The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work s... more The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearan...
As one of the rare observational tools for studying deformation and stress within the Earth, seis... more As one of the rare observational tools for studying deformation and stress within the Earth, seismic anisotropy has been one of the focuses of geophysical studies over the last decade. In order to unravel the anisotropic properties of the crust, the teleseismic receiver functions (RF) methodology has started to be widely applied recently. Such effects of anisotropy on RF were illustrated in theoretical studies, showing the strong backazimuthal dependence of RF on the 3D characteristics of the media sampled by the waves. The use of teleseismic RF has the advantage of not being affected by a heterogeneous depth distribution of local earthquakes, since teleseismic rays sample the entire crust beneath the stations. The application of this technique however, needs to be critically assessed using a suitable field test.
Low values of S-wave velocity characterize the shallow layers of the Earth’s crust. This is parti... more Low values of S-wave velocity characterize the shallow layers of the Earth’s crust. This is particularly the case in basins filled with unconsolidated sediments of recent deposition. Such low-velocity materials trap seismic energy, and can lead to large ground accelerations. Therefore precise knowledge of those low shear-wave velocities in sedimentary basins is important for better understanding seismic hazard, e.g. for modeling seismic wave propagation and reconstruction of shake maps.
In this work we present the application of the global-phase seismic interferometry (GloPSI) techn... more In this work we present the application of the global-phase seismic interferometry (GloPSI) technique to a dataset recorded across the Eastern Alps with the EASI (Eastern Alpine Seismic Investigation) temporary seismic network. GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium and is used here to retrieve information about the crust-mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions where it shows a nearly planar behaviour and corresponds to a strong velocity contrast (i.e. in the northern part of the profile, from the Bohemian Massif to the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence of the boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards and confirm the observations of previous studies suggesting a structurally complex and faulted internal Alpine crustal structure.
We use a novel technique named global-phase seismic interferometry (GloPSI) to image the lithosph... more We use a novel technique named global-phase seismic interferometry (GloPSI) to image the lithospheric structure, and in particular the Moho, below two parallel north-south transects belonging to the GANSSER network (2013–2014). The profiles cross the Himalayan orogenic wedge in Bhutan, a tectonically important area within the largest continent-continent collision zone on Earth that is still undergoing crustal thickening and represents a challenging imaging target for the GloPSI approach. GloPSI makes use of direct waves from distant earthquakes and receiver-side reverberations with near vertical incidence. Reflections are isolated from earthquake recordings by solving a correlation integral and are turned into a reflectivity image of the lithosphere below the arrays. Our results compare favorably with first-order features observed from a previous receiver function (RF) study. We show that a combined interpretation of GloPSI and RF results allows for a more in-depth understanding of ...
SUMMARYTo constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wa... more SUMMARYTo constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear posit...
In this work we present the application of the Global-Phase Seismic Interferometry (GloPSI) techn... more In this work we present the application of the Global-Phase Seismic Interferometry (GloPSI) technique to a data-set recorded across the Eastern Alps with the EASI temporary seismic network (Eastern Alpine Seismic Investigation). GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium, and is used here to retrieve information about the crust-mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions where it shows a nearly planar behaviour (i.e. in the northern part of the profile, from the Bohemian massif to beneath the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence for a typical boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards. On the contrary, our results confirm the observations of previous studies suggesting a structurally complex Moho topography and faulted internal Alpine crustal structure. 1 Introduction As part of the Alpine-Himalayan orogen, the European Alps are the result of the subduction of the Alpine Tethys and European paleomargin beneath the Adriatic microplate and the subsequent continent-continent collision that led to a 200km wide convergence zone with a significant crustal root (e.g. Handy et al., 2015, and references therein). Due to the distinctively different chemistry between the crust and mantle leading to very different physical properties of the crust with respect to the uppermost mantle, the base of the crust denotes a first-order velocity interface and is the seismically best visible subsurface structure in the Earth. First noted by A. Mohorovicic 1910, this velocity interface-called Moho in his honor-,in seismic records from intra-crustal earthquakes produces a specific set of waves that today are well known in seismology as the direct wave Pg, the wide-angle reflection from the Moho (PmP) and the critically refracted wave Pn travelling in the uppermost mantle along the Moho (e.g. Giese et al., 1976). These three specific seismic phases are well visible also in record sections from blasts and thus 1
