Increasing deployment of dense arrays has facilitated detailed structure imaging for tectonic investigation, hazard assessment and resource exploration. Strong velocity heterogeneity and topographic changes have to be considered during passive source imaging. However, it is quite challenging for ray-based methods, such as Kirchhoff migration or the widely used teleseismic receiver function, to handle these problems. In this study, we propose a 3-D passive source reverse time migration strategy based on the spectral element method. It is realized by decomposing the time reversal full elastic wavefield into amplitude-preserved vector P and S wavefields by solving the corresponding weak-form solutions, followed by a dot-product imaging condition to get images for the subsurface structures. It enables us to use regional 3-D migration velocity models and take topographic variations into account, helping us to locate reflectors at more accurate positions than traditional 1-D model-based methods, like teleseismic receiver functions. Two synthetic tests are used to demonstrate the advantages of the proposed method to handle topographic variations and complex velocity heterogeneities. Furthermore, applications to the Laramie array data using both teleseismic P and S waves enable us to identify several south-dipping structures beneath the Laramie basin in southeast Wyoming, which are interpreted as the Cheyenne Belt suture zone and agree with, and improve upon previous geological interpretations.

Significant imbalances in terrestrial water storage (TWS) and severe drought have been observed around the world as a consequence of climate changes. Improving our ability to monitor TWS and drought is critical for water-resource management and water-deficit estimation. We use continuous seismic ambient noise to monitor temporal evolution of near-surface seismic velocity, dv/v, in central Oklahoma from 2013 to 2022. The derived dv/v is found to be negatively correlated with gravitational measurements and groundwater depths, showing the impact of groundwater storage on seismic velocities. Seasonal cycling of dv/v follows atmospheric temperature changes with a phase shift, which can be explained by thermo-elastic strain in the uppermost crust and sedimentary cover. The occurrences of droughts appear simultaneously with the local peaks of dv/v, demonstrating the sensitivity of near-surface seismic velocities to droughts. The results illustrate the potential of using seismic data for monitoring TWS and drought at regional to local scales.

Over the past decade, the seismicity rate in the state of Oklahoma has increased significantly, which has been linked to industrial operations, such as saltwater injection. Taking advantage of induced earthquakes and recently deployed seismometers, we construct a 3-D radially anisotropic seismic velocity model for the crust of Oklahoma by using full waveform inversion. To mitigate the well-known cycle-skipping problem, we use misfit functions based on phase and waveform differences in several frequency bands. Relative velocity perturbations in the inverted model allow us to delineate major geological provinces in Oklahoma, such as the Anadarko and Arkoma Basins, as well as the Cherokee Platform and Shelf. In addition, radial anisotropy in the inverted model reflects deformation within the crust of Oklahoma, which might correlate with sedimentary layers, micro-cracks/fractures, as well as the dominant orientation of anisotropic minerals. The crystalline basement beneath Oklahoma can be inferred from the new velocity model, which enables us to classify induced seismicity in current earthquake catalogs better. Furthermore, synthetic experiments suggest that the new velocity model enables us to better constrain earthquake location in Oklahoma, especially for determining their depths, which are important for investigating induced seismicity.

For seismographic stations with short acquisition duration, the signal-to-noise ratios (SNRs) of ambient noise cross-correlation functions (CCFs) are typically low, preventing us from accurately measuring surface wave dispersion curves or waveform characteristics. In addition, with low-quality CCFs, it is difficult to monitor temporal variations of subsurface physical states or extract relatively weak signals such as body waves. In this study, we propose to use local attributes to improve the SNRs of ambient noise CCFs, which allows us to enhance the quality of CCFs for stations with limited acquisition duration. Two local attributes: local cross-correlation and local similarity, are used in this study. The local cross-correlation allows us to extend the dimensionality of daily CCFs with computational costs similar to global cross-correlation. Taking advantage of this extended dimensionality, the local similarity is then used to measure non-stationary similarity between the extended daily CCFs with a reference stacking trace, which enables us to design better stacking weights to enhance coherent features and attenuate incoherent background noises. Ambient noise recorded by several broadband stations from the USArray in North Texas and Oklahoma, the Superior Province Rifting EarthScope Experiment in Minnesota and Wisconsin and a high-frequency nodal array deployed in the San Bernardino basin are used to demonstrate the performance of the proposed approach for improving the SNR of CCFs.

In order to investigate seismic structures beneath the Australian continent and slab geometry of subduction zones around the Australian plate, we construct a new radially anisotropic shear wavespeed model, namely AU21, by applying full-waveform inversion technique to three-component data from 248 regional earthquakes and recorded by 1,102 seismographic stations. Twenty one preconditioned conjugate gradient iterations are utilized to minimize discrepancies between observed and synthetic seismograms, resulting 32,655 body-wave (15-40 s) and 35,897 surface-wave (25-100 s) measurements in the final iteration. AU21 confirms the sharp contrast in shear wavespeed perturbations between the Phanerozoic eastern continental margin with the Precambrian western and central Australia, and this boundary is offset to the east of the Tasman Line. The lithosphere-asthenosphere boundary is about 250-300 km beneath central and western Australia. A unique weak radial anisotropy layer at 80-150 km is observed beneath the western Australian craton, which may be caused by dipping layers alignments or tilted symmetry axes of anisotropic minerals. Slow anomalies extending down to the uppermost lower mantle beneath east of New Guinea, the Tasman Island, and the Tasman Sea may suggest deep thermal activities beneath these regions, possibly contributing to the formation of the low wavespeed band along the eastern Australian margin. Furthermore, we observe that the Tonga slab is stagnant within the mantle transition zone, while the Kermadec slab penetrates through the 660-km discontinuity down to the lower mantle.