We conduct a high-resolution teleseismic receiver function investigation of the subducting plate interface within the Alaskan forearc beneath Kodiak Island using data collected as part of the Alaska Amphibious Community Seismic Experiment in 2019. The Kodiak node array consisted of 398 nodal geophones deployed at ~200-300 m spacing on northeastern Kodiak Island within the southern asperity of the 1964 Mw9.2 Great Alaska earthquake. Receiver function images at frequencies of 1.2 and 2.4 Hz show a coherent, slightly dipping velocity increase at ~30-40 km depth consistent with the expected slab Moho. In contrast to studies within the northern asperity of the 1964 rupture, we find no evidence for a prominent low-velocity layer above the slab Moho thick enough to be resolved by upgoing P-to-S conversions. These results support evidence from seismicity and geodetic strain suggesting that the 1964 rupture connected northern (Kenai) and southern (Kodiak) asperities with different plate interface properties.

Explosions and earthquakes are effectively discriminated by P/S amplitude ratios for moderate magnitude events (M≥4) observed at regional to teleseismic distances (≥200 km). It is less clear if P/S ratios are effective explosion discriminants for lower magnitudes observed at shorter distances. We report new tests of P/S discrimination using a dense seismic array in a continental volcanic arc setting near Mount St. Helens, with 23 single-fired borehole explosions (ML 0.9-2.3) and 406 earthquakes (ML 1-3.3). The array provides up to 95 three-component broadband seismographs and most source-receiver distances are <120 km. Additional insight is provided by ~3,000 vertical component geophone recordings of each explosion. Potential controls on local distance P/S ratios are investigated, including: frequency range, distance, magnitude, source depth, number of seismographs, and site effects. A frequency band of about 10-18 Hz performs better than lower or narrower bands because explosion-induced S-wave amplitudes diminish relative to P for higher frequencies. Source depth and magnitude exhibited weak influences on P/S ratios. Site responses for earthquakes and explosions are correlated with each other and with shallow crustal Vp and Vs from travel-time tomography. Overall, the results indicate high potential for local distance P/S explosion discrimination in a continental volcanic arc setting, with ≥98% true positives and ≤6.3% false positives when using the array median from ≥16 stations. Performance is reduced for smaller arrays, especially those with ≤4 stations, thereby emphasizing the importance of array data for discrimination of low magnitude explosions.

Accurate measurements of P/S ratios at local-regional scale are often challenged by near-distance failure and/or sparse monitoring. During June 2014 - Aug. 2016 (iMUSH project), the XD array was deployed around the Mount St. Helens (MSH, hear and after) within ~150 km of the summit. The array contains ~80 three-component broadband stations, forming a nearly uniform monitoring system that recorded 23 active shots and 407 of M>1 earthquake within the 2-year period. The determined local magnitudes for the shots range from 0.9 to 2.3. Most earthquakes are deep (>10 km) and the shots are 10m below the surface. Due to the complexity of near surface structure, waveforms of the shots have both strong P and S energy on the transverse components, leaving them identical to earthquakes. This study takes advantage of this integrate dataset to systematically evaluates potential contributing factors to the P/S ratio measurements, including: source type (i.e., explosive vs. shear), distance, depth, and station location (i.e., site effect, coverage). We also compared between different frequency ranges, window choices, methods and component choices (i.e., RTZ vs. LQT) that differentiate the shots from the earthquakes. We observed no clear dependence for P-to-S ratio over depth, azimuth nor magnitude, while other parameters could be optimized to isolate contributions from the source. For example, the P-to-S ratios increase with frequency from 4Hz to 18 Hz and shots show much larger increasements than the earthquakes. We suggest that the near-source challenges could be relieved by: 1) use narrow windows to capture early phase and avoid overlap between S-coda and surface wave, 2) use higher frequency range to enhance body wave signal-to-noise ratio and allow for enough period within narrow phase windows, 3) rotate from the RTZ system to LQT system to maximize P wave energy and/or 4) include the transverse (T) component in P-phase energy calculations. With these optimizations, our averaged ratios are consistent throughout all distances (6-160 km) and clearly separate the shots from the earthquakes. In addition, regardless of the source type, the P-to-S ratios also vary with station location, which could be attributable to site effect and local structures.

The Raton Basin is known as an area of injection induced seismicity for the past two decades, but the reactivated fault zone structures and spatiotemporal response of seismicity to evolving injection have been poorly constrained in the past due to scarce public monitoring. Application of a machine-learning phase picker to four years of continuous data from a local array enables the detection and location of ~38,000 earthquakes. The events between 2016-2020 are ~2.5-6 km below sea level and range from ML<-1 to 4.2. Most earthquakes occur within previously identified ~N-S zones of seismicity, however our new catalog illuminates these zones are composed of many short faults with variable orientations. The two most active zones, the Vermejo Park and Tercio, are potentially linked by small intermediate faults. In total, we find ~60 short (<3 km) basement faults with strikes from WNW to slightly east of N. Faulting mechanisms are predominantly normal but some variability, including reverse dip-slip and oblique-slip, is observed. The Trinidad fault zone that hosted the 2011 Mw 5.3 earthquake is quiescent during 2016-2020, likely in response to decreased wastewater injection after 2012 and the shut-in of two nearby wells in 2015. Unlike some induced seismicity regions with higher injection rates, Raton Basin frequency-magnitude and spatiotemporal statistics are not distinguishable from tectonic seismicity. The similarity suggests that induced earthquakes in the Raton Basin are dominantly releasing tectonic stress.

