A regional study of mantle velocity variations beneath eastern Australia and the southwestern Pacific using short-period recordings of P, S, PcP, ScP and ScS waves produced by Tongan deep earthquakes

Frohlich, Clifford A; Barazangi, Muawia

doi:10.1016/0031-9201(80)90014-x

Cited by 36 publications

(11 citation statements)

References 30 publications

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“…Observations of deep seismicity and source mechanisms in the Tonga subduction zone by Giordini and Woodhouse [1984,1986] appear unfavorable to slab penetrating the 670-km discontinuity, and these authors propose that the deepest part of the slab is displaced laterally by shear flow in the mantle. Travel time analyses in Tonga by Sondergeld et al [1977], Bock [1981], and Ansell and Gubbins [1986] found no indications for high seismic velocities in the deeper parts of the subducting slab, but their view is challenged by other workers [Fitch, 1977;Frohlich and Barazangi, 1980;Fischer et al, 1988], who suggested the presence of 5-10% high-velocity anomalies at depths below 600 Ian.…”

Section: Discussionmentioning

confidence: 51%

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

Zhou¹,

Clayton²

1990

J. Geophys. Res.

143

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We have observed slablike high P and S velocity anomalies around the Wadati‐Benioff zone under island arcs of the northwest Pacific through travel time tomographic inversions. Nineteen years of International Seismological Centre travel time residuals for events and stations in this large region are used. Analyses of resolution and noise show that the images are generally well resolved. The images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth. Near the bottom of the upper mantle, fingering of the slabs, including segmenting and spreading, is indicated. The fast anomalies associated with the Japan, Izu‐Bonin, and Mariana subduction zones tend to flatten to subhorizontal at depth, while downward spreading may occur under parts of the Mariana and Kurile arcs. The fast anomalies below 700 km are not in the shape of a single coherent sheet. The principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high‐velocity slab, even when it bends to subhorizontal at depth. The depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip. Slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the “670‐km” discontinuity.

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Section: Discussionmentioning

confidence: 51%

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

Zhou¹,

Clayton²

1990

J. Geophys. Res.

143

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“…Hot thermal anomalies in the lower mantle below the slabs are a possible explanation. Strong travel-time anomalies have also been reported by Frohlich & Barazangi (1980) for waves reflected at CMB in the South Pacific. These authors invoke mantle heterogeneities beneath deep earthquake zones as a possible explanation.…”

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confidence: 70%

Ellipticity and density at the inner core boundary from subcritical PKiKP and PcP data

Souriau

1989

Geophysical Journal International

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Summary The first part of this article gives new travel‐time data for the phase PKiKP at subcritical distances (Δ < 110°) obtained at the Warramunga array in Northern Australia. A compilation of the PKiKP subcritical travel times obtained from previous studies has also been carried out. In the second part, differential PKiKP‐PcP travel times at short distances (Δ<45°) are used to determine the inner core ellipticity and mean radius, subject to hypotheses on the equilibrium figure of the liquid core and the topography of the core‐mantle boundary (CMB). The coherence of the PKiKP‐PcP travel‐time residuals is increased when Morelli & Dziewonski's CMB topography is taken into account. According to various hypotheses, the values obtained for the ellipticity range from (1.9±2.2)×10−3 to (5.6±1.7)×10−3. This denotes a small inner core flattening corresponding to a decrease of the polar radius from (1.6±1.8) to (5.0±1.5) km with respect to a spherical inner core. The preferred solutions which involve the CMB topography, are consistent with the ellipticity 2.43times10−3 deduced from hydrostatic equilibrium of a rotating inner core. The last part of the paper concerns the amplitude variations of PKiKP. The amplitude ratio PKiKP/PcP at short distances (Δ<45°) is used to constrain the density jump δρ at the inner core boundary. δρ values of 1.35–1.66 g cm−3 are obtained for a quality factor in the liquid core ranging from 10 000 to infinity. These values, as well as a possible stratification in the inner core, are in favour of an inner core composition which is not pure iron.

