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

Abstract

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. ...

Toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity

“…The observation of Kennett et al [2014] of less efficient Po and So propagation in younger lithosphere (higher temperatures and attenuation) would be consistent with the introduction of heterogeneity in the general neighborhood of the ridge crest, with contrasts masked by the presence of strong attenuation but having a stronger influence on the wavefield as the lithosphere cools. Kawakatsu et al [2009] have proposed the presence of a ''mille-feuile'' structure in the oceanic asthenosphere, with elongated pods of partial melt, to explain the contrasts with the lithosphere seen in S wave Geochemistry, Geophysics, Geosystems…”

“…Across the Northwest Pacific region, the apparent velocity for Po is 8.1 km/s and for So is 4.6 km/s. Similar properties as observed for regions of efficient Po and So propagation in other parts of the Pacific basin [ Kennett et al ., ].…”

“…Models which reproduce the character of the Po and So arrivals in 2‐D simulations invoke quasi‐laminar heterogeneity with a von‐Kármán distribution characterized by horizontal correlation length 10–20 km and vertical correlation length 0.5 km in the mantle component of the lithosphere [ Kennett and Furumura , ; Shito et al ., ; Chen et al ., ; Kennett et al ., ]. The details of the crustal model have only a mild influence on the nature of the Po and So after propagation for a few hundred kilometers, because the seismic energy spends little time in this zone.…”

“…The observation of Kennett et al . [] of less efficient Po and So propagation in younger lithosphere (higher temperatures and attenuation) would be consistent with the introduction of heterogeneity in the general neighborhood of the ridge crest, with contrasts masked by the presence of strong attenuation but having a stronger influence on the wavefield as the lithosphere cools.…”

Crustal Heating and Lithospheric Alteration and Erosion Associated With Asthenospheric Upwelling Beneath Southern New England (USA)

“…More recent studies (Kennett et al, ; Kennett & Furumura, ) that model a more comprehensive set of seismograms with 2D waveform simulations have confirmed these early results and, furthermore, have shown that the lithospheric mantle plays the dominant role in transmitting P o and S o energies. Scattering from fine‐scale heterogeneities in the lithospheric mantle with high (20:1) horizontal‐to‐vertical aspect ratios reinforces the effects of the crust and water column, subsequently producing complex high‐frequency arrivals with long codas.…”