scholarly journals Transversely Isotropic Lower Crust of Variscan Central Europe imaged by Ambient Noise Tomography of the Bohemian Massif

2020 ◽  
Author(s):  
Jiří Kvapil ◽  
Jaroslava Plomerová ◽  
Hana Kampfová Exnerová ◽  
Vladislav Babuška ◽  
György Hetényi ◽  
...  
Keyword(s):  
Bohemian Massif ◽  
Lower Crust ◽  
Ambient Noise ◽  
Group Velocity Dispersion ◽  
Transverse Isotropy ◽  
Regional Scale ◽  
Velocity Model ◽  
Significant Feature ◽  
Ambient Noise Tomography ◽  
Low Velocity

Abstract. Recent development of ambient noise tomography, in combination with increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the crust of the Bohemian Massif (BM). The velocity model with a cell size of 22 km is built by conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ~0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The model provides compelling evidence for a regional-scale of velocity distribution. The Cadomian part of the region has a thinner crust, while the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickenings. A significant feature of the presented model is the velocity drop interface (VDI) modelled in the lower part of the crust. We explain this feature by anisotropic fabric of the lower crust, which is characterized as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the NW SE shortening of the crust and the late Variscan strike slip movements along the NE SW oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho, which indicate an extrusion of mantle rocks into the lower crust, can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

scholarly journals Transversely isotropic lower crust of Variscan central Europe imaged by ambient noise tomography of the Bohemian Massif

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1051-1074
Author(s):  
Jiří Kvapil ◽  
Jaroslava Plomerová ◽  
Hana Kampfová Exnerová ◽  
Vladislav Babuška ◽  
György Hetényi ◽  
...  
Keyword(s):  
Bohemian Massif ◽  
Lower Crust ◽  
Ambient Noise ◽  
Regional Scale ◽  
Crustal Thickening ◽  
Scale Model ◽  
Significant Feature ◽  
Ambient Noise Tomography ◽  
North East ◽  
The North

Abstract. The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

scholarly journals Velocity structure of the Whataroa Valley using Ambient Noise Tomography

2021 ◽  
Author(s):  
◽  
Andy McNab
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Shear Wave Velocity ◽  
Correlation Functions ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Ambient Noise Tomography

<p>This thesis applies ambient noise tomography to investigate the shallow structure of the Whataroa Valley. Ambient noise techniques are applied to continuous seismic recordings acquired on 158 geophones deployed during the Whataroa Active Source Seismic Experiment. Despite only having four days of data, a robust shear-wave velocity model is calculated using a phase-weighted stacking approach to improve the cross-correlation functions' signal-to-noise ratios, allowing for robust velocity measurements to be obtained between periods of 0.3 and 1.8\,s. This yields a database of 12,500 vertical component cross correlation functions and the corresponding Rayleigh wave phase and group velocity dispersion curves. Linearised straight-ray tomography is applied to phase and group velocity dispersion measurements at periods ranging from periods of 0.3 to 1.8\,s. The tomography reveals a velocity that decreases from the vicinity of the DFDP-2B borehole to the centre of the valley. This is interpreted to be the geologic basement deepening towards the centre of the valley. A Monte-Carlo inversion technique is used to jointly invert Rayleigh-wave phase and group velocity dispersion curves constructed from phase and group velocity tomography maps of successively higher periods. Linear interpolation of the resulting 1D shear-wave velocity profiles produces a pseudo-3D velocity model of the uppermost 1,000\,m of the Whataroa Valley. Using sharp increases in velocity to represent lithological change, we interpret two velocity contours at 1,150 and 1,250\,m/s as potential sediment-basement contacts. Depth isocontours of these velocities reveal that the basement deepens towards the centre of the valley, reaching a maximum depth of 400 or 600\,m for the 1,150 and 1,250\,m/s velocity contours respectively. These depths indicate strong glacial over-deepening and have implications for future drilling projects in the Whataroa Valley. A sharp velocity increase of 200\,m/s also occurs at 100\,m depth at the DFDP-2B borehole. We interpret this to be a change in sedimentary rock lithology from fluvial gravels to lacustrine silty sands, related to a change in sedimentary depositional environment.</p>

scholarly journals Velocity structure of the Whataroa Valley using Ambient Noise Tomography

