ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉Reliable instrument recoverability and data quality rely on accurate estimates of instrument locations on the seafloor. However, freely available software for this estimation does not currently exist. We present OBSrange, an open‐source tool for robustly locating ocean‐bottom seismometers (OBSs) on the seafloor using acoustic transponder ranging data. Available in both MATLAB and Python (see 〈a href="https://pubs.geoscienceworld.org/srl#sc6Data%20and%20Resources"〉Data and Resources〈/a〉), the algorithm inverts two‐way acoustic ranging travel‐time data for instrument location, depth, and average water sound speed with the ability to accurately account for ship velocity, ray refraction through the water column specific to the region, and a known lateral offset between the ship’s Global Positioning System (GPS) receiver and acoustic transponder. The tool provides comprehensive estimates of model parameter uncertainty including bootstrap uncertainties for all four parameters as well as an F‐test grid search providing a 3D confidence ellipsoid around each station. We validate the tool using a synthetic travel‐time dataset and find average horizontal location errors on the order of ∼4  m for an instrument at 5000 m depth. An exploration of survey geometries shows significant variation in location precision depending on the pattern chosen. We explore the trade‐off between survey length and location uncertainty to quantitatively inform cruise planning strategies. The optimal survey radius for resolving instrument location depends on water depth and desired precision and nominally ranges from 0.75–1 nautical mile (NM) at 5000 m water depth to ∼0.25  NM at 500 m depth. Radial legs toward and away from the instrument are crucial for resolving the depth‐water velocity trade‐off, and thus circle surveys should be avoided. Line surveys, common for active source experiments, are unable to resolve the instrument location orthogonal to the survey line. We apply our tool to the 2018 Young Pacific OBS Research into Convecting Asthenosphere (ORCA) deployment in the south Pacific yielding an average root mean square data misfit of 1.96 ms with an average instrument drift of ∼170  m. Observed drifts reveal a clockwise rotation pattern of ∼500  km diameter that resembles a cyclonic mesoscale gyre observed in the geostrophic flow field, suggesting a potential application of accurate instrument drifts as a novel proxy for depth‐integrated flow through the water column.〈/span〉

Description: Trade-offs between velocity and anisotropy heterogeneity complicate the interpretation of differential traveltime data and have the potential to bias isotropic tomographic models. By constructing a simple parametrisation to describe an elastic tensor with hexagonal symmetry, we find analytic solutions to the Christoffel equations in terms of fast and slow horizontal velocities that allow us to simultaneously invert differential traveltime data and splitting data from teleseismic S arrivals to recover 3-D velocity and anisotropy structure. This technique provides a constraint on the depth-extent of shallow anisotropy, otherwise absent from interpretations based on SKS splitting alone. This approach is well suited to the young Woodlark Rift, where previous studies have found strong velocity variation and substantial SKS splitting in a continental rift with relatively simple geometry. This study images a low-velocity rift axis with ≤4 per cent spreading-parallel anisotropy at 50–100 km depth that separates regions of pre-existing lithospheric fabric, indicating the synchronous development of extensional crystallographic preferred orientation and lithospheric thinning. A high-velocity slab fragment north of the rift axis is associated with strike-parallel anisotropic fast axes, similar to that seen in the shallow mantle of some subduction zones. In addition to the insights provided by the anisotropy structure, the improvement in fit to the differential traveltime data demonstrates the merit to a joint inversion that accounts for anisotropy.

