ALBERT

Description: We present a new surface-wave tomograpic study of the broad European and Mediterranean region. Our goal is to enhance the resolution of previously published group velocity models using new data from European permanent seismic networks and a dense broad-band array in Northern Apennines (RETREAT). We measure fundamental mode Rayleigh and Love wave group velocities from long period seismograms recorded at regional distance (between 600 and 7000 km). Our measurement technique is based on iterative application of multiple filters and phase-matched filters; we accurately estimate dispersion curves for more than 1500 Rayleigh wave and about 850 Love wave paths in the period range 35 s - 170 s. Consistency of measurements is evaluated by comparing ray clusters from sample earthquakes to closely spaced RETREAT stations. In the whole data set, measurement errors in group velocity decrease with increasing distance, and show to be caused by inaccuracy in the estimate of group arrival time. We calculate maps of Love and Rayleigh group velocity at selected periods by linear tomographic inversion, accounting for group arrival time errors and evaluating a posteriori group slowness errors. Data coverage in this region is not uniform, and it is highly influenced by the uneven distribution of earthquakes and seismic stations. We therefore build a laterally heterogeneous reference model by inverting a global data set of group velocity derived from the phase velocity library of Ekstrom et al., 1997. Use of this reference as an a priori model during inversion improves preliminary data coverage at the borders of our study region, and warrants consistency with global models. The implications of different regularization constraints (mathematically equivalent to norm damping or smoothing with different criteria) are analyzed and compared. Group velocity maps confirm the large scale geological lineaments known for the region: short periods maps differentiate well among thinner oceanic and thicker continental crust; the most dominant feature in long period maps is the difference between the fast Precambrian East European Platform and the low velocity signature of central Europe and western Mediterranean, separated by a sharp gradient in correspondence of the Tornquist-Tesseyre Zone. The seismically active Tethyan Belt is clearly marked by a continuous slow anomaly. Smaller scale, possibly thermally related, low velocity anomalies are found under Iceland and Mid-Atlantic Ridge, Rhine Graben and Tyrrhenian Back-Arc basin, while the Hellenic Arc is characterized by fast velocity.

Description: SPICE EC FP6 Marie Curie RTN NERIES INFRAST-2.1-026130

Description: Submitted

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Description: We present a new surface-wave tomographic study of the broad European and Mediterranean region. Our goal is to enhance the resolution of previously published group velocity models using new data from European permanent seismic networks and a dense broad-band array in Northern Apennines (RETREAT). We measure fundamental mode Rayleigh and Love wave group velocities from long-period seismograms recorded at regional distance (between 600 and 7000 km). Our measurement technique is based on iterative application of multiple filters and phase-matched filters; we accurately estimate dispersion curves for more than 1500 Rayleigh wave and about 850 Love wave paths in the period range 35–170 s. Consistency of measurements is evaluated by comparing ray clusters from sample earthquakes to closely spaced RETREAT stations. In the whole data set, measurement errors in group velocity decrease with increasing distance and show to be caused by inaccuracy in the estimate of group arrival time. We calculate maps of Love and Rayleigh group velocity at selected periods by linear tomographic inversion, accounting for group arrival time errors and evaluating a posteriori group slowness errors. Data coverage in this region is not uniform, and it is highly influenced by the uneven distribution of earthquakes and seismic stations. We therefore build a laterally heterogeneous reference model by inverting a global data set of group velocity derived from the phase velocity library of Ekström et al. (1997). Use of this reference as an a priori model during inversion improves preliminary data coverage at the borders of our study region and warrants consistency with global models. The implications of different regularization constraints (mathematically equivalent to norm damping or smoothing with different criteria) are analysed and compared. Group velocity maps confirm the large-scale geological lineaments known for the region: short-periods maps differentiate well among thinner oceanic and thicker continental crust; the most dominant feature in long-period maps is the difference between the fast Precambrian East European Platform and the low velocity signature of central Europe and western Mediterranean, separated by a sharp gradient in correspondence of the Tornquist–Tesseyre Zone. The seismically active Tethyan Belt is clearly marked by a continuous slow anomaly. Smaller scale, possibly thermally related, low velocity anomalies are found under Iceland and Mid-Atlantic Ridge, Rhine Graben and Tyrrhenian back-arc basin, whereas the Hellenic Arc is characterized by fast velocity.