2021 ◽  
Author(s):  
◽  
Andy McNab
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Shear Wave Velocity ◽  
Correlation Functions ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Ambient Noise Tomography

<p>This thesis applies ambient noise tomography to investigate the shallow structure of the Whataroa Valley. Ambient noise techniques are applied to continuous seismic recordings acquired on 158 geophones deployed during the Whataroa Active Source Seismic Experiment. Despite only having four days of data, a robust shear-wave velocity model is calculated using a phase-weighted stacking approach to improve the cross-correlation functions' signal-to-noise ratios, allowing for robust velocity measurements to be obtained between periods of 0.3 and 1.8\,s. This yields a database of 12,500 vertical component cross correlation functions and the corresponding Rayleigh wave phase and group velocity dispersion curves. Linearised straight-ray tomography is applied to phase and group velocity dispersion measurements at periods ranging from periods of 0.3 to 1.8\,s. The tomography reveals a velocity that decreases from the vicinity of the DFDP-2B borehole to the centre of the valley. This is interpreted to be the geologic basement deepening towards the centre of the valley. A Monte-Carlo inversion technique is used to jointly invert Rayleigh-wave phase and group velocity dispersion curves constructed from phase and group velocity tomography maps of successively higher periods. Linear interpolation of the resulting 1D shear-wave velocity profiles produces a pseudo-3D velocity model of the uppermost 1,000\,m of the Whataroa Valley. Using sharp increases in velocity to represent lithological change, we interpret two velocity contours at 1,150 and 1,250\,m/s as potential sediment-basement contacts. Depth isocontours of these velocities reveal that the basement deepens towards the centre of the valley, reaching a maximum depth of 400 or 600\,m for the 1,150 and 1,250\,m/s velocity contours respectively. These depths indicate strong glacial over-deepening and have implications for future drilling projects in the Whataroa Valley. A sharp velocity increase of 200\,m/s also occurs at 100\,m depth at the DFDP-2B borehole. We interpret this to be a change in sedimentary rock lithology from fluvial gravels to lacustrine silty sands, related to a change in sedimentary depositional environment.</p>

Sensitivity enhancement of the ambient noise tomography and analysis of seasonal variations of noise sources to refine velocities in the lower crust in central Europe

2021 ◽  
Author(s):  
Jiri Kvapil ◽  
Jaroslava Plomerova ◽  
AlpArray Working Group
Keyword(s):  
Central Europe ◽  
Seasonal Variations ◽  
Lower Crust ◽  
Ambient Noise ◽  
Regional Scale ◽  
Dispersion Curves ◽  
Small Scale ◽  
Ambient Noise Tomography ◽  
Noise Sources

&lt;p&gt;The capability of the ambient noise tomography (ANT) to image subtle regional-scale velocity variations &lt;span&gt;in &lt;/span&gt;the lower crust is limited by strong directionality of ambient noise sources in central Europe, which affects the quality of dispersion curves. Significant decrease of sensitivity kernels and sparse coverage of long interstation ray-pathes result in lower resolution at longer periods and thus &lt;span&gt;increase&lt;/span&gt; uncertainty of the inversion solution &lt;span&gt;in&lt;/span&gt; depth. &lt;span&gt;If&lt;/span&gt; these well-known ANT &lt;span&gt;limitations&lt;/span&gt; are properly addressed, the ANT is able to retrieve reliable high-resolution 3&amp;#8209;D shear velocities of the lower crust.&lt;/p&gt;&lt;p&gt;In this study we focus on seasonal variations of ambient noise sources in selected sites in different tectonic settings. We analyse ambient noise sources on continusly recorded wavefields from permanent observatories and temporary stations of AlpArray passive experiment with its complementary experiment and PACASE. These seismic networks with densely-spaced stations are well-suited for detailed analysis of period-dependent directionality of ambient noise sources and their effects on FTAN appearance and consequently on the quality of dispersion curves. In the second part of this study, we advocate a concept of layer-stripping during the stochastic inversion (enhanced ANT). It proved to be an efficient technique to explore the model space, particularly in the lower part of the crust. We discuss the sensitivity of the enhanced ANT to the imaged small-scale velocity features in the lower part of the crust, as well as the sensitivity to the sharp or gradational Moho in the models.&lt;/p&gt;

