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The top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active‐source methods with limited depth penetration, or low‐resolution teleseismic methods. To access short wavelengths at greater depth, we use high‐frequency P‐to‐S (PS, 1–15 Hz) mode‐converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high‐amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low‐velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (A_{PS_P}) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. A_{PS_P}changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High A_{PS_P}cannot be explained by a velocity step or a single low‐velocity, high Vp/Vs layer, but requires several alternating high and low‐velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones.

 

Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) comprised 75 ocean-bottom seismometers and 30 land stations and covered about 650 km along the segment of the subduction zone that includes Kodiak Island, the Alaska Peninsula and the Shumagin Islands between May 2018 and September 2019. This unprecedented onshore-offshore dataset provided an opportunity to compile a greatly enhanced earthquake catalog for the region by both increasing the number of detected earthquakes and improving the accuracy of their source parameters. We use all available regional and AACSE campaign seismic data to compile an earthquake catalog for the region between Kodiak and the Shumagin Islands including the Alaska Peninsula (51° N–59° N, 148° W–163° W). We apply the same processing and reporting standards to additional picks and events as the Alaska Earthquake Center currently uses for compilation of the authoritative regional earthquake catalog. Over 7200 events (both newly detected and previously reported) have been processed with AACSE data. We added about 30% more events, 60% more phase picks, lowered the magnitude of completeness by about 0.2 on average across the region, and improved location errors. All data have been published in public data archives. In addition, we test the machine-learning earthquake detection and picking algorithm EarthquakeTransformer (EQT) on the AACSE seismic dataset, comparing EQT-determined P and S picks with the new catalog. EQT is entirely trained on land data, whereas AACSE is amphibious. Overall, EQT finds 59% of P and 63% of S arrivals in the catalog within 300 km epicentral distance. The percent of catalog picks detected by EQT varies inversely with earthquake epicentral distance, and EQT performs particularly poorly on data from earthquakes recorded by instruments in the outer rise. 

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. These data show anomalous amplitude variations with complex azimuthal dependence at frequencies as low as 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. 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 and consistent amplitude anomalies are observed along the arc, suggesting that the Juan de Fuca slab may be continuous from Canada to northern California.

 

Volcanic arcs consist of many distinct vents that are ultimately fueled by the common melting processes 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 (MSH), Mt. Adams (MA), and Mt. Rainier, based on ambient noise data from 234 seismographs. Simultaneous inversion of Rayleigh and Love wave dispersion constrains the isotropic shear velocity (Vs) and identifies radially anisotropic structures. IsotropicVsshows two sub‐parallel low‐Vszones (∼3.45–3.55 km/s) at ∼15–30 km depth with one connecting Mt. Rainier to MA, and another connecting MSH 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, from vertical fractures like dikes, is prevalent in this part of the Cascadia, but is interrupted by positive radial anisotropy, from subhorizontal features like sills, extending vertically beneath MA and Mt. Rainier at ∼10–30 km depth and weaker and west‐dipping positive anisotropy beneath MSH. The positive anisotropy regions are adjacent to rather than co‐located with the isotropic low‐Vsanomalies. Ascending melt that stalled and mostly crystallized in sills with possible compositional differences from the country rock may explain the near‐averageVsand positive radial anisotropy adjacent to the active deep crustal magma reservoirs.

 

Abstract The plate interface undergoes two transitions between seismogenic depths and subarc depths. A brittle-ductile transition at 20–50 km depth is followed by a transition to full viscous coupling to the overlying mantle wedge at ∼80 km depth. We review evidence for both transitions, focusing on heat-flow and seismic-attenuation constraints on the deeper transition. The intervening ductile shear zone likely weakens considerably as temperature increases, such that its rheology exerts a stronger control on subduction-zone thermal structure than does frictional shear heating. We evaluate its role through analytic approximations and two-dimensional finite-element models for both idealized subduction geometries and those resembling real subduction zones. We show that a temperature-buffering process exists in the shear zone that results in temperatures being tightly controlled by the rheological strength of that shear zone’s material for a wide range of shear-heating behaviors of the shallower brittle region. Higher temperatures result in weaker shear zones and hence less heat generation, so temperatures stop increasing and shear zones stop weakening. The net result for many rheologies are temperatures limited to ≤350–420 °C along the plate interface below the cold forearc of most subduction zones until the hot coupled mantle is approached. Very young incoming plates are the exception. This rheological buffering desensitizes subduction-zone thermal structure to many parameters and may help explain the global constancy of the 80 km coupling limit. We recalculate water fluxes to the forearc wedge and deep mantle and find that shear heating has little effect on global water circulation. 

