scholarly journals Structural features and shear-velocity structure of the “Pacific Anomaly”

2009 ◽  
Vol 114 (B2) ◽  
Author(s):  
Yumei He ◽  
Lianxing Wen
Keyword(s):  
Velocity Structure ◽  
Shear Velocity ◽  
Structural Features ◽  
The Pacific

The upper mantle beneath the Aleutian Island Arc from pure-path Rayleigh-wave dispersion data

1972 ◽  
Vol 62 (6) ◽  
pp. 1439-1453 ◽  
Author(s):  
K. H. Jacob ◽  
K. Hamada
Keyword(s):  
Velocity Structure ◽  
Shear Velocity ◽  
Wave Dispersion ◽  
Island Arc ◽  
Aleutian Island ◽  
Low Velocity ◽  
The Pacific ◽  
Average Shear ◽  
Tectonically Active ◽  
Group Velocities

abstract Group velocities of Rayleigh waves recorded at a long-period seismograph station on Amchitka Island were obtained for mixed tectonic paths across the Pacific and the Aleutian Island Arc Ridge. The mixed-path group velocities for periods between 20 and 60 sec were then separated into pure-oceanic and pureridge path group velocities. The group velocities for the pure paths along the Aleutian ridge are on the average 0.36 km/sec lower than those for the purely oceanic paths across the Pacific. Inversion of the pure-ridge group velocities yields almost continental shear velocities in the crust, a very gradual crust-mantle transition at depths between 20 and 40 km, a thin lithospheric lid of uppermost mantle material between 30 and 70 km with relatively low maximum shear velocities approaching 4.4 km/sec, and a very pronounced low-velocity zone at depths below 70 km with an average shear velocity of 4.1 km/sec. The computed shear velocity structure beneath the Aleutian Ridge is compared to models for other tectonically active and stable regions.

A new 3-D mantle density model from recent normal-mode measurements

2021 ◽  
Author(s):  
Rûna van Tent ◽  
Arwen Deuss ◽  
Andreas Fichtner ◽  
Lars Gebraad ◽  
Simon Schneider ◽  
...  
Keyword(s):  
Normal Mode ◽  
Lower Mantle ◽  
Velocity Structure ◽  
Normal Modes ◽  
Shear Velocity ◽  
Boundary Region ◽  
Splitting Function ◽  
The Pacific ◽  
Compositional Variations ◽  
Body Tides

<p>Constraints on the 3-D density structure of Earth’s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (M<sub>w</sub> > 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.</p><p>A large number of new normal-mode splitting function measurements has become available since the last density models of the entire mantle were published. Here, we show the models from our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.</p>

Integrated geophysical modelling of terranes and other structural features along the western Canadian margin

1992 ◽  
Vol 29 (7) ◽  
pp. 1492-1508 ◽  
Author(s):  
S. A. Dehler ◽  
R. M. Clowes
Keyword(s):  
Seismic Refraction ◽  
Vancouver Island ◽  
Structural Features ◽  
Pacific Rim ◽  
Data Set ◽  
Reflection Seismic ◽  
Barkley Sound ◽  
The Pacific ◽  
Wrangellia Terrane ◽  
Lower Crustal

An integrated geophysical data set has been used to develop structural models across the continental margin west of Vancouver Island, Canada. A modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous Crescent and Pacific Rim terranes. These terranes in turn abut against the pre-Tertiary Wrangellia terrane that constitutes most of the island. Gravity and magnetic anomaly data, constrained by seismic reflection, seismic refraction, and other data, were interpreted to determine the offshore positions of these terranes and related features. Iterative 2.5-dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features. An average model was then developed to represent this part of the margin. The Pacific Rim terrane appears to be continuous and close to the coastline along the length of Vancouver Island, consistent with emplacement by strike-slip motion along the margin. The Westcoast fault, the boundary between the Pacific Rim and Wrangellia terranes, is interpreted to be 15 km farther seaward than in previous interpretations in the region of Barkley Sound. The Crescent terrane forms a thin landward-dipping slab along the southern half of the Vancouver Island margin, and cannot be confirmed along the northern part. Model results suggest the slab has buckled into an anticline beneath southern Vancouver Island and Juan de Fuca Strait, uplifting high-density lower crustal or upper mantle material close to the surface to produce the observed intense positive gravity anomaly. This geometry is consistent with emplacement of the Crescent terrane by oblique subduction.