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“…A rather high Q has also been found beneath station DRV at Dumont d’Urville on the Antarctic margin (Choudhury & Dorel 1973). Frohlich & Barazangi (1980) have estimated Q in the lower mantle between Fiji‐Tonga and Australia from a comparison of ( ScP ‐ PcP ) and ( ScS ‐ PcS ) waves, and obtain a value of 1050 at 1 Hz. This estimation justifies our assumption that most of the attenuation can be ascribed to the upper mantle, despite a longer path in the lower mantle: the contribution of the upper mantle to the global attenuation is close to 75 per cent.…”

Section: Discussionmentioning

confidence: 99%

“…This method has several advantages: it directly takes into account source and propagation effects, it allows us to work at a single frequency, and it provides a good control on possible undesirable contributions due to nearby phases, scattered phases, high noise or strong focusing–defocusing effects. This method is applied to the Australian–Antarctic region, where global attenuation models retrieved from surface waves have a limited resolution, and where only few short period ScS wave studies have been performed (Choudhury & Dorel 1973; Frohlich & Barazangi 1980). The region is well sampled by high quality ScS n waves thanks to the deep focus events of the nearby subduction zones, and to the availability of data from several permanent broad‐band stations.…”

Section: Introductionmentioning

confidence: 99%

Seismic attenuation in the eastern Australian and Antarctic plates, from multiple ScS waves

Souriau¹,

Rivera

Maggi

et al. 2012

Geophysical Journal International

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SUMMARY The attenuation of seismic shear waves in the mantle beneath the eastern Australian and Antarctic plates is analysed using a large data set of multiple ScSn waves, reflected n times at the core–mantle boundary and (n–1) times at the surface. The data are the transverse components of deep earthquakes from the subduction zones north and east of Australia, recorded at stations in Antarctica, Australia, Indonesia, New Caledonia and New Zealand. The data are filtered with narrow bandpass filters at five frequencies in the range 0.013–0.040 Hz. The ScSn+1/ScSn amplitude ratios of successive ScS phases are compared to the ratios computed for synthetic seismograms for the same paths and same focal mechanisms, to eliminate the effects of source radiation and geometric attenuation. The synthetic seismograms are computed from a summation of toroidal modes for the 1‐D reference model PREM. The observed to computed spectral ratios appear consistent for similar paths. They reveal that the attenuation is not frequency dependent, that the contribution of scattering to attenuation is low, and that the PREM model is a valuable reference model for the study region at the considered frequencies. An inversion of the data at 0.026 Hz is performed to retrieve the quality factor Q in the upper mantle, in regions defined using a priori constraints inferred from seismic shear velocities. Q‐values close to those of PREM are found beneath the Australian and Antarctic cratons, lower values beneath the Eastern Australian Phanerozoic margin, and very low values beneath the oceanic region between Australia and Antarctica, where ridges and a triple junction are present. The Australian–Antarctic Discordance along the South‐Indian ridge appears as an exception with a Q‐value close to those of stable continents. The highest Q‐values are found beneath the subduction zones, a feature which is not apparent in global attenuation models possibly because of its narrow lateral extension, and because it extends at depths larger than those sampled by surface waves. Despite limitations due to the uneven distribution of the ScSn bounce points at the surface and to the difficulty of collecting a large number of high quality data, our approach appears very promising. It is complementary to the more widely used determination of seismic attenuation using surface waves because it provides increased depth coverage, and a broader spectral coverage. It therefore has a considerable potential in future investigations of mantle structure and dynamics.

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A regional study of mantle velocity variations beneath eastern Australia and the southwestern Pacific using short-period recordings of P, S, PcP, ScP and ScS waves produced by Tongan deep earthquakes

Cited by 36 publications

References 30 publications

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

P and S wave travel time inversions for subducting slab under the island arcs of the northwest Pacific

Ellipticity and density at the inner core boundary from subcritical PKiKP and PcP data

Seismic attenuation in the eastern Australian and Antarctic plates, from multiple ScS waves

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