Description: 〈span〉〈div〉SUMMARY〈/div〉Despite their importance as a fundamental constraint on Earth properties, regional-scale measurements of body-wave seismic attenuation are scarce. This is partially a result of the difficulty in producing robust estimates of attenuation. In this paper, we focus on measuring differential attenuation on records of teleseismic P waves. We examine a unique dataset of 5 records of the North Korean nuclear test of 2017 measured at five broadband seismic stations deployed within a few meters of each other but using different installation procedures. Given their extreme proximity, we expect zero differential intrinsic attenuation between the different records. However, we find that different attenuation measurement methods and implementation parameters in fact produce significant apparent differential attenuation (Δt*). Frequency-domain methods yield a wide range of Δt* estimates between stations, depending on measurement bandwidth and nuances of signal processing. This measurement instability increases for longer time windows. Time domain methods are largely insensitive to the frequency band being considered but are sensitive to the time window that is chosen. We determine that signal-generated noise can affect measurements in both the frequency and time domain. In some cases, the range of results amounts to a significant fraction of the range of differential attenuation across the conterminous United States as determined by a recent study. We suggest some approaches to manage the inherent instability in these measurements and recommend best practices to confidently estimate body wave attenuation.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Despite their importance as a fundamental constraint on Earth properties, regional-scale measurements of body-wave seismic attenuation are scarce. This is partially a result of the difficulty in producing robust estimates of attenuation. In this paper, we focus on measuring differential attenuation on records of teleseismic 〈span〉P〈/span〉 waves. We examine a unique data set of five records of the North Korean nuclear test of 2017 measured at five broad-band seismic stations deployed within a few metres of each other but using different installation procedures. Given their extreme proximity, we expect zero differential intrinsic attenuation between the different records. However, we find that different attenuation measurement methods and implementation parameters in fact produce significant apparent differential attenuation (Δ〈span〉t〈/span〉*). Frequency-domain methods yield a wide range of Δ〈span〉t〈/span〉* estimates between stations, depending on measurement bandwidth and nuances of signal processing. This measurement instability increases for longer time windows. Time domain methods are largely insensitive to the frequency band being considered but are sensitive to the time window that is chosen. We determine that signal-generated noise can affect measurements in both the frequency and time domain. In some cases, the range of results amounts to a significant fraction of the range of differential attenuation across the conterminous United States as determined by a recent study. We suggest some approaches to manage the inherent instability in these measurements and recommend best practices to confidently estimate body wave attenuation.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Reliable instrument recoverability and data quality rely on accurate estimates of instrument locations on the seafloor. However, freely available software for this estimation does not currently exist. We present OBSrange, an open‐source tool for robustly locating ocean‐bottom seismometers (OBSs) on the seafloor using acoustic transponder ranging data. Available in both MATLAB and Python (see 〈a href="https://pubs.geoscienceworld.org/srl#sc6Data%20and%20Resources"〉Data and Resources〈/a〉), the algorithm inverts two‐way acoustic ranging travel‐time data for instrument location, depth, and average water sound speed with the ability to accurately account for ship velocity, ray refraction through the water column specific to the region, and a known lateral offset between the ship’s Global Positioning System (GPS) receiver and acoustic transponder. The tool provides comprehensive estimates of model parameter uncertainty including bootstrap uncertainties for all four parameters as well as an F‐test grid search providing a 3D confidence ellipsoid around each station. We validate the tool using a synthetic travel‐time dataset and find average horizontal location errors on the order of ∼4  m for an instrument at 5000 m depth. An exploration of survey geometries shows significant variation in location precision depending on the pattern chosen. We explore the trade‐off between survey length and location uncertainty to quantitatively inform cruise planning strategies. The optimal survey radius for resolving instrument location depends on water depth and desired precision and nominally ranges from 0.75–1 nautical mile (NM) at 5000 m water depth to ∼0.25  NM at 500 m depth. Radial legs toward and away from the instrument are crucial for resolving the depth‐water velocity trade‐off, and thus circle surveys should be avoided. Line surveys, common for active source experiments, are unable to resolve the instrument location orthogonal to the survey line. We apply our tool to the 2018 Young Pacific OBS Research into Convecting Asthenosphere (ORCA) deployment in the south Pacific yielding an average root mean square data misfit of 1.96 ms with an average instrument drift of ∼170  m. Observed drifts reveal a clockwise rotation pattern of ∼500  km diameter that resembles a cyclonic mesoscale gyre observed in the geostrophic flow field, suggesting a potential application of accurate instrument drifts as a novel proxy for depth‐integrated flow through the water column.〈/span〉

Description: At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative’s Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays ( tS*~1.7 s and T S ~ 2 s) in a narrow zone 〈50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor ( Q s ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies.

Description: Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long-time-scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress-independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory-derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high-pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations. © 2017. American Geophysical Union. All Rights Reserved.