The focus of this study is the mantle structure beneath the Apennines, and aims to understanding ... more The focus of this study is the mantle structure beneath the Apennines, and aims to understanding how deep processes are connected to shallow deformations. We present new observations from a rich receiver function data set from stations located along the North and Central Apennine chain, and use it for comparison and to strengthen the observations of previous seismic tomography images. The two methodologies define a low shear wave velocity zone (decrease of Vs in the order of 5%) and an increase of Vp/Vs (about 3%) in the shallow mantle between 50 and 90 km depth beneath the orogenic belt. The low Vs melt zone is not restricted to the mantle beneath the Quaternary volcanic areas, as previously thought, but is detected under the whole central Apennines suggesting future broad effects on a large scale. Our interpretation of the teleseismic RFs and tomography, reveals consistently a diffuse mantle upwelling beneath the Apennines, and we hypothesize that slab-derived fluids might interact with the sub-lithospheric mantle generating melts that accumulate at the top of the mantle feeding post-collisional extension. This mechanism can be potentially applied to other cases of extension that spread over wide continental regions.
The upper crust of the KTB (Kontinentales Tiefbohrprogramm) area in the Southeastern Germany is a... more The upper crust of the KTB (Kontinentales Tiefbohrprogramm) area in the Southeastern Germany is a focal point for the Earth Science community due to the huge amount of information collected throughout the last 30 yr. In this study, we explore the crustal structure of the KTB area through the application of the Receiver Function (RF) technique to a new data set recorded by nine temporary seismic stations and one permanent station. We aim to unravel the isotropic structure and compare our results with previous information from the reflection profiles collected during the initial site investigations. Due to the large amount of information collected by previous studies, in terms of P-wave velocity, depth and location of major reflectors, depth reconstruction of major faults zones, this area represents a unique occasion to test the resolution capability of a passive seismological study performed by the application of the RF. We aim to verify which contribution could be given by the application of the RF technique, for future studies, in order to get clear images of the deep structure and up to which resolution. The RF technique has apparently not been applied in the area before, yet it may give useful additional insight in subsurface structure, particularly at depths larger than the maximum depth reached by drilling, but also on structures in the upper crust, around the area that has been studied in detail previously. In our results v S-depth profiles for stations located on the same geological units display common features and show shallow S-wave velocities typical of the outcropping geological units (i.e. sedimentary basin, granites and metamorphic rocks). At around 10 km depth, we observe a strong velocity increase beneath all stations. For the stations located in the centre of the area, this variation is weaker, which we assume to be the signature of the main tectonic suture in the area (i.e. the Saxothuringian-Moldanubian suture), along a west-to-east extended region, may be due to the presence of the allochthonous klippe trapped between the main crustal terrains that came in touch during the Variscan orogeny. In the lower crust we see only small variations throughout the area, at the resolution that is possible with a small temporary experiment with just 10 stations.
Seismic anisotropy is a unique observational tool for remotely studying deformation and stress wi... more Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the threedimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf-Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0-4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of micarich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.
The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work s... more The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/ or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems.
• The seismic structure of the crust across the deformation zone associated with the tear in the ... more • The seismic structure of the crust across the deformation zone associated with the tear in the Ionian slab is investigated, between Southern Calabria and Sicily (Italy) • The isotropic S-wave velocities profiles show the clear differences at the boundaries of the deformation zone, pointing out the different origin for the crust in the two regions • Anisotropy at mid-crustal depth develops both in Sicily and in Southern Calabria, with identical geometrical parameters, and shows connections with the deformation due to the differential movement of the retreating slab
ABSTRACT The crustal velocity structure in a region of central Apennines of Italy at the hinge be... more ABSTRACT The crustal velocity structure in a region of central Apennines of Italy at the hinge between the highly stretched portion of the Monte Argentario promontory and the magmatic province of the Tolfa Domes Complex (Northern Latium) is discussed in this study. S-wave velocities at depth have been constrained by the modeling of P-wave Receiver Functions (RF) from both temporary and permanent broadband seismic stations. The computer 3D Vs models show a thin crust (19–25 km) made of a shallow and thin sedimentary cover, a very high velocity and anisotropic layer related to a metamorphic basement, and a low Vs anisotropic layer in the middle-lower crust above a shallow Moho discontinuity modeled at about 20 km depth. The volcano-tectonic evolution of this portion of Tyrrhenian back-arc margin has been strongly influenced by its peculiar crustal architecture. The low-Vs layer acted as a shear zone in the middle-lower crust during the Tyrrhenian extension, also helping the development of Plio-Quaternary magmatism. Our findings potentially give new constraints on the evolution of the area and to the general comprehension of back-arc development in collisional regions.
