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Abstract 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. 

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. 

Abstract 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. 

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. 

SUMMARY 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° periodicity), a few studies have also demonstrated strong but spurious $$1\psi $$ azimuthal anisotropy (360° 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. 

Abstract The crustal structure in south‐central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike‐slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month‐long north‐south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi‐component noise cross‐correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2‐D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low‐velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low‐velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low‐velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low‐velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low‐velocity zone as a previously unknown subduction‐related crustal magma reservoir located beneath the DVG. 

Abstract Near‐surface seismic velocity structure plays a critical role in ground motion amplification during large earthquakes. In particular, the local Vp/Vs ratio strongly influences the amplitude of Rayleigh waves. Previous studies have separately imaged 3D seismic velocity and Vp/Vs ratio at seismogenic depth, but lack regional coverage and/or fail to constrain the shallowest structure. Here, we combine three datasets with complementary sensitivity in a Bayesian joint inversion for shallow crustal shear velocity and near‐surface Vp/Vs ratio across Southern California. Receiver functions–including with an apparent delayed initial peak in sedimentary basins, and long considered a nuisance in receiver function imaging studies–highly correlate with short‐period Rayleigh wave ellipticity measurements and require the inclusion of a Vp/Vs parameter. The updated model includes near‐surface low shear velocity more in line with geotechnical layer estimates, and generally lower than expected Vp/Vs outside the basins suggesting widespread shallow fracturing and/or groundwater undersaturation. 

Abstract Steamboat Geyser in Yellowstone National Park is the tallest active geyser on Earth and is believed to have hydrologic connection to Cistern Spring, a hydrothermal pool ∼100 m southwest from the geyser vent. Despite broad scientific interest, rare episodic Steamboat eruptions have made it difficult to study its eruption dynamics and underground plumbing architecture. In response to the recent reactivation of Steamboat, which has produced more than 130 eruptions since March 2018, we deployed a dense seismic nodal array surrounding the enigmatic geyser in the summer of 2019. The array recorded abundant 1–5 Hz hydrothermal tremor originating from phase‐transition events within both Steamboat Geyser and Cistern Spring. To constrain the spatiotemporal distribution of the tremor sources, an interferometric‐based polarization analysis was developed. The observed tremor locations indicate that the conduit beneath Steamboat is vertical and extends down to ∼120 m depth and the plumbing of Cistern includes a shallow vertical conduit connecting with a deep, large, and laterally offset reservoir ∼60 m southeast of the surface pool. No direct connection between Steamboat and Cistern plumbing structures is found. The temporal variation of tremor combined within situtemperature and water depth measurements of Cistern reveals interaction between Steamboat and Cistern throughout the eruption/recharge cycles. The observed delayed responses of Cistern Spring in reaction to Steamboat eruptions and recharge suggest that the two plumbing structures may be connected through a fractured/porous medium instead of a direct open channel, consistent with our inferred plumbing structure. 

Abstract The Formosa array, with 137 broadband seismometers and ∼5 km station spacing, was deployed recently in Northern Taiwan. Here by using eight months of continuous ambient noise records, we construct the first high‐resolution three‐dimensional (3‐D) shear wave velocity model of the crust in the area. We first calculate multi‐component cross‐correlations to extract robust Rayleigh wave signals. We then determine phase velocity maps between 3 and 10 s periods using Eikonal tomography and measure Rayleigh wave ellipticity at each station location between 2 and 13 s periods. For each location, we jointly invert the two types of Rayleigh wave measurements with a Bayesian‐based inversion method for a one‐dimensional shear wave velocity model. All piecewise continuous one‐dimensional models are then used to construct the final 3‐D model. Our 3‐D model reveals upper crustal structures that correlate well with surface geological features. Near the surface, the model delineates the low‐velocity Taipei and Ilan Basins from the adjacent fast‐velocity mountainous areas, with basin geometries consistent with the results of previous geophysical exploration and geological studies. At a greater depth, low velocity anomalies are observed associated with the Linkou Tableland, Tatun Volcano Group, and a possible dyke intrusion beneath the Southern Ilan Basin. The model also provides new geometrical constraints on the major active fault systems in the area, which are important to understand the basin formation, orogeny dynamics, and regional seismic hazard. The new 3‐D shear wave velocity model allows a comprehensive investigation of shallow geologic structures in the Northern Taiwan. 

Abstract We present observations and modeling of spatial eigen‐functions of resonating waves within fault zone waveguide, using data recorded on a dense seismic array across the San Jacinto Fault Zone (SJFZ) in southern California. The array consists of 5‐Hz geophones that cross the SJFZ with ~10–30 m spacing at the Blackburn Saddle near the Hemet Stepover. Wavefield snapshots after theSwave arrival are consistent for more than 50 near‐fault events, suggesting that this pattern is controlled by the fault zone structure rather than source properties. Data from example event with high signal to noise ratio show three main frequency peaks at ~1.3, ~2.0, and ~2.8 Hz in the amplitude spectra of resonance waves averaged over stations near the fault. The data are modeled with analytical expressions for eigen‐functions of resonance waves in a low‐velocity layer (fault zone) between two quarter‐spaces. Using a grid search‐based method, we investigate the possible width of the waveguide, location within the array, and shear wave velocities of the media that fit well the resonance signal at ~1.3 Hz. The results indicate a ~300 m wide damaged fault zone layer with ~65%Swave velocity reduction compared to the host rock. The SW edge of the low‐velocity zone is near the mapped fault surface trace, indicating that the damage zone is asymmetrically located at the regionally faster NE crustal block. The imaging resolution of the fault zone structure can be improved by modeling fault zone resonance modes and trapped waves together.