Seismic Attenuation at the Equatorial Mid‐Atlantic Ridge Constrained by Local Rayleigh Wave Analysis From the PI‐LAB Experiment

Saikia, Utpal; Rychert, C.; Harmon, Nicholas; Kendall, J.‐M.

doi:10.1029/2021gc010085

Cited by 9 publications

(6 citation statements)

References 110 publications

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“…Despite recent methodological advances on land, seismic imaging in marine environments lags due to challenges associated with the relatively noisy seafloor environment and often sparse station coverage compared to terrestrial seismic deployments. This is true especially for studies of Rayleigh wave attenuation across arrays of ocean‐bottom seismometers (OBSs), where only a handful of observations have been made to date (e.g., Ma et al., 2020; Ruan et al., 2018; Saikia et al., 2021; Yang & Forsyth, 2006). To our knowledge, all existing regional Rayleigh wave attenuation observations made in the oceans were measured using the TPW method.…”

Section: Introductionmentioning

confidence: 99%

Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography

Russell

Dalton

2022

JGR Solid Earth

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Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods <35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (Qμ ${Q}_{\mu }$ > 1,500) overlying a highly attenuating asthenospheric layer (Qμ ${Q}_{\mu }$ ∼ 50 to 70). At JdF, we find a broad peak in attenuation (Qμ ${Q}_{\mu }$ ∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure.

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Section: Introductionmentioning

confidence: 99%

Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography

Russell

Dalton

2022

JGR Solid Earth

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“…We do not account for phase changes of the aluminious phases in the calculations although the spinel -garnet transition is encompassed somewhat between the lherzolite and pyrolite models. We apply a frequency dependent attenuation correction to the S-wave velocity output from the codes using both the approach and the 1-D attenuation structure 62 . The velocities were calculated for a range of temperatures at a given pressure corresponding to the depth in the model and the temperature corresponding to each velocity in the model was determined by interpolation.…”

Section: Methodsmentioning

confidence: 99%

Broad fault zones enable deep fluid transport and limit earthquake magnitudes

Leptokaropoulos

Rychert

Harmon

et al. 2022

Preprint

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Constraining the controlling factors of fault rupture is fundamental to understanding the earthquake cycle. Fluids can influence earthquake locations and magnitudes, although the exact pathways of fluids through the lithosphere are not well-known. Ocean transform faults are an ideal laboratory to study the factors controlling fault ruptures and fluid pathways given their relative simplicity. Here, we analyse seismicity recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment, centred around the Chain Fracture Zone. We find that earthquakes beneath mapped morphological transpressional features occur deeper than the brittle-ductile transition predicted by simple thermal models but elsewhere occur shallower. These features are characterised by multiple parallel fault segments and step overs, high b-values, gaps in large historical earthquakes, and seismic velocity structures consistent with hydrothermal alteration. This suggests that broader fault damage zones preferentially facilitate fluid transport into the lithosphere. Although this cools the mantle, it also reduces the potential for large earthquakes at punctuated locations (barriers). These barriers divide the transform into asperity segments that are shorter, thereby limiting the earthquake magnitudes in these regions.

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“…These kernels assume a homogeneous Earth, and therefore do not include the effects of structural heterogeneity like the previously described approaches. Although the kernels could easily be extended to 3‐D, we investigate the 2‐D case here, as a first step and for the sake of simplicity and also given that many tectonic environments, for instance rifts (e.g., Armitage et al., 2015; Chambers et al., 2021, 2019; Lavayssiere et al., 2018; Rychert et al., 2012), ridges (e.g., Harmon et al., 2020, Harmon, Wang et al., 2021, 2018; Agius et al., 2021, 2018; Eakin et al., 2018; Rychert et al., 2021, 2018; Rychert et al., 2020; Saikia et al., 2021b, 2021a; Wang et al., 2020), or subduction zone trenches (e.g., Harmon et al., 2013, Harmon, Rychert et al., 2021, 2008; Chichester et al., 2020; Cooper et al., 2020; Harmon & Rychert, 2015; Rychert et al., 2008; Schlaphorst et al., 2021; Syracuse et al., 2008) are often characterized by at least some structures that are expected to be relatively 2‐D. The idea is to determine the utility of using the simplest and computationally efficient way to implement a kernel and also recover the magnitude of velocity discontinuities.…”

Section: Introductionmentioning

confidence: 99%

2‐D Analytical P‐to‐S and S‐to‐P Scattered Wave Finite Frequency Kernels

Harmon

Rychert

Xie

et al. 2022

Geochem Geophys Geosyst

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Scattered wave imaging provides a powerful tool for understanding Earth's structure. The development of finite frequency kernels for scattered waves has the potential for improving the resolution of both the structure and magnitude of discontinuities in S‐wave velocity. Here we present a 2‐D analytical expression for teleseismic P‐to‐S and S‐to‐P scattered wave finite‐frequency kernels for a homogeneous medium. We verify the accuracy of the kernels by comparing to a spectral element method kernel calculated using SPECFEM2D. Finally, we demonstrate the ability of the kernels to recover seismic velocity discontinuities with a variety of shapes including a flat discontinuity, a discontinuity with a sharp step, a discontinuity with a smooth bump, and an undulating discontinuity. We compare the recovery using the kernel approach to expected recovery assuming the classical common conversion point (CCP) stacking approach. We find that the P‐to‐S kernel increases recovery of all discontinuity structures in comparison to CCP stacking especially for the shallowest discontinuity in the model. The S‐to‐P kernel is less successful but can be useful for recovering the curvature of shallow discontinuity undulations. Finally, although we observe some variability in the amplitude of the kernels along the discontinuities, the kernels show some potential for recovering the magnitude of the velocity contrast across a discontinuity.

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Seismic Attenuation at the Equatorial Mid‐Atlantic Ridge Constrained by Local Rayleigh Wave Analysis From the PI‐LAB Experiment

Cited by 9 publications

References 110 publications

Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography

Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography

Broad fault zones enable deep fluid transport and limit earthquake magnitudes

2‐D Analytical P‐to‐S and S‐to‐P Scattered Wave Finite Frequency Kernels

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