Receiver functions (RFs) analyzed at two permanent broadband seismic stations operating in the ep... more Receiver functions (RFs) analyzed at two permanent broadband seismic stations operating in the epicentral area of the M w 6.3, 2009 L'Aquila earthquake (central Italy) yield insight on crustal structure along the fault rupture. The harmonic decomposition of RFs highlights a subsurface structure in which both isotropic and anisotropic features are present. We model the waveforms using recently developed Monte Carlo methods. The retrieved models display a common depth structure, between 10 and 40 km depth, consistent with the under-thrusting of the Adria lithosphere underneath the Apennines belt. Along the fault, in the uppermost crust, the S wave velocity structure is laterally heterogeneous. Right above the hypocenter, we find a 4-6 km thick, very high S wave velocity body (V s as high as 4.2 km/s) that is absent in the SE portion of the fault, where the earthquake propagated. The high-V s body is coincident with the area of fewer aftershocks and is anticorrelated with the maximum slip patches of the earthquake, as modeled by differential interferometric synthetic aperture radar (DInSAR) and strong motion data. We interpret this high-V s body as a high-strength barrier responsible for the high peak ground motion in the near field, observed in the L'Aquila city and surroundings, and for the complexity in the rupture evolution. The retrieved seismic S wave velocity of this body far exceeds common V s values in the upper crust and it is more compatible with values observed in mafic basement rocks.
Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zon... more Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zone are imaged using a new method for the analysis of teleseismic receiver functions (RFs). More than 13,000 P-wave coda of teleseismic records from the 2003-2007 Retreating-Trench, Extension, and Accretion Tectonics (RETREAT) experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of Mediterranean evolution. A new methodology for the analysis of receiver functions is developed, which combines both migration and harmonic decomposition of the receiver function data set. The migration technique follows a classical "Common Conversion Point" scheme and helps to focus on a crucial depth range (20-70 km) where the mantle wedge develops. Harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of P-to-S converted phases. The separation of the back-azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures. The application of our method to the RETREAT data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the Northern Apennines chain, where previous studies failed to clearly retrieve the geometry of the subducted interfaces. We present our results in closely spaced profiles across and along the Northern Apennines chain to highlight the convergence of the Tyrrhenian and the Adriatic microplates which differ in their crustal structure where the Adriatic microplate subducts beneath Tuscany and the Tyrrhenian sea. A signature of the dipping Adriatic Moho is clearly observed beneath the Tyrrhenian Moho in a large portion of the forearc region. In the area where the two Mohos overlap, our new analysis reveals the presence of an anisotropic body above the subducted Moho. There is a strong Ps converted phase with anisotropic characteristics from the top of the Adriatic plate to a depth of at least 80 km. Because the Ps conversion occurs much deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic crust seems unlikely as a causative factor. Rather, the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting.
The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work s... more The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearan...
As one of the rare observational tools for studying deformation and stress within the Earth, seis... more As one of the rare observational tools for studying deformation and stress within the Earth, seismic anisotropy has been one of the focuses of geophysical studies over the last decade. In order to unravel the anisotropic properties of the crust, the teleseismic receiver functions (RF) methodology has started to be widely applied recently. Such effects of anisotropy on RF were illustrated in theoretical studies, showing the strong backazimuthal dependence of RF on the 3D characteristics of the media sampled by the waves. The use of teleseismic RF has the advantage of not being affected by a heterogeneous depth distribution of local earthquakes, since teleseismic rays sample the entire crust beneath the stations. The application of this technique however, needs to be critically assessed using a suitable field test.
