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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. CASIE21-OBS: An Open-Access, OBS Controlled-Source Seismic Data Set for Investigating the Structure and Properties of the Cascadia Accretionary Wedge and the Downgoing Explorer-Juan de Fuca-Gorda Plate System

   https://doi.org/10.1785/0220230010  

   Canales, Juan Pablo; Miller, Nathaniel C.; Baldwin, Wayne; Carbotte, Suzanne M.; Han, Shuoshuo; Boston, Brian; Jian, Hanchao; Collins, John; Lizarralde, Dan (May 2023, Seismological Research Letters)

   Abstract Geological processes at subduction zones and their associated geohazards (e.g., megathrust earthquakes, submarine landslides, tsunamis, and arc volcanism) are, to a large extent, controlled by the structure, physical properties and fluid content of the subducting plate, the accreted sediments, and the overriding plate. In these settings, modern seismic modeling and imaging techniques based on controlled-source, multicomponent ocean-bottom seismometer (OBS) data are some of the best tools available for determining the subseafloor elastic properties, which can be linked to the aforementioned properties. Here, we present CASIE21-OBS, a controlled-source marine wide-angle OBS data set recently collected across the Cascadia convergent margin as part of the larger CAscadia Seismic Imaging Experiment 2021 (CASIE21). The main component of CASIE21 is a long-offset multichannel seismic (MCS) survey of the Cascadia margin conducted in June–July 2021 onboard R/V M.G. Langseth (cruise MGL2104) aiming to characterize the incoming plate, the plate interface geometry and properties, and the overlying sediment stratigraphy and physical properties. CASIE21-OBS was conducted during R/V M.G. Langseth cruise MGL2103 (May 2021) and R/V Oceanus cruise OC2106A (June–July 2021). It consisted of 63 short-period four-component OBSs deployed at a total 120 stations along 10 across-trench profiles extending from ∼50 km seaward of the deformation front to the continental shelf, and from offshore northern Vancouver Island to offshore southern Oregon. The OBSs recorded the airgun signals of the CASIE21-MCS survey as well as natural seismicity occurring during the deployment period (24 May 2021 19:00 UTC–9 July 2021 09:00 UTC). The OBS data are archived and available at the Incorporated Research Institutions for Seismology Data Management Center under network code YR_2021 for continuous time series (miniSEED) and identifier 21-008 for assembled data set (SEG-Y). 

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3. Time Correction of Ocean-Bottom Seismometers Using Improved Ambient Noise Cross Correlation of Multicomponents and Dual-Frequency Bands

   https://doi.org/10.1785/0220200358  

   Tian, Jinyu; Lin, Jian; Zhang, Fan; Xu, Min; Zhang, Yayun; Guo, Laiyin; Zeng, Xin (January 2021, Seismological Research Letters)

   null (Ed.)

   Abstract An effective approach was developed for identifying and correcting ocean-bottom seismometer (OBS) time errors through improving ambient noise cross-correlation function (NCCF) analysis and combination with other methods. Significant improvements were illustrated through analyzing data from a passive-source seismic experiment in the southwestern sub-basin of the South China Sea. A novel method was first developed that can effectively identify errors in the sampling frequency of the OBS instruments. The traditional NCCF method was then expanded by increasing the analyzed data spectrum from a single-frequency band to dual-frequency band pairs, thus doubling the number of available data points and substantially improving the time correction quality. For data with relatively low signal-to-noise ratios, the average time errors were reduced from the original average values of 60–80 ms by the conventional methods to <40  ms using the improved approaches. The new multistep procedure developed in this study has general applicability to analysis of other OBS experiments. The demonstrated significant improvements in the data quality are critical for advancing seismic tomography and other modern marine geophysical studies that require high accuracy in the OBS data. 

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4. Seismic Structure, Gravity Anomalies and Flexure Along the Emperor Seamount Chain

   https://doi.org/10.1029/2020JB021109  

   Watts, A. B.; Grevemeyer, I.; Shillington, D. J.; Dunn, R. A.; Boston, B.; Gómez de la Peña, L. (March 2021, Journal of Geophysical Research: Solid Earth)

   Abstract The Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s⁻¹, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s⁻¹, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s⁻¹. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating. 

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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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