First step towards an integrated geophysical-geological model of the W-Alps: A new Vs model from transdimensional ambient-noise tomography

2021 ◽  
Author(s):  
Ahmed Nouibat ◽  
Laurent Stehly ◽  
Anne Paul ◽  
Romain Brossier ◽  
Thomas Bodin ◽  
...  
Keyword(s):  
Group Velocity ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Ambient Noise ◽  
Receiver Functions ◽  
Time Inversion ◽  
Geological Model ◽  
Ambient Noise Tomography ◽  
Low Velocity

&lt;p&gt;&lt;span&gt;We have successfully derived a new &lt;/span&gt;&lt;span&gt;3-D&lt;/span&gt;&lt;span&gt; high resolution shear wave velocity model of the crust and uppermost mantle of a large part of W-Europe from transdimensional&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;ambient-noise tomography. This model is intended to contribute to the development of the first &lt;/span&gt;&lt;span&gt;3-D&lt;/span&gt;&lt;span&gt; crustal-scale integrated geophysical-geological model of the W-Alps to deepen understanding of orogenesis and its relationship to mantle dynamics. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We used an exceptional dataset of 4 years of vertical-component, daily seismic noise records (2015 - 2019) of more than 950 permanent broadband seismic stations located in and around the Greater Alpine region, complemented by 490 temporary stations from the AlpArray sea-land seismic network and 110 stations from Cifalps dense deployments.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We firstly performed a &lt;/span&gt;&lt;span&gt;2-D&lt;/span&gt;&lt;span&gt; data-driven transdimensional travel time inversion for group velocity maps from 4 to 150 s (Bodin &amp; Sambridge, 2009). The data noise level was treated as a parameter of the inversion problem, and determined within a Hierarchical Bayes method. We used Fast Marching Eikonal solver (Rawlinson &amp; Sambridge, 2005) jointly with the reversible jump algorithm to update raypath geometry during inversion. In the inversion of group velocity maps for shear-wave velocity, we set up a new formulation of the&lt;/span&gt;&lt;span&gt; approach proposed by Lu et al (2018) by including group velocity uncertainties. Posterior probability distributions on &lt;/span&gt;&lt;span&gt;Vs&lt;/span&gt;&lt;span&gt; and interfaces were estimated by exploring a set of 130 millions synthetic &lt;/span&gt;&lt;span&gt;4-&lt;/span&gt;&lt;span&gt;layer &lt;/span&gt;&lt;span&gt;1-D Vs&lt;/span&gt;&lt;span&gt; models that allow for &lt;/span&gt;&lt;span&gt;low-velocity zones&lt;/span&gt;&lt;span&gt;&lt;em&gt;.&lt;/em&gt;&lt;/span&gt;&lt;span&gt; The obtained probabilistic model was refined using a linearized inversion&lt;/span&gt;&lt;span&gt;&lt;em&gt;. &lt;/em&gt;&lt;/span&gt;&lt;span&gt;For the ocean-bottom seismometers of the Ligurian-Provencal basin, we applied a specific processing to clean daily noise signals from instrumental and oceanic noises (Crawford &lt;/span&gt;&lt;span&gt;&amp;&lt;/span&gt;&lt;span&gt; Webb, 2000) and adapted the inversion for Vs to include the water column.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Our Vs model evidences strong variations of the crustal structure along strike, particulary in the subduction complex. The European crust includes lower crustal low-velocity zones and a Moho jump of ~8-12 km beneath the W-boundary of the external crystalline massifs. We observe a deep LVZ&lt;em&gt; &lt;/em&gt;structure (50 - 80 km) in the prolongation&lt;em&gt; &lt;/em&gt;of the European continental subduction beneath the Ivrea body. The striking fit between the receiver functions ccp migrated section across the Cifalps profile and this new Vs model validate its reliability.&lt;/p&gt;

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