Description: NERIES INFRAST-2.1-026130 SPICE EC FP6 Marie Curie RTN

Description: Published

Description: 1050-1066

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Description: Earthquake occurrence stems from a complex interaction of processes that are still partially unknown. This lack of knowledge is revealed by the different statistical distributions that have been so far proposed, and by the different beliefs about the role of some key components as the tectonic setting, fault recurrence, seismic clusters, and fault interaction. Here, we explore these issues through a numerical model based on a realistic interacting fault system. We use an active fault system in Central Italy responsible for moderate to large earthquakes, where geometric and kinematic parameters of each structure can be confidently assessed. Then, we generate synthetic catalogs by modeling different seismogenic processes and allowing co- and post-seismic fault interaction. The comparison of synthetic and real seismic catalogs highlights many interesting features: (i) synthetic seismic catalogs reproduce the short-term clustering and the long-term modulation observed in the historical catalog of the last centuries; (ii) a recurrent model of earthquake occurrence on faults is more effective than a Poisson model to explain such short-term and long-term time features; (iii) a realistic fault pattern is a key component to generate stochasticity in the seismic catalog, preventing a systematic time ”synchronization” of strongly coupled faults; (iv) such a stochasticity may put strong limits to the forecasting capability of models based on fault interaction, even though the latter is a key component of the process. Finally, the model allows explicit predictions on future paleoseismological observations to be made.

Description: In press

Description: 3.1. Fisica dei terremoti

Description: 3.2. Tettonica attiva

Description: JCR Journal

Description: open

Description: Earthquake occurrence stems from a complex interaction of processes that are still partially unknown. This lack of knowledge is revealed by the different statistical distributions that have been so far proposed and by the different beliefs about the role of some key components as the tectonic setting, fault recurrence, seismic clusters, and fault interaction. Here, we explore these issues through a numerical model based on a realistic interacting fault system. We use an active fault system in central Italy responsible for moderate to large earthquakes, where geometric and kinematic parameters of each structure can be confidently assessed. Then, we generate synthetic catalogs by modeling different seismogenic processes and allowing coseismic and postseismic fault interaction. The comparison of synthetic and real seismic catalogs highlights many interesting features: (1) synthetic seismic catalogs reproduce the short-term clustering and the long-term modulation observed in the historical catalog of the last centuries; (2) a recurrent model of earthquake occurrence on faults is more effective than a Poisson model to explain such short-term and long-term time features; (3) a realistic fault pattern is a key component to generate stochasticity in the seismic catalog, preventing a systematic time ‘‘synchronization’’ of strongly coupled faults; (4) such a stochasticity may put strong limits to the forecasting capability of models based on fault interaction, even though the latter is a key component of the process. Finally, the model allows explicit predictions on future paleoseismological observations to be made.

