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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.

 

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. 

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.

 

Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high‐resolution tomographic images of the crust and upper mantle. Rayleigh‐wave phase‐velocity maps and three‐dimensional images of shear velocity (Vs), generated from ambient noise and earthquake surface waves, show that west of MSH the middle‐lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0–4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, middle‐lower crustVsis low (3.45–3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wave speeds, producing a strong Moho. The sharp boundary in crust and mantleVswithin a few kilometers of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes.

 

Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances.