South Pacific Upper mantle surface waves (original) (raw)
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Physics of the Earth and Planetary Interiors, 1981
Long-period Rayleigh wave group velocity dispersion curves are presented for paths across the east Pacific Ocean. The records are from the IPG site at Pamatai (Tahiti) and the IDA station at Nana. For the first time, direct-path observations of group velocities up to 300 s have been obtained. This study shows that for young oceanic regions group velocities are low even at long periods. The observations are interpreted in terms of an S-wave velocity model by a generalized inversion scheme. In the models for young ages, the low-velocity zone under the asthenosphere lid is well developed, with a strong velocity gradient at the bottom of this zone, followed at larger depths by a plateau representing a lower-velocity zone, and a marked gradient at 400 km.
Geophysical Journal International
We examine P-wave velocity structure at the base of the mantle beneath the western Pacific, near the western edge of the Pacific Large-Low Shear Velocity Province (LLSVP), using high-quality seismograms provided by a large-scale mobile broad-band seismic observation in northeastern China (the NECESSArray project). Forward modelling using the reflectivity method is conducted to explain the variation of P-wave traveltimes as a function of epicentral distance near the core shadow zone. Additionally, PcP-P traveltimes are examined to enlarge the survey area. As a result, a rapid variation of P-wave velocity is detected at the base of the mantle. Regions of thin (20-50 km thick) and low velocity (−2 to −5 per cent) layers at the base of the mantle are intersected by an 80-km-thick region with a high velocity (+2 per cent). A slightly fast region exists at the northwest of the region with the thin low-velocity layer. These layers are typically separated by several hundred kilometres and would be difficult to explain by thermal effects alone. These observations suggest that very complicated thermochemical reactions occur near the edge of the Pacific LLSVP.
Journal of Geophysical Research, 1989
Abslracl. Group and phase velocity distributions of Rayleigh wave fundamental modes are determined in the Atlantic Ocean using a method of regionMization without a priori constraints, for a period range from 50 to 200 s. Eighty-six direct source-station paths are studied, and 38 out of the best data from Weidner's (1974) phase velocities are added to our data set for periods lower than 100 s. The importance of shallow layer corrections is described in detail. The general features of the distributions agree with global models, and smaller-scMe heterogeneities are revealed. A spatial resolution of 1500 km is needed to detect fine lateral heterogeneities in the Atlantic. The small lateral extent of young oceanic regions and strong lateral velocity gradients do not allow to quantify velocity versus age relationships using our data set. On the other hand, the regionalization without a priori constraints of phase velocities confirms the presence of a high-velocity anomaly beneath the central part of the Atlantic for periods greater than 100 s. and Mitchell [1981] found that the best regionalization consisted of two regions younger and older' than 65 Woodhouse and Dziewonski's [1984] M84C model, which includes crustal corrections, shows an asymmetric North Atlantic upper mantle where a broad southwestnortheast trending low-velocity anomaly extends from. Central America to the northernmost region of the North Atlantic, down to 250 km depth. In contrast, N akanishi and Anderson [1984] found 0.10 km/s higher shear wave velocities in the western region of the North Atlantic upper mantle than in the eastern region. This discrepancy between various global tomographic models is also present, in this depth range, in the South Atlantic, where Woodhouse and Dziewonski [1984] obtained a low-velocity anomaly and Nakanishi and Anderson [1984] and Tanimoto [1986] observed high shear wave velocities. Down to 150 km depth, the representation of upper mantle heterogeneities, with the no a priori constraints regionalization of Nataf et al. [1986], also shows higher shear wave velocities in the western part of the South Atlantic than M84C.
Mantle P-wave velocity structure beneath the Hawaiian hotspot
Earth and Planetary Science Letters, 2011
Three-dimensional images of P-wave velocity structure beneath the Hawaiian Islands, obtained from a network of seafloor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a high-velocity anomaly in the shallow upper mantle that is parabolic in map view. Low velocities continue downward to the mantle transition zone between 410 and 660 km depth and extend into the topmost lower mantle, although the resolution of lower mantle structure from this data set is limited. Comparisons of inversions with separate data sets at different frequencies suggest that contamination by water reverberations is not markedly biasing the P-wave imaging of mantle structure. Many aspects of the P-wave images are consistent with independent tomographic images of S-wave velocity in the region, but there are some differences in upper mantle structure between P-wave and S-wave velocities. Inversions without station terms show a southwestward shift in the location of lowest P-wave velocities in the uppermost mantle relative to the pattern for shear waves, and inversions with station terms show differences between P-wave and S-wave velocity heterogeneity in the shallow upper mantle beneath and immediately east of the island of Hawaii. Nonetheless, the combined data sets are in general agreement with the hypothesis that the Hawaiian hotspot is the result of an upwelling, high-temperature plume. The broad upper-mantle low-velocity region beneath the Hawaiian Islands may reflect the diverging "pancake" at the top of the upwelling zone; the surrounding region of high velocities could represent a downwelling curtain; and the low-velocity anomalies southeast of Hawaii in the transition zone and topmost lower mantle are consistent with predictions of plume tilt.