Low values of S-wave velocity characterize the shallow layers of the Earth’s crust. This is parti... more Low values of S-wave velocity characterize the shallow layers of the Earth’s crust. This is particularly the case in basins filled with unconsolidated sediments of recent deposition. Such low-velocity materials trap seismic energy, and can lead to large ground accelerations. Therefore precise knowledge of those low shear-wave velocities in sedimentary basins is important for better understanding seismic hazard, e.g. for modeling seismic wave propagation and reconstruction of shake maps.
In this work we present the application of the global-phase seismic interferometry (GloPSI) techn... more In this work we present the application of the global-phase seismic interferometry (GloPSI) technique to a dataset recorded across the Eastern Alps with the EASI (Eastern Alpine Seismic Investigation) temporary seismic network. GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium and is used here to retrieve information about the crust-mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions where it shows a nearly planar behaviour and corresponds to a strong velocity contrast (i.e. in the northern part of the profile, from the Bohemian Massif to the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence of the boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards and confirm the observations of previous studies suggesting a structurally complex and faulted internal Alpine crustal structure.
We use a novel technique named global-phase seismic interferometry (GloPSI) to image the lithosph... more We use a novel technique named global-phase seismic interferometry (GloPSI) to image the lithospheric structure, and in particular the Moho, below two parallel north-south transects belonging to the GANSSER network (2013–2014). The profiles cross the Himalayan orogenic wedge in Bhutan, a tectonically important area within the largest continent-continent collision zone on Earth that is still undergoing crustal thickening and represents a challenging imaging target for the GloPSI approach. GloPSI makes use of direct waves from distant earthquakes and receiver-side reverberations with near vertical incidence. Reflections are isolated from earthquake recordings by solving a correlation integral and are turned into a reflectivity image of the lithosphere below the arrays. Our results compare favorably with first-order features observed from a previous receiver function (RF) study. We show that a combined interpretation of GloPSI and RF results allows for a more in-depth understanding of ...
SUMMARYTo constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wa... more SUMMARYTo constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. Four years of teleseismic earthquake data were processed, from 723 temporary and permanent broad-band stations of the AlpArray deployment including ocean-bottom seismometers, providing a spatial coverage that is unprecedented. The technique is applied automatically (without human intervention), and it thus provides a reproducible image of anisotropic structure in and around the Alpine region. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the Central Alps. The spatial variation of splitting delay times is particularly interesting though. On one hand, there is a clear posit...
In this work we present the application of the Global-Phase Seismic Interferometry (GloPSI) techn... more In this work we present the application of the Global-Phase Seismic Interferometry (GloPSI) technique to a data-set recorded across the Eastern Alps with the EASI temporary seismic network (Eastern Alpine Seismic Investigation). GloPSI aims at rendering an image of the lithosphere from the waves that travel across the core before reaching the seismic stations (i.e. PKP, PKiKP, PKIKP). The technique is based on the principle that a stack of autocorrelations of transmission responses mimics the reflection response of a medium, and is used here to retrieve information about the crust-mantle boundary, such as its depth and topography. We produce images of the upper lithosphere using 64 teleseismic events. We notice that with GloPSI, we can well image the topography of the Moho in regions where it shows a nearly planar behaviour (i.e. in the northern part of the profile, from the Bohemian massif to beneath the Northern Calcareous Alps). Below the higher crests of the Alpine chain, and the Tauern Window in particular, we cannot find evidence for a typical boundary between crust and mantle. The GloPSI results indicate the absence of an Adriatic crust made of laterally continuous layers smoothly descending southwards. On the contrary, our results confirm the observations of previous studies suggesting a structurally complex Moho topography and faulted internal Alpine crustal structure. 