Description: Published

Description: B01307

Description: 3.1. Fisica dei terremoti

Description: 3.2. Tettonica attiva

Description: JCR Journal

Description: reserved

Description: The broad European and Mediterranean region is characterized by an extremely com- plex tectonic setting, driven by the ma jor convergence between Eurasian and African plates. A detailed model of the upper mantle in this region is fundamental to improve our understanding of its geodynamical evolution. Seismic tomography can help to ad- dress this problem modeling seismic speed anomalies, that can be related to different tectonic features, such as continental roots, rifting areas, magmatic provinces, plumes or subducting slabs. Due to high seismicity rates and dense seismograph coverage, this region has been the sub ject of many tomographic studies from regional to local scale. Traveltime high resolution models of P-wave speed anomalies [Spakman et al., 1993; Piromal lo and Morel li , 2003] have illuminated the deep structure of the mantle, but at shallow depth they often suffer from uneven ray coverage, being strongly dependent on station and epicenter distribution. Regional S-wave velocity models have been re- trieved from the analysis of surface wave group or phase velocity [Ritzwol ler and Levshin , 1998; Vil lase˜ nor et al., 2001], from waveform inversion of surface waves [Marquering and Snieder , 1996] or both surface and body waves [Marone et al., 2004]. However, the non-optimal distribution of observatories and seismic sources has affected regional to- mographic models. Global models derived from surface wave data image the large-scale structures of the region, but their resolution is insufficient to describe its complexity [Shapiro and Ritzwol ler , 2002; Boschi and Ekstr¨ om , 2002; Ritsema et al., 1999; Zhou et al., 2006; Trampert and Woodhouse , 1995]. Global models with finer parameteriza- tion on Mediterranean [Boschi et al., 2004] have been proposed and recent modeling of surface waves from ambient noise gave new insights into the shallowest European upper mantle [Yang et al., 2006]. The increased availability of high quality seismic records from permanent observato- ries and from the recent temporary deployment RETREAT in the Northern Apennines gave us the opportunity to exploit new data, that can provide new and finer constraints to the tomographic problem. We present in this thesis a new surface wave tomography study, aimed at exploiting the high sensitivity of these waves to shallow structure and their wide spatial coverage in the complex sources-stations distribution of the European and Mediterranean domain. The inverse problem of obtaining a VS three-dimensional model from analysis of surface wave can be solved in different ways. [Marone et al., 2004] use the partitioned waveform inversion of [Van der Lee and Nolet , 1997], where the 1-D average S-velocity structure along each path is first determined by non-linear waveform fitting, and in a second step the 1-D path averaged structures are combined in a damped least-squares linear inversion for a 3-D S-velocity model. [Shapiro and Ritzwol ler , 2002] in a first step estimate 2-D dispersion maps with a linear tomographic inversion of path average fundamental mode group and phase velocities, and afterwards apply a Monte-Carlo method to perform the non-linear inversion of the dispersion curves at each geographical point and retrieve the 3-D shear-velocity model. [Boschi and Ekstr¨om , 2002] carry out a single non-linear inversion of phase anomaly measurements making use of JWKB ray-theoretical sensitivity kernels computed in a reference 3-D model. [Zhou et al., 2006] invert long period fundamental mode phase delays with finite-frequency 3-D Born approximation kernels, calculated in a reference 1-D model. We will proceed with a 2 steps scheme, first inverting group path averaged speeds for a regionalized group velocity model assuming a linearized ray theoretical wave propagation. In a second step, we will use the group velocity maps as data to perform a non-linear iterative depth inversion for the local 1-D structure, accounting for the lateral variations of the Crust. This thesis presents a new model along with a discussion of the robustness and resolution of its main features. We will firstly present the group velocity measurement technique and an analysis of measurement errors (Chapter 2), then we will introduce the linear inversion of the regional data starting from a reference global model, with an accurate examination of the implication of different regularization constraints (Chapter 3). Group velocity maps will then be shown and discussed. Subsequently we will invert the group velocity for the Vs structure of upper mantle (Chapter 4). Our resulting 3-D radially anisotropic model will be discussed in detail and compared with other published global and regional models.

Description: Universita' di Bologna

Description: Published

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: open

Description: We here exploit fundamental mode Rayleigh and Love seismic wave information and the high resolution satellite global gravity model GGM02C to obtain a 1° × 1° 3-D image of: (a) upper-mantle isotropic shear-wave speeds; (b) densities; and (c) density-vS coupling below the European plate (20°N–90°N) (40°W–70°E). The 3-D image of the density-vS coupling provides unprecedented detail of information on the compositional and thermal contributions to density structures. The accurate and high-resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models. The correlation between residual topography and mantle residual gravity anomalies defines three large-scale regions where upper mantle dynamics produce surface expression: the East European Craton; the eastern side of the Arabian Plate; and the Mediterranean Basin. The effects of mantle convection are also clearly visible at: (1) the Eastern Sirt Embayment; (2) the West African Craton northern margins; (3) the volcanically active region of the Canarian Archipelago; (4) the northern edge of the Central European Volcanic Province; and (5) the Northeastern part of the Atlantic Ocean, between Greenland and Iceland. Strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression. Although both thermal and additional dependencies have been incorporated into the density model, convective down-welling in the mantle below the East European Craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography.

