Rayleigh Wave Attenuation and Amplification Measured at Ocean-Bottom
Seismometer Arrays using Helmholtz Tomography
Abstract
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 (OBS). 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 (Q_μ>1500)
overlying a highly attenuating asthenospheric layer
(Q_μ~50–70). At JdF, we find a broad peak in
attenuation (Q_μ~50–60) 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.