1 Introduction As part of the Alpine-Himalayan orogen, the European Alps are the result of the subduction of the Alpine Tethys and European paleomargin beneath the Adriatic microplate and the subsequent continent-continent collision that led to a 200km wide convergence zone with a significant crustal root (e.g. Handy et al., 2015, and references therein). Due to the distinctively different chemistry between the crust and mantle leading to very different physical properties of the crust with respect to the uppermost mantle, the base of the crust denotes a first-order velocity interface and is the seismically best visible subsurface structure in the Earth. First noted by A. Mohorovicic 1910, this velocity interface-called Moho in his honor-,in seismic records from intra-crustal earthquakes produces a specific set of waves that today are well known in seismology as the direct wave Pg, the wide-angle reflection from the Moho (PmP) and the critically refracted wave Pn travelling in the uppermost mantle along the Moho (e.g. Giese et al., 1976). These three specific seismic phases are well visible also in record sections from blasts and thus 1
The focus of this study is the mantle structure beneath the Apennines, and aims to understanding ... more The focus of this study is the mantle structure beneath the Apennines, and aims to understanding how deep processes are connected to shallow deformations. We present new observations from a rich receiver function data set from stations located along the North and Central Apennine chain, and use it for comparison and to strengthen the observations of previous seismic tomography images. The two methodologies define a low shear wave velocity zone (decrease of Vs in the order of 5%) and an increase of Vp/Vs (about 3%) in the shallow mantle between 50 and 90 km depth beneath the orogenic belt. The low Vs melt zone is not restricted to the mantle beneath the Quaternary volcanic areas, as previously thought, but is detected under the whole central Apennines suggesting future broad effects on a large scale. Our interpretation of the teleseismic RFs and tomography, reveals consistently a diffuse mantle upwelling beneath the Apennines, and we hypothesize that slab-derived fluids might interact with the sub-lithospheric mantle generating melts that accumulate at the top of the mantle feeding post-collisional extension. This mechanism can be potentially applied to other cases of extension that spread over wide continental regions.
The upper crust of the KTB (Kontinentales Tiefbohrprogramm) area in the Southeastern Germany is a... more The upper crust of the KTB (Kontinentales Tiefbohrprogramm) area in the Southeastern Germany is a focal point for the Earth Science community due to the huge amount of information collected throughout the last 30 yr. In this study, we explore the crustal structure of the KTB area through the application of the Receiver Function (RF) technique to a new data set recorded by nine temporary seismic stations and one permanent station. We aim to unravel the isotropic structure and compare our results with previous information from the reflection profiles collected during the initial site investigations. Due to the large amount of information collected by previous studies, in terms of P-wave velocity, depth and location of major reflectors, depth reconstruction of major faults zones, this area represents a unique occasion to test the resolution capability of a passive seismological study performed by the application of the RF. We aim to verify which contribution could be given by the application of the RF technique, for future studies, in order to get clear images of the deep structure and up to which resolution. The RF technique has apparently not been applied in the area before, yet it may give useful additional insight in subsurface structure, particularly at depths larger than the maximum depth reached by drilling, but also on structures in the upper crust, around the area that has been studied in detail previously. In our results v S-depth profiles for stations located on the same geological units display common features and show shallow S-wave velocities typical of the outcropping geological units (i.e. sedimentary basin, granites and metamorphic rocks). At around 10 km depth, we observe a strong velocity increase beneath all stations. For the stations located in the centre of the area, this variation is weaker, which we assume to be the signature of the main tectonic suture in the area (i.e. the Saxothuringian-Moldanubian suture), along a west-to-east extended region, may be due to the presence of the allochthonous klippe trapped between the main crustal terrains that came in touch during the Variscan orogeny. In the lower crust we see only small variations throughout the area, at the resolution that is possible with a small temporary experiment with just 10 stations.
Seismic anisotropy is a unique observational tool for remotely studying deformation and stress wi... more Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the threedimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf-Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0-4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of micarich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.
The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work s... more The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/ or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems.