Description: DATEC MERG-CT-2007-046522 and NERIES INFRAST-2.1-026130

Description: Published

Description: B09401

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: restricted

Description: We present a new 3-D transversely isotropic shear wave velocity model of the European and Mediter ranean upper mantle obtained by analysis of surface waves. Data used are fundamental- mode Rayleigh and Love group velocity measurements in the period range 35–170 s, taken on seismograms recorded by European stations for regional earthquakes. The tomographic inversion to map the 3-D earth structure is split into two steps. First, we regionalize the group velocity dispersion measurements, obtaining distinct geographical group velocity maps at different periods; then, each local dispersion curve is inverted separately to find the shear wave velocity structure at depth. The inversion benefits from using a priori information from a 3-D global mantle model (S20RTS) and a new detailed European crustal model (EPcrust) to constrain the shallower layers. The inversion scheme follows a non-linear iterative algorithm by which Rayleigh and Love group slowness are inverted simultaneously for the best-fitting isotropic Voigt shear wave speed and radial anisotropy parameter. Final merging of the vS profiles results in a new higher resolution 3-D model of European upper mantle. We find that Western Europe and Mediter ranean Sea are mainly characterized by relatively low velocities, strongly contrasting with the fast roots of the Eastern European Craton. Many regional scale structures are also evident in the model, thus providing insights into the complex geodynamic framework of the European continent. Most prominent are the low-velocity West Mediter ranean spreading basins and European Cenozoic rift system, and seismically fast features connected to subduction of Adria microplate, Hellenic Arc and Calabrian Arc. Radial anisotropy does not vary very significantly with respect to the PREM profile, as available data only resolve lateral variations to a limited degree due to trade-off with velocity. EPmantle has the potential to provide a reliable seismological reference for the upper-mantle structure in the broad European region.

Description: Published

Description: 469-484

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Description: We collect and compare available three-dimensional seismological models of the earth’s upper mantle beneath the broad European and Mediterranean region, to quan- tify how well they agree. The zone we considered covers the territory from the Mid- Atlantic Ridge to the Urals, and from North Africa to the North Pole, covering an area corresponding to about one-sixth of the earth’s surface. Most of the models ac- tually cover the whole globe, but we restrict the analysis to our study sector. Avail- able tomographic P- and S-wave speed models have been computed fitting different data-sets and following a variety of inversion techniques and strategies, and may bear consequences or bias connected to the specific data-set used, or the choice made by the author. An extensive comparative investigation may thus contribute to clarify our knowledge of the deep earth structure beneath continents. The visual, qualitative level of agreement is usually rather good, particularly for the larger-scale features, such as the signatures of the East European and West African Cratons, the Mid-Atlantic Ridge, the Red Sea Rift system, the Alpine-Hymalayan belt. These traits can be identified in all models. However, quantitative comparisons do not always show high consistency among models. Model amplitudes vary considerably, and correlation analysis is not always satisfactory. We also test the ability of different models to fit group and/or phase velocity measurements, that were not used for their derivation. The test of com- patibility among different models and data-sets is a necessary preliminary step for the creation of a seismological reference earth model.

Description: Published

Description: Vienna-Austria

Description: open

Description: We present a new 3-D transversely isotropic shear wave velocity model of the European and Mediterranean upper mantle obtained by analysis of surface waves. Data used are fundamental-mode Rayleigh and Love group velocity measurements in the period range 35–170 s, taken on seismograms recorded by European stations for regional earthquakes. The tomographic inversion to map the 3-D earth structure is split into two steps. First, we regionalize the group velocity dispersion measurements, obtaining distinct geographical group velocity maps at different periods; then, each local dispersion curve is inverted separately to find the shear wave velocity structure at depth. The inversion benefits from using a priori information from a 3-D global mantle model (S20RTS) and a new detailed European crustal model (EPcrust) to constrain the shallower layers. The inversion scheme follows a non-linear iterative algorithm by which Rayleigh and Love group slowness are inverted simultaneously for the best-fitting isotropic Voigt shear wave speed and radial anisotropy parameter (vSH−vSV). Final merging of the vS profiles results in a new higher resolution 3-D model of European upper mantle. We find that Western Europe and Mediterranean Sea are mainly characterized by relatively low velocities, strongly contrasting with the fast roots of the Eastern European Craton. Many regional scale structures are also evident in the model, thus providing insights into the complex geodynamic framework of the European continent. Most prominent are the low-velocity West Mediterranean spreading basins and European Cenozoic rift system, and seismically fast features connected to subduction of Adria microplate, Hellenic Arc and Calabrian Arc. Radial anisotropy does not vary very significantly with respect to the PREM profile, as available data only resolve lateral variations to a limited degree due to trade-off with velocity. EPmantle has the potential to provide a reliable seismological reference for the upper-mantle structure in the broad European region.