A 3‐D Shear Velocity Model of the Crust and Uppermost Mantle Beneath Alaska Including Apparent Radial Anisotropy

We present a 3D probabilistic model of shear wave velocity and radial anisotropy of the European crust and uppermost mantle mainly focusing on the Alps and the Apennines.The model is built using continuous seismic noise recorded between 2010 and 2018 at 1521 broadband stations, including the AlpArray network (Hete&#769;nyi et al., 2018).We use a large dataset of more than 730&#160;000 couples of stations representing as many virtual source-receiver pairs. For each path, we calculate the cross-correlation of continuous vertical- and transverse-components of the noise records in order to get the Green&#8217;s function. From the Green&#8217;s function, we then obtain the group velocity dispersion curves of Love and Rayleigh waves in the period range 5 to 149 s.Our 3D model is built in two steps. First, the dispersion data are used in a linearized least square inversion providing 2D maps of group velocity in Europe at each period. These maps are obtained using the same coverage for Love and Rayleigh waves. Dispersion curves for both Love and Rayleigh waves are then extracted from the maps, at each geographical point. In a second step, these curves are jointly inverted to depth for shear velocity and radial anisotropy. The inversion in done within a Bayesian Monte-Carlo framework integrating some a priori information coming either from PREM (Dziewonski and Anderson 1961) or the recent 3D shear wave model of Lu et al. 2018 performed for the same region.Therefore, this joint inversion of Rayleigh and Love data allows us to derive a new 3D model of shear velocity and radial anisotropy of the European crust and uppermost mantle. The isotropic part of our model is consistent with the shear velocity model of Lu et al. 2018. The 3D radial anisotropy model of the region adds new constraints on the deformation of the lithosphere in Europe. Here we present and discuss this new radial anisotropy model, with particular emphasis on the Apennines.

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A 3-D shear velocity model of the crust and uppermost mantle beneath the United States from ambient seismic noise

Geophysical Journal International ◽

10.1111/j.1365-246x.2009.04125.x ◽

2009 ◽

Vol 177 (3) ◽

pp. 1177-1196 ◽

Cited By ~ 72

Author(s):

G. D. Bensen ◽

M. H. Ritzwoller ◽

Y. Yang

Keyword(s):

United States ◽

Seismic Noise ◽

Shear Velocity ◽

Velocity Model ◽

The United States ◽

Ambient Seismic Noise ◽

Uppermost Mantle

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Lithospheric architecture of the Ligurian Basin from seismic travel time tomography

10.5194/egusphere-egu21-2822 ◽

2021 ◽

Author(s):

Anke Dannowski ◽

Heidrun Kopp ◽

Ingo Grevemeyer ◽

Grazia Caielli ◽

Roberto de Franco ◽

...

Keyword(s):

Seismic Data ◽

Velocity Model ◽

P Wave ◽

Central Basin ◽

Uppermost Mantle ◽

Mantle Boundary ◽

Oceanic Spreading ◽

Refraction Seismic ◽

Back Arc ◽

Ligurian Basin

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, seismicity indicates the closure of this back-arc basin. In the basin, earthquake clusters occur in the lower crust and uppermost mantle and are related to re-activated, inverted, normal faults created during rifting.To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile comprises 35 OBS and three land stations at Corsica to give a complete image of the continental thinning including the necking zone.The majority of the refraction seismic data show mantle phases with offsets up to 70 km. The arrivals of seismic phases were picked and used to generate a 2-D P-wave velocity model. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The P-wave velocities in the crust reach 6.6 km/s at the base. The uppermost mantle shows velocities >7.8 km/s. The crust-mantle boundary becomes shallower from ~18 km to ~12 km depth within 30 km from Corsica towards the basin centre. The velocity model does not reveal an axial valley as expected for oceanic spreading. Further, it is difficult to interpret the seismic data whether the continental lithosphere was thinned until the mantle was exposed to the seafloor. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. The distribution of earthquakes and their fault plane solutions, projected along our seismic velocity model, is in-line with the counter-clockwise opening of the Ligurian Basin.

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Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

Bulletin of the Seismological Society of America ◽

10.1785/bssa0740041263 ◽

1984 ◽

Vol 74 (4) ◽

pp. 1263-1274

Author(s):

Lawrence H. Jaksha ◽

David H. Evans

Keyword(s):

New Mexico ◽

Wave Velocity ◽

Lower Crust ◽

Seismic Waves ◽

Seismic Refraction ◽

Velocity Model ◽

P Wave ◽

Uppermost Mantle ◽

P Wave Velocity ◽

Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

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Evaluation of 3D shear-wave anisotropy based on elastic-wave velocity variations around the borehole

Geophysics ◽

10.1190/geo2020-0523.1 ◽

2021 ◽

pp. 1-47

Author(s):

Song Xu ◽

Xiao-Ming Tang ◽

Carlos Torres-Verdín ◽

Zhen Li ◽

Yuanda Su

Keyword(s):

Shear Wave ◽

Elastic Anisotropy ◽

Anisotropy Parameter ◽

Primary Source ◽

Shear Velocity ◽

Fracture Network ◽

Elastic Wave Velocity ◽

Azimuthal Anisotropy ◽

Conventional Procedure ◽

Radial Anisotropy

Aligned fractures/cracks in rocks are a primary source of elastic anisotropy. In an azimuthally anisotropic formation surrounding a borehole, shear waves split into fast and slow waves that propagate along the borehole and are recorded by a borehole logging tool. However, when the formation has conjugate fractures with orthogonal strike directions, the azimuthal anisotropy vanishes. Hence, azimuthal anisotropy measurements may not be adequate to detect orthogonal fracture sets. We develop a method for obtaining azimuthal and radial shear-wave anisotropy parameters simultaneously from four-component array waveforms. The method utilizes a velocity tomogram around the borehole. Azimuthal and radial anisotropy were determined by integrating shear velocity radiation profile along the radial direction at different azimuthal angles. Results indicate that this approach is reliable for estimating anisotropy properties in aligned crack systems. The advantage of this interpretation method is shown in multiple conjugate crack systems. Field data processing examples are used to verify the application of the proposed technique. Comparison of results against those obtained with a conventional procedure shows that the new method can not only provide estimates of azimuthal anisotropy, but also of the radial anisotropy parameter, which is important in fracture network evaluation.

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

10.26686/wgtn.16985062.v1 ◽

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

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.

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