• The seismic structure of the crust across the deformation zone associated with the tear in the ... more • The seismic structure of the crust across the deformation zone associated with the tear in the Ionian slab is investigated, between Southern Calabria and Sicily (Italy) • The isotropic S-wave velocities profiles show the clear differences at the boundaries of the deformation zone, pointing out the different origin for the crust in the two regions • Anisotropy at mid-crustal depth develops both in Sicily and in Southern Calabria, with identical geometrical parameters, and shows connections with the deformation due to the differential movement of the retreating slab
ABSTRACT The crustal velocity structure in a region of central Apennines of Italy at the hinge be... more ABSTRACT The crustal velocity structure in a region of central Apennines of Italy at the hinge between the highly stretched portion of the Monte Argentario promontory and the magmatic province of the Tolfa Domes Complex (Northern Latium) is discussed in this study. S-wave velocities at depth have been constrained by the modeling of P-wave Receiver Functions (RF) from both temporary and permanent broadband seismic stations. The computer 3D Vs models show a thin crust (19–25 km) made of a shallow and thin sedimentary cover, a very high velocity and anisotropic layer related to a metamorphic basement, and a low Vs anisotropic layer in the middle-lower crust above a shallow Moho discontinuity modeled at about 20 km depth. The volcano-tectonic evolution of this portion of Tyrrhenian back-arc margin has been strongly influenced by its peculiar crustal architecture. The low-Vs layer acted as a shear zone in the middle-lower crust during the Tyrrhenian extension, also helping the development of Plio-Quaternary magmatism. Our findings potentially give new constraints on the evolution of the area and to the general comprehension of back-arc development in collisional regions.
Receiver functions (RFs) analyzed at two permanent broadband seismic stations operating in the ep... more Receiver functions (RFs) analyzed at two permanent broadband seismic stations operating in the epicentral area of the M w 6.3, 2009 L'Aquila earthquake (central Italy) yield insight on crustal structure along the fault rupture. The harmonic decomposition of RFs highlights a subsurface structure in which both isotropic and anisotropic features are present. We model the waveforms using recently developed Monte Carlo methods. The retrieved models display a common depth structure, between 10 and 40 km depth, consistent with the under-thrusting of the Adria lithosphere underneath the Apennines belt. Along the fault, in the uppermost crust, the S wave velocity structure is laterally heterogeneous. Right above the hypocenter, we find a 4-6 km thick, very high S wave velocity body (V s as high as 4.2 km/s) that is absent in the SE portion of the fault, where the earthquake propagated. The high-V s body is coincident with the area of fewer aftershocks and is anticorrelated with the maximum slip patches of the earthquake, as modeled by differential interferometric synthetic aperture radar (DInSAR) and strong motion data. We interpret this high-V s body as a high-strength barrier responsible for the high peak ground motion in the near field, observed in the L'Aquila city and surroundings, and for the complexity in the rupture evolution. The retrieved seismic S wave velocity of this body far exceeds common V s values in the upper crust and it is more compatible with values observed in mafic basement rocks.
Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zon... more Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zone are imaged using a new method for the analysis of teleseismic receiver functions (RFs). More than 13,000 P-wave coda of teleseismic records from the 2003-2007 Retreating-Trench, Extension, and Accretion Tectonics (RETREAT) experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of Mediterranean evolution. A new methodology for the analysis of receiver functions is developed, which combines both migration and harmonic decomposition of the receiver function data set. The migration technique follows a classical "Common Conversion Point" scheme and helps to focus on a crucial depth range (20-70 km) where the mantle wedge develops. Harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of P-to-S converted phases. The separation of the back-azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures. The application of our method to the RETREAT data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the Northern Apennines chain, where previous studies failed to clearly retrieve the geometry of the subducted interfaces. We present our results in closely spaced profiles across and along the Northern Apennines chain to highlight the convergence of the Tyrrhenian and the Adriatic microplates which differ in their crustal structure where the Adriatic microplate subducts beneath Tuscany and the Tyrrhenian sea. A signature of the dipping Adriatic Moho is clearly observed beneath the Tyrrhenian Moho in a large portion of the forearc region. In the area where the two Mohos overlap, our new analysis reveals the presence of an anisotropic body above the subducted Moho. There is a strong Ps converted phase with anisotropic characteristics from the top of the Adriatic plate to a depth of at least 80 km. Because the Ps conversion occurs much deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic crust seems unlikely as a causative factor. Rather, the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting.
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Papers by Irene Bianchi