Spectral Analyses of High-Frequency Pn and Sn Phases Observed at Great Distances in the Western Pacific

Walker, Daniel A.; McCreery, Charles S.; Sutton, George H.; Duennebier, F. K.

doi:10.1126/science.199.4335.1333-a

Cited by 41 publications

(33 citation statements)

References 10 publications

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“…This process helps to explain the observations of higher apparent Q s than Q p for the Po and So phases (e.g. Walker et al 1983) that are hard to reconcile with normal intrinsic attenuation (Jackson 2007).…”

Section: P O / S O P Ro Pa G At I O N Pat T E R N S I N T H E Pa C I mentioning

confidence: 93%

High-frequency Po/So guided waves in the oceanic lithosphere: II—heterogeneity and attenuation

Furumura

Zhao²

2014

Geophysical Journal International

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S U M M A R YIn the western Pacific, high-frequency seismic energy is carried to very great distances from the source. The Po and So phases with observed seismic velocities characteristic of the mantle lithosphere have complex and elongated waveforms that are well explained by a model of stochastic heterogeneity. However, in the eastern part of the Pacific Basin equivalent paths show muted Po and weak, or missing, So. Once established, it is hard to eliminate such guided Po and So energy in the mantle lithosphere by purely structural effects. Even sharp changes in lithospheric thickness or complex transitions at fracture zones only weaken the mantle ducted wave trains, but Po and So remain distinct. In contrast, the effect of attenuation is much more severe and can lead to suppression of the So phase to below the noise level after passage of a few hundred kilometres. The differing characteristics of Po and So across the Pacific can therefore be related directly to the thermal state via the enhanced attenuation in hotter regions, such as the spreading ridges and backarc regions.Key words: Guided waves; Seismic attenuation; Computational seismology; Wave scattering and diffraction; Pacific Ocean. I N T RO D U C T I O NIn Paper I (Kennett & Furumura 2013), we described the characteristics of the propagation of the high-frequency mantle phases Po and So. These oceanic Pn and Sn phases can be observed after propagation over many thousands of kilometres from the source, retaining high frequencies but acquiring a long and complex coda. Paper I concentrated on the way in which these characteristics can be sustained by fine-scale heterogeneity in the oceanic lithosphere that reinforces the influence of multiple P reverberations in the ocean and sediments recognized by Sereno & Orcutt (1985, 1987. A form of quasi-laminar heterogeneity with horizontal correlation lengths around 10 km and vertical correlation lengths of about 0.5 km provides a good representation for efficient propagation of the Po and So wavefield. This class of stochastic heterogeneity creates a strong scattering environment within the lithosphere that helps to sustain the Po and So phases over long distances. Propagation of So is most effective in thick old lithosphere, for example, in the northwest Pacific Plate. Amplitudes of So are reduced significantly by propagation through thinner lithosphere in the Philippine Sea Plate.In this paper, we look at the entire Pacific Basin and map out the propagation patterns for Po and So, which have the general characteristic of much more efficient propagation in the western sector than in the east, which is much less well sampled by stations and * Now at: Dipartimento di Matematica 'Federigo Enriques', Università degli studi di Milano, Italy. suitable sources. There are stronger changes in the nature of So than Po. For the same frequency, S waves have a shorter wavelength than P waves, and so the So phase is more sensitive to the effects of both lateral variations in lithospheric structure and seismic attenuation. ...

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Section: P O / S O P Ro Pa G At I O N Pat T E R N S I N T H E Pa C I mentioning

confidence: 93%

High-frequency Po/So guided waves in the oceanic lithosphere: II—heterogeneity and attenuation

Furumura

Zhao²

2014

Geophysical Journal International

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“…Estimates of Q p and its frequency dependence for P n in the western Pacific vary widely and are based on a variety of different techniques including falloff rates of the coda, reduction in amplitude with distance along a linear array, maximum distance the phase is observed, and spectral ratios [Oliver and Isacks, 1967;Walker et al, 1983;Butler et al, 1987;Sereno and Orcutt, 1987;Brandsdóttir and Menke, 1988;Mallick and Frazer, 1990;Roth et al, 1999;Kennett and Furumura, 2013;Shito et al, 2013], although the general consensus is that Q p for P n in the lithosphere at 10 Hz is on the order of 1000 or greater. The thickness of the low-attenuation lithosphere is even less clear.…”

Section: 1002/2013jb010589mentioning

confidence: 99%

Upper mantle Q structure beneath old seafloor in the western Pacific

2014

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The Pacific Lithosphere Anisotropy and Thickness Experiment ocean bottom seismometer array south of the Shatsky Rise allows imaging the attenuation structure of the upper mantle in the oldest parts of the Pacific Ocean. The array consisted of eight seismometers with a lateral extent of 200 km by 600 km. Intermediate-and deep-focus earthquake sources in the Mariana and Izu-Bonin subduction zones provide paths that probe different depths beneath the seafloor. Acceleration spectra from the vertical component of P waves are inverted for the attenuation operator and corner frequency. P n propagation indicates very high Q p values in the lithosphere, of order 1000, while path-average Q p values in the upper mantle are on the order of 400. The variety of source epicentral distances and depths enables us to deduce the vertical Q p À1 structure of the upper mantle. Using the amplitude spectra directly in a weighted least squares problem that accounts for the effects of triplications in the traveltime curves, we invert for Q p À1 as a function of depth. We demonstrate that Q p is frequency dependent and adopt a power law dependence with coefficient α = 0.27. We resolve three distinct layers using a reference frequency of 1 Hz with Q p on the order of 145 at depths 100-250 km, Q p on the order of 350 at depths 250-410 km, and very high Q p with attenuation indistinguishable from zero for depths below 410 km. The P waves have a component of scattering, and thus, Q p À1 represents the maximum intrinsic attenuation beneath normal undisturbed oceanic lithosphere.

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“…Attenuation limits the frequencies of teleseismic body waves to less than a few hertz, except for Po and So, which travel within the high-Q oceanic lithosphere. These arrivals can have energy at frequencies as high as 15-20 Hz at distances greater than 30 ø [Walker et al, 1983;Butler et al, 1987]. The long (1-2 min) codas associated with these phases have been ascribed to either scattering by small-scale heterogeneities [Richards and Menke, 1983; Menke and Chen, 1984; Novelo-Casanova and Butler, 1986] or as the result of propagation in leaky organ-pipe modes of the lithospheric waveguide Orcutt, 1985, 1987].…”

Section: Vlf Acoustics Band (5-50 Hz)mentioning

confidence: 99%