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1. The Alaska Amphibious Community Seismic Experiment

   https://doi.org/10.1785/0220200189  

   Barcheck, Grace; Abers, Geoffrey A.; Adams, Aubreya N.; Bécel, Anne; Collins, John; Gaherty, James B.; Haeussler, Peter J.; Li, Zongshan; Moore, Ginevra; Onyango, Evans; et al (August 2020, Seismological Research Letters)

   Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an ∼700  km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262  m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the R/V Sikuliaq in May and July 2018 and retrieved on two cruises aboard the R/V Sikuliaq and R/V Langseth in August–September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May–June 2019, and an active-source seismic survey on the R/V Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 Mw 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories. 

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2. Characterizing Sub‐Seafloor Seismic Structure of the Alaska Peninsula Along the Alaska‐Aleutian Subduction Zone

   https://doi.org/10.1029/2024jb029862  

   Zheng, Mengjie; Sheehan, Anne F; Liu, Chuanming; Wu, Mengyu; Ritzwoller, Michael H (November 2024, Journal of Geophysical Research: Solid Earth)

   Abstract A shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading. 

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3. Observations of Earth’s Normal Modes on Broadband Ocean Bottom Seismometers

   https://doi.org/10.3389/feart.2021.679958  

   Laske, Gabi (June 2021, Frontiers in Earth Science)

   null (Ed.)

   It is generally thought that high noise levels in the oceans inhibit the observation of long-period earthquake signals such as Earth’s normal modes on ocean bottom seismometers (OBSs). Here, we document the observation of Earth’s gravest modes at periods longer than 500 s (or frequencies below 2 mHz). We start with our own 2005–2007 Plume-Lithosphere-Undersea-Mantle Experiment (PLUME) near Hawaii that deployed a large number of broadband OBSs for the first time. We collected high-quality normal mode spectra for the great November 15, 2006 Kuril Islands earthquake on multiple OBSs. The random deployment of instruments from different OBS groups allows a direct comparison between different broadband seismometers. For this event, mode S 0 6 (1.038 mHz) consistently rises above the background noise at all OBSs that had a Nanometrics Trillium T-240 broadband seismometer. We also report observations of other deployments in the Pacific ocean that involved instruments of the U.S. OBS Instrument Pool (OBSIP) where we observe even mode S 0 4 (0.647 mHz). Earth’s normal modes were never the initial target of any OBS deployment, nor was any other ultra-low-frequency signal. However, given the high costs of an OBS campaign, the fact that data are openly available to future investigators not involved in the campaign, and the fact that seismology is evolving to investigate ever-new signals, this paper makes the case that the investment in a high-quality seismic sensor may be a wise one, even for a free-fall OBS. 

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4. TOQUES: An Ocean-Bottom Seismometer Dataset on the Central Queen Charlotte Fault, Southeast Alaska

   https://doi.org/10.1785/0220240380  

   Gase, Andrew C; Roland, Emily C; Worthington, Lindsay L; Walton, Maureen_A L; Bostock, Michael G; Nedimović, Mladen R; Collins, John A; Brandl, Collin C; Jaye_Oliva, Sarah; Sun, Tianhaozhe (March 2025, Seismological Research Letters)

   Abstract Plate boundaries in the oceans are often poorly monitored. Though typically less remote than the deep sea, shallow marine environments with seafloor depths <0.5 km can be especially challenging for seismic experiments due to natural and anthropogenic hazards and noise sources that can affect instrument survival and data quality. The Queen Charlotte fault (QCF) is part of a transform plate boundary that follows the continental shelf of the Alaska Panhandle and central British Columbia. This fault system accommodates dextral slip between the Pacific and North American plates and has hosted several historic Mw > 7 earthquakes. In August 2021, we deployed 28 broadband ocean-bottom seismometers (OBSs) along the central QCF for the “Transform Obliquity along the Queen Charlotte Fault and Earthquake Study” (TOQUES) to investigate fault architecture and local seismicity. Deployment depths varied between 0.2 and 2.5 km below sea level, with half of the instruments deployed in shallow water (<0.5 km depth). We describe the scientific motivations for the TOQUES broadband OBS array, present data metrics, and discuss factors that influence data quality and instrument survival. We show that many opportunities exist for scientific study of shallow marine environments and the solid earth. Despite concerns that shallow water was responsible for the risk of data or instrument loss, direct relationships between instrument success and water depth are inconclusive. Rather, instrument success may be more related to the ability of different instrument designs to withstand shallow-water conditions. 

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5. Determining the Orientation of Ocean-Bottom Seismometers on the Seafloor and Correcting for Polarity Flipping via Polarization Analysis and Waveform Modeling

   https://doi.org/10.1785/0220190239  

   Zhu, Gaohua; Yang, Hongfeng; Lin, Jian; You, Qingyu (February 2020, Seismological Research Letters)

   null (Ed.)

   Abstract The erroneous flipping of polarity in seismic records of ocean-bottom seismometers (OBSs) could go unnoticed and undiagnosed because it is coupled with unknown horizontal orientation of OBS instruments on the seafloor. In this study, we present detailed approaches to first identify potential errors in the flipping polarity of individual OBS instruments, and then determine the correct orientation of OBS instruments on the seafloor. We first conduct a series of tests by artificially flipping the polarity of seismic records of the Global Seismographic Network stations to determine the effects on orientation estimates, utilizing polarization characteristics of teleseismic P and Rayleigh waves, respectively. The tests demonstrate that erroneous polarity reversal in seismic recording could cause false estimates and reverse radial (R) and tangential (T) components. We determine the sensor orientations through comparing the observed waveforms to the synthetic waveforms, which could solve the ambiguity of R and T directions caused by potential erroneous polarity reversal of OBS data. We then apply the approaches to an OBS data set collected in the southern Mariana subduction zone to obtain the correct orientation for 9 out of 12 OBS instruments. 

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