized—along the edge of the Wyoming Craton in the western U.S. Eight of the nine earthquakes occurred >15 km beneath the Moho where temperatures are likely above 700°C. We infer a mixture of brittle and ductile (thermal runaway) source processes facilitated by elevated strain rates from regional or edge‐driven mantle convection, which is thought to be a primary force behind crustal seismicity in the Intermountain West.

The stability of hazardous volcanic systems is strongly influenced by the uppermost magma storage depth and volatile exsolution1-3. Despite abundant evidence for an upper crustal magma reservoir beneath Yellowstone caldera4-7, its depth and the properties at its top have not been well constrained. New controlled-source seismic imaging illuminates a sharp reflective cap of the magma reservoir approximately 3.8 km beneath the northeastern caldera. Magma ascent to such low pressure is expected to drive volatile exsolution and potentially localized accumulation of bubbles near the top of the reservoir8,9, but this process typically remains hidden in contemporary volcanic systems. P-wave and P-to-S-wave reflections from the sharp top of the Yellowstone magma reservoir indicate that a mixture of supercritical fluid and magma fills the pore space at the cap of the approximately 3-8-km-deep low-shear-velocity layer imaged by seismic tomography6,7. The results are consistent with partial retention of bubbles exsolved from an upper crustal reservoir with ongoing magma supply from a volatile-enriched mantle source. Bubble accumulation can eventually lead to reservoir instability2,8, but the bubble volume fraction seismically estimated at the top of the reservoir today is lower than typical estimates of pre-eruptive conditions for rhyolites1,10,11, and measurements of the hydrothermal system document high fluxes of magmatic volatiles escaping to the surface12-15. We infer that the magma reservoir is in a stable state of efficient bubble ascent into the hydrothermal system on the basis of estimates that it is a crystal-rich (less than 30% porosity) reservoir for which dynamic modelling favours channelized bubble escape that prevents instability8.

ABSTRACT Seismic tomography is the most abundant source of information about the internal structure of the Earth at scales ranging from a few meters to thousands of kilometers. It constrains the properties of active volcanoes, earthquake fault zones, deep reservoirs and storage sites, glaciers and ice sheets, or the entire globe. It contributes to outstanding societal problems related to natural hazards, resource exploration, underground storage, and many more. The recent advances in seismic tomography are being translated to nondestructive testing, medical ultrasound, and helioseismology. Nearly 50 yr after its first successful applications, this article offers a snapshot of modern seismic tomography. Focused on major challenges and particularly promising research directions, it is intended to guide both Earth science professionals and early-career scientists. The individual contributions by the coauthors provide diverse perspectives on topics that may at first seem disconnected but are closely tied together by a few coherent threads: multiparameter inversion for properties related to dynamic processes, data quality, and geographic coverage, uncertainty quantification that is useful for geologic interpretation, new formulations of tomographic inverse problems that address concrete geologic questions more directly, and the presentation and quantitative comparison of tomographic models. It remains to be seen which of these problems will be considered solved, solved to some extent, or practically unsolvable over the next decade.

The recent developments in array-based surface-wave tomography have made it possible to directly measure apparent phase velocities through wave front tracking. While directionally dependent measurements have been used to infer intrinsic $2\psi $ azimuthal anisotropy (with a 180 degrees periodicity), a few studies have also demonstrated strong but spurious $1\psi $ azimuthal anisotropy (360 degrees periodicity) near major structure boundaries particularly for long period surface waves. In such observations, Rayleigh waves propagating in the direction perpendicular to the boundary from the slow to the fast side persistently show a higher apparent velocity compared to waves propagating in the opposite direction. In this study, we conduct numerical and theoretical investigations to explore the effect of scattering on the apparent Rayleigh-wave phase velocity measurement. Using 2-D spectral-element numerical wavefield simulations, we first reproduce the observation that waves propagating in opposite directions show different apparent phase velocities when passing through a major velocity contrast. Based on mode coupling theory and the locked mode approximation, we then investigate the effect of the scattered fundamental-mode Rayleigh wave and body waves interfering with the incident Rayleigh wave separately. We show that scattered fundamental-mode Rayleigh waves, while dominating the scattered wavefield, mostly cause short wavelength apparent phase velocity variations that could only be studied if the station spacing is less than about one tenth of the surface wave wavelength. Scattered body waves, on the other hand, cause longer wavelength velocity variations that correspond to the existing real data observations. Because of the sensitivity of the $1\psi $ apparent anisotropy to velocity contrasts, incorporating such measurements in surface wave tomography could improve the resolution and sharpen the structural boundaries of the inverted model.