Variations ofPwave speeds in the mantle transition zone beneath the northern Philippine Sea
Journal of Geophysical Research, 1997
Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern comer of the Philippine Sea, within a region of approximately 500 x 500 km 2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in V, across the 660-km discontinuity from approximately 6% to 3%. Such a configuration •s consistent wath the interpretanon that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel-Mg-Fe-perovskite transition. between the upper and the lower mantle [e.g., Anderson, 1989; Lay, 1994; Silver et at., 1988]. Several recent studies showed evidence that the extent of slab penetration into the lower mantle seems to vary laterally along subduction zones such that at least some subducted slabs remain stagnant in the upper mantle [e.g., Glennon and Chen, 1995a; Ding and Grand, 1994], while others plunge steeply into the lower mantle [e.g., Jordan, 1977; Creager and Jordan, 1984, 1986]. For instance, on the basis of long-period (-20 s) converted and reflected body waves, Shearer and Masters [ 1992] and Shearer [ 1991 ] contended that the topography of the 660-km discontinuity shows
Structure of the upper mantle in Asia from phase and group velocities of Rayleigh waves
Izvestiya, Physics of the Solid Earth, 2008
Dispersion curves of phase velocities of Rayleigh waves are determined by the method of frequency-time analysis in a range of periods of 10-200 s from data of 43 interstation traces in Central Asia. Because the joint use of phase and group velocities significantly decreases the uncertainty in the determination of S wave velocity structures, the same traces were used for calculating group velocities from tomographic reconstructions obtained in Kozhevnikov, 2003, 2006] and determining average velocity structures along these traces. The velocity structures were calculated by the Monte Carlo and linear inversion methods, which gave consistent results. Using velocity values obtained at fixed depths by the 2-D tomography method, lateral variations in velocities at these depths were estimated, which allowed us to construct smoothed vertical velocity structures at some points in the region. The resulting structures were used as initial approximations for constructing local velocity structures solely from previously obtained local dispersion curves of group velocities in the area (32 ° -56 ° N, 80 ° -120 ° E). Based on these structures, we mapped the lateral distribution of velocity variations at upper mantle depths of 75-400 km and along three vertical profiles. The inferred velocity variations are in good agreement with data on the tectonics of the region. PACS numbers: 91.30.Ab
Variations of P wave speeds in the mantle transition zone beneath the northern Philippine Sea
Journal of Geophysical Research, 1997
Using waveforms and travel times from deep earthquakes, we constructed 16 seismic profiles, each of which constrains the radial variation in Vp over a small area beneath the northern Philippine Sea. Taken together, the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high Vp (up to 3% faster than average Earth models) originally suggested by travel time tomography. Unlike travel time tomography, which relies heavily on arrival times of the direct P phase, we utilize the waveforms and move-out of later arrivals that mainly sample the mantle transition zone of interest. Our results identify three important characteristics of the northern Philippine Sea anomaly that are distinct from previous results. First, being approximately a subhorizontal, laterally uniform feature, the anomaly is localized beneath the northwestern comer of the Philippine Sea, within a region of approximately 500 x 500 km 2 immediately east of the Ryukyu arc. Second, the anomaly is well constrained to occur in the lower portion of the transition zone, extending all the way down to the 660-km discontinuity. Third, the presence of such a distinct anomaly reduces the contrast in V, across the 660-km discontinuity from approximately 6% to 3%. Such a configuration •s consistent wath the interpretanon that the anomaly is caused by a remnant of subducted slab, as negative buoyancy should rest the slab just above the 660-km discontinuity where resistance to subduction is expected from a negative Clapeyron slope during the spinel-Mg-Fe-perovskite transition.
3D S-wave velocity pattern in the upper mantle beneath the continent of Asia from Rayleigh wave data
Physics of the Earth and Planetary Interiors, 2003
Group velocities of Rayleigh waves in the 10-150 s period range along about 1200 paths across Central Asia and Siberia are used to obtain group velocity maps of the region. To image the lateral variations of group velocities we used the tomography method developed earlier for spherical surface [PEPI 122 (2000) 19]. The method is supplemented by the opportunity to estimate the resolving power of the data. The locally averaged dispersion curves have been then inverted to vertical S-wave velocity profiles up to depth of 500 km at different sites in the region, which thus image a 3D velocity distribution. In order to reduce the number of unknown parameters we assumed the relationships between S-wave and P-wave velocities and densities to be those of the PREM model. The 3D S-wave velocity pattern is displayed in the form of maps at depths from 50 to 350 km with a step of 50 km, and of 2D vertical velocity sections along some profiles crossing different tectonic units. The most remarkable feature of the 3D velocity distribution is the presence of a high-velocity rise at the depths of 250-350 km beneath southern Siberia and western China, which borders upon the low-velocity asthenosphere along the longitude about 105 • E. Collision of the high-and low-velocity substances results in pressing-out the viscous asthenosphere material in the weakened zone of the Baikal rift, where the lithosphere is thinned. The rise of the asthenosphere in this zone probably produces mantle upwelling, and leads to uplift of the Khangay Plateau.
Geophysical Research Letters, 1983
The propagation of Rayleigh waves along 44 direct paths across the Pacific Ocean is studied and a regionalization of the dispersion according to the age of the sea floor is performed. The group velocity dispersion curves characteristic of each one of four age regions considered are obtained for periods up to 300 s. These curves show an increase of group velocities with age over the whole period range considered. The shear wave velocity models inferred yield a lithosphere thickening with age and show the presence below the lithosphere of two zones where negative velocity anomalies decrease with increasing age : the low velocity zone and a zone at a depth of 350 kin.