High frequency seismic waves and slab structures beneath Italy

Sun, Daoyuan; Miller, Meghan; Agostinetti, Nicola Piana; Asimow, Paul D.; Li, Dunzhu

doi:10.1016/j.epsl.2014.01.034

Cited by 23 publications

(30 citation statements)

References 61 publications

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“…Subducted slabs also have the capacity to duct high-frequency energy from deep events, with most notable effects seen in old slabs, for example, Japan (Furumura & Kennett 2005), Indonesia and Italy (Sun et al 2014), but also for younger slabs in southwestern Japan and Taiwan (Chen et al 2013). This guided energy too can be explained with a similar model of heterogeneity.…”

Section: O N C L U S I O N Smentioning

confidence: 81%

“…Sun et al (2014) discuss a number of mechanisms that might help to preserve such heterogeneity. As noted in Paper I, our preferred class of lithosphere heterogeneity has a strong resemblance to a frozen version of the mille-feuille model proposed by Kawakatsu et al (2009) for asthenospheric structure including melt pods beneath the Philippine Sea Plate.…”

Section: O N C L U S I O N Smentioning

confidence: 99%

“…This guided energy too can be explained with a similar model of heterogeneity. In the stress and thermal states prevailing in the subducted slab, it is difficult to achieve slab-parallel alignment (Sun et al 2014), and therefore the heterogeneity must be imported with the incoming oceanic lithosphere. The successful modelling of Shito et al (2013) of Po and So arrivals from deep events in the subducted Pacific Plate recorded at ocean bottom seismometers, using the same heterogeneity structure in both the subduction zone and the oceanic lithosphere provides strong support to a common origin of the heterogeneity, as also proposed by Sun et al (2014).…”

Section: O N C L U S I O N Smentioning

confidence: 99%

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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: O N C L U S I O N Smentioning

confidence: 81%

Section: O N C L U S I O N Smentioning

confidence: 99%

Section: O N C L U S I O N Smentioning

confidence: 99%

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

Furumura

Zhao²

2014

Geophysical Journal International

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“…Obviously, the wider the seismic velocity distribution, the higher is the radial anisotropy (Figure ). For skew‐normal distributions analogous to those inferred from the scattering of P waves propagating through the lithosphere (Furumura & Kennett, ; Sun et al, ), radial anisotropy is only about 0.2% when the distribution is symmetric, and progressively decreases with increasing asymmetry of the distribution.…”

Section: Quantification Of Extrinsic Seismic Anisotropymentioning

confidence: 90%

“…Symmetric and asymmetric distributions are given by

w ((), x) = \frac{2}{ω} ϕ ((), \frac{x - ξ}{ω}) normalΦ ((), α ((), \frac{x - ξ}{ω}))

, where

ϕ ((), x) = \frac{1}{\sqrt{2 π}} e^{- \frac{x^{2}}{2}}

normalΦ ((), x) = \frac{1}{2} ([], 1 + erf ((), \frac{x}{\sqrt{2}}))

, ξ is the location (Vs = 4,619 m/s; Vp= 8,000 m/s; Poisson ratio = 0.25; density = 3,300 kg/m 3 ), ω is the scale (for symmetric—Gaussian—distributions it is the standard deviation; shown as percentage of ξ ), α is the shape (−2, 0, 2 for left‐skewed, symmetric, right‐skewed distributions). Inset: Vp distributions with ω = 2.5% (as from P wave scattering data in the lithosphere; e.g., Furumura & Kennett, ; Sun et al, ), which generate radial anisotropy of about 0.1–0.2%. In a two‐component system with ω = 2.5% and 50:50 phase proportions, w Vs (4,537 m/s, 4,700 m/s) = w Vp (7,859 m/s, 8,141 m/s) = (0.5, 0.5), and radial anisotropy is about 0.1 (%).…”

Section: Quantification Of Extrinsic Seismic Anisotropymentioning

confidence: 99%

Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle

et al. 2019

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Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi‐laminated structures observed in subducting slabs using P and S wave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1–0.2% of Vp and Vs radial anisotropy). More generally, rock‐scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain‐scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid‐ocean ridge basalt‐rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2–3% Vp and 3–6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660‐km discontinuity could be possibly related to grain‐scale shape‐preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.

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

Furumura

2015

Geochem Geophys Geosyst

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The character of the high‐frequency seismic phases Po and So, observed after propagation for long distances in the oceanic lithosphere, requires the presence of scattering from complex structure in 3‐D. Current models use stochastic representations of seismic structure in the oceanic lithosphere. The observations are compatible with quasi‐laminate features with horizontal correlation length around 10 km and vertical correlation length 0.5 km, with a uniform level of about 2% variation through the full thickness of the lithosphere. Such structures are difficult to explain with petrological models, which would favor stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts. Petrological evidence mostly points to smaller‐scale features than suggested by seismology. The models from the different fields have been derived independently, with various levels of simplification. Fortunately, it is possible to gently modify the seismological model toward stronger basal heterogeneity, but there remains a need for some quasi‐laminate structure throughout the mantle component of the oceanic lithosphere. The new models help to bridge the gulf between the different viewpoints, but ambiguities remain.

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High frequency seismic waves and slab structures beneath Italy

Cited by 23 publications

References 61 publications

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

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

Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle

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

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