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.

Slow earthquakes are mainly distributed in regions surrounding seismogenic zones along the plate boundaries of subduction zones. In the Central American subduction zone, large regular interplate earthquakes with magnitudes of 7–8 occur repeatedly around the Nicoya Peninsula, in Costa Rica, and a tsunami earthquake occurred off Nicaragua, just north of Costa Rica, in 1992. To clarify the spatial distribution of various slip behaviors at the plate boundary, we detected and located very low frequency earthquakes (VLFEs) around the Nicoya Peninsula using a grid-search matched-filter technique with synthetic templates based on a regional three-dimensional model. VLFEs were active in September 2004 and August 2005, mainly near the trench axis, updip of the seismogenic zone. The distribution of VLFEs overlapped with large slip areas of slow slip events. Low frequency tremor signals were also found in high-frequency seismogram envelopes within the same time windows as detected VLFEs; thus, we also investigated the energy rates of tremors accompanied by VLFEs. The range of scaled energy, which is the ratio of the seismic energy rate of a tremor to the seismic moment rate of accompanying VLFE and related to the rupture process of seismic phenomena, was 10-9–10-8. The along-dip separation of shallow slow and large earthquakes and the range of the scaled energy off Costa Rica are similar to those in shallow slow earthquakes in Nankai, which shares a similar thermal structure along the shallow plate boundary.

Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short-term arrays. Parallel teleseismic travel-time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local-earthquake attenuation studies but at 10´ lower frequencies. The pattern is seen both across the area of normal Pacific subduction in the Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction-dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction-related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high-frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local-earthquake body waves. Differences between P and S attenuation suggest primarily shear-modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction-related structure, complementary to travel times.

Slow earthquakes are generally distributed in regions surrounding seismogenic zones along the plate boundaries of subduction zones. In the Costa Rica subduction zone, large regular interplate earthquakes with a magnitude of 7–8 occur repeatedly, and a tsunami earthquake occurred in the northern part in 1992. To clarify the spatial distribution of various slip behaviors at the plate boundary in the Costa Rica subduction zone, we detected and located very low frequency earthquakes (VLFEs) using a grid-search matched-filter technique with synthetic templates based on a regional three-dimensional model. VLFEs were activated in September 2004 and August 2005, and most of the VLFEs were located near the trench axis at a depth range of 5–10 km, the updip of the seismogenic zone. The spatial distribution of VLFEs complements the slip areas of large earthquakes and the tsunami earthquake. Low frequency tremor signals were also found in high-frequency seismogram envelopes within the same time windows of detected VLFEs; thus, we also investigated the energy rates of tremors accompanied by VLFEs. The range of scaled energy, which is the ratio of the seismic energy rate of a tremor to the seismic moment rate of accompanying VLFE, was 10-9–10-8. This value is similar to that in shallow slow earthquakes in the Nankai subduction zone. The similarity of characteristics and distribution of shallow slow earthquakes in the Costa Rica and Nankai subduction zones may be due to common tectonic features, such as age, temperature, or the presence of accretionary prisms.

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.

Seismic wave amplitudes have tremendous sensitivity to subduction structure; however, they are affected by attenuation, scattering and focusing, and have therefore been sparsely used compared with traveltimes. We measure and model teleseismic body wave amplitudes recorded at a dense broadband array in the Washington Cascades (iMUSH). These data show anomalous amplitude variations with complex azimuthal dependence at the low frequency of 0.05 Hz, accompanied by significant multipathing. We demonstrate using spectral-element numerical simulations that focusing of the teleseismic wavefield by the Juan de Fuca slab is responsible for some of the amplitude anomalies. The focusing effects can contaminate the apparent differential attenuation measurements and produce at least 20% of the inferred attenuation signal. The focusing results in complex azimuthal patterns that produce different phase and amplitude variations than does intrinsic attenuation, which should allow separation of elastic (focusing) and anelastic effects. Our results indicate that the amplitudes are sensitive to the subducting slab geometry and subduction structure, and can be used to refine seismic images. Ubiquitous focusing effects are observed along the arc, suggesting a continuous Juan de Fuca slab from Canada to northern California.

England and May (2021) present a model comparison for the forearc thermal structure in subduction zones that employs a finite element modeling technique and a new variation on approximate equations. We have some comments on various claims made in this published paper.