SUMMARY Taiwan, one of the most active orogenic belts in the world, undergoes orogenic processes that can be elucidated by the doubly vergent wedge model, explaining the extensive island-wide geological deformation. To provide a clearer depiction of its cross-island orogenic architecture, we apply ambient noise tomography across an east–west linear seismic array in central Taiwan, constructing the first high-resolution 2-D shear velocity model of the upper crust in the region. We observe robust fundamental- and higher-mode Rayleigh waves, with the latter being mainly present in the western Coastal Plain. We develop a multimode double-beamforming method to determine local phase velocities across the array between 2- and 5-s periods. For each location, we jointly invert all available fundamental- and higher-mode phase velocities using a Bayesian-based inversion method to obtain a 1-D model. All 1-D models are then combined to form a final 2-D model from the surface to ∼10 km depth. Our newly developed 2-D model clearly delineates major structural boundaries and fault geometries across central Taiwan, thereby corroborating the previously proposed pro-wedge and retro-wedge models while offering insight into regional seismic hazards.

Abstract Doublet Pool is an active hydrothermal feature in Yellowstone National Park, USA. Approximately every half hour, it thumps for about 10 min due to bubbles collapsing at the base of the pool. To understand its thermodynamics and sensitivity to external factors, we performed a recurring multiple‐year passive seismic experiment. By linking recorded hydrothermal tremor with active thumping, we determine the onset and end of thumping, and the duration of silence between each thumping cycle. The silence interval decreased from around 30 min before November 2016 to around 13 min in September 2018. This change followed unusual thermal activity on the surrounding Geyser Hill. On a shorter time scale, wind‐driven evaporative cooling can lengthen the pre‐thumping silence interval. Based on energy conservation, we determine the heating rate and heat needed to initiate thumping to be 3–7 MW and ∼6 GJ, respectively.

Understanding the distribution and mobility of crystal mushes within modern magmatic systems is crucial to volcanic hazard assessments as distinct pockets of mobile magma may become interconnected and lead to melt accumulation on shorter time scales than magma that is broadly distributed in a homogeneous mush. Here, we reveal that Yellowstone's upper-crustal magma reservoir in the top 20 km is heterogeneous in both melt concentration and texture. We exploit ambient noise in an unprecedented dense temporary seismic network to jointly constrain vertically- and horizontally-polarized shear wave speeds to create enhanced 3D isotropic and anisotropic shear velocity models. Our models show an exceptionally low-velocity (>20% reduction) layer 4–7 km beneath the surface, situated near the top of the reservoir previously-imaged by earthquake P-wave tomography. The presence of strong radial anisotropy (20%) within this layer indicates the uppermost portion of the modern Yellowstone reservoir is organized as a sill complex, with up to 28% of melt fraction in horizontally-elongated volumes at depths where rhyolite was commonly stored before past eruptions. The findings demonstrate that high-resolution anisotropic imaging through a dense seismic network can constrain both magma distribution and reservoir texture, which are important to understand the evolution and hazard assessment of volcanic systems like Yellowstone.

Rayleigh wave ellipticity and phase velocity measurement from ambient noise were used to estimate a 2-D Vs profile in Central Alaska across the Alaska Range

We construct a 3D shear velocity model of the Salt Lake Valley using Rayleigh waves excited by the 31 March 2020 Mw 6.5 central Idaho earthquake recorded on a 168-station temporary nodal geophone network and the 49-station permanent regional network. The temporary array—deployed in response to the March 18 Mw 5.7 Magna earthquake—serendipitously recorded clear surface waves between 10 and 20 s period from the Idaho event at ∼500 km epicentral distance, from which we measure both Rayleigh wave phase velocity and ellipticity (H/V ratio). In addition, we employ multicomponent earthquake coda cross correlation to extend the measurements down to 5 s period. Because Rayleigh wave ellipticity features outstanding shallow sensitivity, we invert for a 3D upper crust VS model of the Salt Lake Valley. Our model shows basin structure in general agreement with and complements the current Community Velocity Model, which is mostly constrained by borehole and gravity measurements. Our model thus provides critical information for future earthquake hazard assessment studies, which require detailed shallow velocity structure.