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1. Examining Alaska’s Earthquakes on Land and Sea

   Abers, G.A. (March 2019, Eos)

   North America’s largest earthquakes and most powerful volcanic eruptions occur along the Alaska Peninsula subduction zone, a meeting of two tectonic plates that sweeps an arc across the North Pacific margin between Alaska and Russia. However, studies that would help us understand these hazards are few and far between in this remote, sparsely populated region. A major shoreline-crossing community seismic experiment, now under way, spans the Alaska Peninsula subduction zone, with the intention of filling gaps in our knowledge of this region. Information that we collect along this margin can provide direct information about many firstorder questions about subduction zone processes that influence earthquakes and volcanism. 

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2. Enhanced Regional Earthquake Catalog with Alaska Amphibious Community Seismic Experiment Data

   https://doi.org/10.1785/0220220226  

   Ruppert, Natalia A.; Barcheck, Grace; Abers, Geoffrey A. (November 2022, Seismological Research Letters)

   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. 

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3. Updated concepts of seismic gaps and asperities to assess great earthquake hazard along South America

   https://doi.org/10.1073/pnas.2216843119  

   Lay, Thorne; Nishenko, Stuart P. (December 2022, Proceedings of the National Academy of Sciences)

   So far in this century, six very large–magnitude earthquakes ( M W ≥ 7.8) have ruptured separate portions of the subduction zone plate boundary of western South America along Ecuador, Peru, and Chile. Each source region had last experienced a very large earthquake from 74 to 261 y earlier. This history led to their designation in advance as seismic gaps with potential to host future large earthquakes. Deployments of geodetic and seismic monitoring instruments in several of the seismic gaps enhanced resolution of the subsequent faulting processes, revealing preevent patterns of geodetic slip deficit accumulation and heterogeneous coseismic slip on the megathrust fault. Localized regions of large slip, or asperities, appear to have influenced variability in how each source region ruptured relative to prior events, as repeated ruptures have had similar, but not identical slip distributions. We consider updated perspectives of seismic gaps, asperities, and geodetic locking to assess current very large earthquake hazard along the South American subduction zone, noting regions of particular concern in northern Ecuador and Colombia (1958/1906 rupture zone), southeastern Peru (southeasternmost 1868 rupture zone), north Chile (1877 rupture zone), and north-central Chile (1922 rupture zone) that have large geodetic slip deficit measurements and long intervals (from 64 to 154 y) since prior large events have struck those regions. Expanded geophysical measurements onshore and offshore in these seismic gaps may provide critical information about the strain cycle and fault stress buildup late in the seismic cycle in advance of the future great earthquakes that will eventually strike each region. 

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4. Shear Wave Splitting and Mantle Flow Beneath Alaska

   https://doi.org/10.1029/2019JB018329  

   McPherson, A_M; Christensen, D_H; Abers, G_A; Tape, C. (April 2020, Journal of Geophysical Research: Solid Earth)

   Abstract Shear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion. 

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5. Seismic images of the accretionary prism of the Hikurangi margin, New Zealand, reveal its structural variations and influences on seismogenic character

   Van_Avendonk, H; Gase, A; Bangs, N (December 2022, AGU)

   The Hikurangi margin of New Zealand exhibits contrasting slip behavior from south to north. Whereas the southern Hikurangi margin has a locked plate boundary that can potentially produce large megathrust earthquakes, the northern section of this margin accommodates plate motion by creep and episodic shallow slow-slip events. To investigate these different modes of slip we examine the geometry of the plate boundary and consolidation state of the materials along the plate interface. We use marine seismic reflection data from the SHIRE project to image the reflectivity and seismic velocity structure along 20 profiles across the accretionary wedge of the Hikurangi subduction zone of New Zealand. These active-source seismic data were gathered in 2017 with the R/V Marcus Langseth using a 6,600 in3 seismic source and 12 km long receiver array. We carried out streamer tomography on the SHIRE profiles where we integrated seismic velocity constraints from stacking the reflection data along all SHIRE transects. The seismic velocity images and prestack depth migrations together characterize the nature of incoming basement, sediment subduction and accretion, and faulting and compaction of the accretionary wedge. Our seismic velocity models show that a layer of sediment,with seismic wavespeeds of ~3.0 km/s, is entrained beneath the accretionary prism in the southern Hikurangi margin, but there is no coherent subducted sediment layer to the north. This is a significant result, because it implies that the sediment layer covers basement roughness and forms a smoother plate boundary in the south. In addition, the deepest sediments on the incoming plate in the southern Hikurangi margin are believed to be quartz-rich turbidites, which are prone to unstable slip along the plate boundary. In contrast, the accretionary prism of the northern Hikurangi margin exhibits more variation in accretionary wedge thrust geometry due to interactions with large seamounts on the downgoing oceanic basement. These findings are consistent with the geodetically locked nature of a smooth, quartz-rich plate boundary along the southern Hikurangi subduction zone, and the creeping nature of a heterogeneous plate boundary along the Hikurangi margin to the north. 

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