An oceanic plateau, the Yakutat terrane, has entered the subduction system across southcentral Alaska. Its down‐dip fate and relationship to overlying volcanism is still debated. Broadband seismometers from the Wrangell Volcanism and Lithospheric Fate (WVLF) temporary experiment were deployed with <20 km spacing across southcentral Alaska to study this region. An array‐based deconvolution procedure is used to isolate the scatteredPandScoda of teleseismicPwaves for imaging discontinuity structure. This procedure is applied to WVLF and other dense seismic arrays across southcentral Alaska in a manner that accounts for near‐surface wavespeed variations. Two imaging techniques are employed: two‐dimensional migration and three‐dimensional common‐conversion‐point (CCP) stacking. Migrating the scattered phases along WVLF stations shows the ∼18 ± 4 km thick Yakutat crust subducting beneath the Wrangell Volcanic field to the NNE. It is offset from the Alaska‐Aleutian seismic zone laterally by 250 km to the southeast at 100 km depth, and dips more steeply (45°). At depths <45 km, CCP stacking reveals that the Yakutat crust is continuous for over 450 km along strike. This shallow continuity and deeper offset suggest a tear in the subducting Yakutat slab at depths >45 km, around 146°W. CCP stacking also reveals a continuous thin low‐velocity layer atop the underthrust Yakutat crust for >450 km along strike, at all depths <35 km. The uniform low‐velocity thrust zone indicates consistent properties through multiple rupture‐zone segments, showing that low‐velocity channels generally correspond with subduction megathrusts.

 

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.

 

Abstract The 24 January 2016 Iniskin, Alaska earthquake, at Mw 7.1 and 111 km depth, is the largest intermediate‐depth earthquake felt in Alaska, with recorded accelerations reaching 0.2g near Anchorage. Ground motion from the Iniskin earthquake is underpredicted by at least an order of magnitude near Anchorage and the Kenai Peninsula, and is similarly overpredicted in the back‐arc north and west of Cook Inlet. This is in strong contrast to the 30 November 2018 earthquake near Anchorage that was also Mw 7.1 but only 48 km deep. The Anchorage earthquake signals show strong distance decay and are generally well predicted by ground‐motion prediction equations. Smaller intermediate‐depth earthquakes (depth>70  km and 3<M<6.4) with hypocenters near the Iniskin mainshock show similar patterns in ground shaking as the Iniskin earthquake, indicating that the shaking pattern is due to path effects and not the source. The patterns indicate a first‐order role for mantle attenuation in the spatial variability of strong motion. In addition, along‐slab paths appear to be amplified by waveguide effects due to the subduction of crust at >1  Hz; the Anchorage and Kenai regions are particularly susceptible to this amplification due to their fore‐arc position. Both of these effects are absent in the 2018 Anchorage shaking pattern, because that earthquake is shallower and waves largely propagate in the upper‐plate crust. Basin effects are also present locally, but these effects do not explain the first‐order amplitude variations. These analyses show that intermediate‐depth earthquakes can pose a significant shaking hazard, and the pattern of shaking is strongly controlled by mantle structure. 

Abstract We present two new seismic velocity models for Alaska from joint inversions of body-wave and ambient-noise-derived surface-wave data, using two different methods. Our work takes advantage of data from many recent temporary seismic networks, including the Incorporated Research Institutions for Seismology Alaska Transportable Array, Southern Alaska Lithosphere and Mantle Observation Network, and onshore stations of the Alaska Amphibious Community Seismic Experiment. The first model primarily covers south-central Alaska and uses body-wave arrival times with Rayleigh-wave group-velocity maps accounting for their period-dependent lateral sensitivity. The second model results from direct inversion of body-wave arrival times and surface-wave phase travel times, and covers the entire state of Alaska. The two models provide 3D compressional- (VP) and shear-wave velocity (VS) information at depths ∼0–100  km. There are many similarities as well as differences between the two models. The first model provides a clear image of the high-velocity subducting plate and the low-velocity mantle wedge, in terms of the seismic velocities and the VP/VS ratio. The statewide model provides clearer images of many features such as sedimentary basins, a high-velocity anomaly in the mantle wedge under the Denali volcanic gap, low VP in the lower crust under Brooks Range, and low velocities at the eastern edge of Yakutat terrane under the Wrangell volcanic field. From simultaneously relocated earthquakes, we also find that the depth to the subducting Pacific plate beneath southern Alaska appears to be deeper than previous models. 

In southcentral Alaska, the Alaska‐Aleutian Wadati‐Benioff zone (WBZ) shows high seismicity rates west of 147°W. Further east, the Wrangell volcanic field (WVF) has some of the world's largest continental volcanoes but there is equivocal evidence for a WBZ. We deployed a dense seismometer array around the WVF between 2016 and 2018 and used the data to increase the number of detected earthquakes using an autodetection and location algorithm. One‐dimensional velocity inversion and double‐difference earthquake location further improve earthquake locations. Subcrustal earthquakes form a narrow band of dipping seismicity—a weak but clear WVF WBZ—which strikes parallel to the volcanic trend and dips highly oblique to plate motion. The WVF WBZ is continuous from the coast to a depth of 100 km beneath Mount Wrangell. Earthquakes shallower than 40 km are continuous between the two WBZs, indicating continuity of the subducting Yakutat terrane across the region. However, the earthquakes deeper than 40 km are offset by hundreds of kilometers, which may indicate a slab tear separating the Alaska–Aleutian WBZ from the WVF WBZ. Seismicity rates differ by over 2 orders of magnitude between the separate WBZs, despite the similar incoming plate, with the relatively seismically quiescent WBZ underlying the much more prolific WVF. Higher slab‐surface temperatures beneath the WVF, due to flow around the slab edges and the oblique geometry, may lead to low seismicity rates within subducting crust, as seen in other warm slabs, but abundant water is still transported to subarc depths within the mantle wedge.