A challenge in interpreting the origins of seismic anisotropy in deformed continental crust is that composition and rheology vary with depth. We investigated anisotropy in the northeastern Basin and Range where prior studies found prevalent depth-averaged positive radial anisotropy (Vsh > Vsv). This study focuses on depth-dependence of anisotropy and potentially distinct structures beneath three metamorphic core complexes (MCC’s). Rayleigh and Love wave dispersion were measured using ambient noise interferometry and Bayesian Markov Chain Monte Carlo inversions for Vs structure were tested with several (an)isotropic parameterizations. Acceptable data fits with minimal introduction of anisotropy are achieved by models with anisotropy concentrated in the middle crust. The peak magnitude of anisotropy from the mean of the posterior distributions ranges from 3.5-5% and is concentrated at 8-20 km depth. Synthetic tests with one uniform layer of anisotropy best reproduce the regional mean results with 9% anisotropy at 6-22 km depth. Both magnitudes are feasible based on exhumed middle crustal rocks. The three MCC’s exhibit ~5% higher isotropic upper crustal Vs, likely due to their anomalous levels of exhumation, but no distinctive (an)isotropic structures at deeper depths. Regionally pervasive middle crustal positive radial anisotropy is interpreted as a result of sub-horizontal foliation of mica-bearing rocks deformed near the top of the ductile deformation regime. Decreasing mica content with depth and more broadly distributed deformation at lower stress levels may explain diminished lower crustal anisotropy. Absence of distinctive deep crustal Vs beneath the MCC’s suggests over-printing by ductile deformation since the middle Miocene.

Valles Caldera was formed by large rhyolitic eruptions at ~1.6 and 1.23 Ma and it hosts post-caldera rhyolitic deposits as young as ~70 ka, but the contemporary state of the magmatic system is unclear. Local seismicity beneath Valles Caldera is rare and shear-velocity (Vs) structure has not been previously imaged. Here, we present the first local Vs tomography beneath Valles Caldera using ambient noise Rayleigh dispersion from a ~71 km transect of nodal seismographs with mean spacing of ~750 m. An ~6 km wide low-Vs anomaly (Vs<2.1 km/s) is located at ~3-10 km depth within the 1.23 Ma caldera’s ring fracture. Assuming magma in textural equilibrium, the new tomography suggests that melt fractions up to ~17-22% may be present within the upper crustal depth range where previously erupted rhyolites were stored.

Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth-integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P-to-S converted phases from the mantle transition zone boundaries to identify anisotropy within the ~400-700 km deep layer. Significant anisotropy is found near slab gaps. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite.

Volcanic arcs consist of many distinct vents that are ultimately fueled by the common process of melting in the subduction zone mantle wedge. Seismic imaging of crustal scale magmatic systems can provide insight into how melt is organized in the deep crust and eventually focused beneath distinct vents as it ascends and evolves. Here we investigate the crustal-scale structure beneath a section of the Cascades arc spanning four major stratovolcanoes: Mt. Hood, Mt. St. Helens, Mt. Adams, and Mt. Rainier, based on ambient noise interferometry measurements from 234 seismographs. Simultaneous inversion of Rayleigh and Love wave dispersion better constrain the isotropic shear velocity (Vs) and identify the unusual occurrence of radially anisotropic structures. Isotropic Vs shows two sub-parallel low-Vs zones at ~15-30 km depth with one connecting Mt. Rainier to Mt. Adams, and another connecting Mt. St. Helens to Mt. Hood, which are interpreted as deep crustal magma reservoirs containing up to ~2.5-6% melt, assuming near-equilibrium melt geometry. Negative radial anisotropy is prevalent in this part of the Cascadia margin, but is interrupted by positive radial anisotropy extending vertically beneath Mt. Adams and Mt. Rainier at ~10-30 km depth and weaker positive anisotropy beneath Mt. St. Helens with a west dipping. The positive anisotropy regions are adjacent to rather than co-located with the isotropic low-Vs anomalies. Ascending melt that stalled and mostly crystallized in sills with possible compositional difference from the country rock may explain the near-average Vs and positive radial anisotropy adjacent to the active deep crustal magma reservoirs.

Classification of local-distance, low-magnitude seismic events is challenging because signals can be numerous and difficult to characterize with approaches developed for larger magnitude events observed at greater distances. Yet, accurate classification is important to studies of earthquake processes and detection of potential underground nuclear tests. Here, we combine two classification metrics: the three-component ratio of high-frequency P/S amplitudes and the difference between local and coda duration magnitudes (ML-MC). The metrics use different parts of the high-frequency wavefield and exhibit complementary sensitivity for classification of M~0.5–4 natural earthquakes and borehole explosions, which are the best analog for underground nuclear explosions. Using means from bootstrap resampling across four diverse geologic settings, joint classification achieves >94.4% true positives and <8.4% false positives when using >=8 seismographs within 200 km. This high performance is obtained without local site corrections, indicating that the method may be transportable for local event classification.

Seismicity in the Raton Basin over the past two decades suggests reactivation of basement faults due to wastewater injection. In the summer of 2018, 96 short-period three-component nodal instruments were installed in a highly active region of the basin for a month. A machine-learning based phase picker-(PhaseNet) was adopted and identified millions of picks, which were associated with events using an automated algorithm – REAL (Rapid Earthquake Association and Location). After hypocenter relocation with hypoDD, the earthquake catalog contains 9259 M -2.2 – 3 earthquakes focused at depths of 4-6km. Magnitude of completeness (Mc) varies from -1 at night to -0.5 in daytime, likely reflecting noise variation modulated by wind. The clustered hypocenters with variable depths and focal mechanisms suggest a complex network of basement faults. Frequency-magnitude statistics and the spatiotemporal evolution of seismicity are comparable to tectonic systems.