scholarly journals Imaging New Zealand's Crustal Structure Using Ambient Seismic Noise Recordings from Permanent and Temporary Instruments

2021 ◽  
Author(s):  
◽  
Yannik Behr
Keyword(s):  
Seismic Noise ◽  
Cross Correlation ◽  
Velocity Structure ◽  
Shear Velocity ◽  
Ambient Seismic Noise ◽  
Uppermost Mantle ◽  
Noise Sources ◽  
Noise Field ◽  
Rayleigh And Love Waves ◽  
Noise Cross Correlation

<p>We use ambient seismic noise to image the crust and uppermost mantle, and to determine the spatiotemporal characteristics of the noise field itself, and examine the way in which those characteristics may influence imaging results. Surface wave information extracted from ambient seismic noise using cross-correlation methods significantly enhances our knowledge of the crustal and uppermost mantle shear-velocity structure of New Zealand. We assemble a large dataset of three-component broadband continuous seismic data from temporary and permanent seismic stations, increasing the achievable resolution of surface wave velocity maps in comparison to a previous study. Three-component data enables us to examine both Rayleigh and Love waves using noise cross-correlation functions. Employing a Monte Carlo inversion method, we invert Rayleigh and Love wave phase and group velocity dispersion curves separately for spatially averaged isotropic shear velocity models beneath the Northland Peninsula. The results yield first-order radial anisotropy estimates of 2% in the upper crust and up to 15% in the lower crust, and estimates of Moho depth and uppermost mantle velocity compatible with previous studies. We also construct a high-resolution, pseudo-3D image of the shear-velocity distribution in the crust and uppermost mantle beneath the central North Island using Rayleigh and Love waves. We document, for the first time, the lateral extent of low shear-velocity zones in the upper and mid-crust beneath the highly active Taupo Volcanic Zone, which have been reported previously based on spatially confined 1D shear-velocity profiles. Attributing these low shear-velocities to the presence of partial melt, we use an empirical relation to estimate an average percentage of partial melt of < 4:2% in the upper and middle crust. Analysis of the ambient seismic noise field in the North Island using plane wave beamforming and slant stacking indicates that higher mode Rayleigh waves can be detected, in addition to the fundamental mode. The azimuthal distributions of seismic noise sources inferred from beamforming are compatible with high near-coastal ocean wave heights in the period band of the secondary microseism (~7 s). Averaged over 130 days, the distribution of seismic noise sources is azimuthally homogeneous, indicating that the seismic noise field is well-suited to noise cross-correlation studies. This is underpinned by the good agreement of our results with those from previous studies. The effective homogeneity of the seismic noise field and the large dataset of noise cross-correlation functions we here compiled, provide the cornerstone for future studies of ambient seismic noise and crustal shear velocity structure in New Zealand.</p>

scholarly journals Higher mode surface waves and their bearing on the structure of the earth's mantle

1964 ◽  
Vol 54 (1) ◽  
pp. 161-182
Author(s):  
Robert L. Kovach ◽  
Don L. Anderson
Keyword(s):  
Group Velocity Dispersion ◽  
Shear Velocity ◽  
Wave Dispersion ◽  
Abrupt Increase ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
The Pacific ◽  
The Pacific Ocean ◽  
Mode Group ◽  
Velocity Zone

abstract A detailed numerical investigation of surface wave dispersion and particle motion associated with the higher Love and Rayleigh modes over realistic earth models has been carried out as a preliminary to the routine use of these waves in studies of the crust-mantle system. The suggestion that the so-called channel waves, such as the Lg, Li, and Sa phases, can be interpreted by higher mode group velocity dispersion curves is verified in detail. Furthermore, Sa should have a higher velocity across shield areas than across normal continental areas and a higher velocity across continents than across oceans. Higher mode Rayleigh wave data are presented for long oceanic paths to Pasadena. The observed data favor the CIT 11 model of Anderson and Toksöz (1963) over the 8099 model of Dorman et al. (1960) and indicate that under the Pacific Ocean the low-velocity zone extends to a depth perhaps as deep as 400 km followed by an abrupt increase in shear velocity.

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