Converted wave tomography: Developing a new inversion method for 3-D crustal shear wâve velocities, with application to the Central Alps

Passive seismological investigations typically image the Earth’s crust with direct P-waves or ambient noise correlation yielding S-wave information. While the first method requires local earthquakes to achieve high resolution, in the second method the depth penetration strongly depends on the recording network’s aperture. In this thesis I develop a new inversion method and implement the related software in which teleseismic P-to-S converted waves (receiver functions) are exploited to construct a fully 3-D structural and shear-wave velocity model of the crust. This method does not require local earthquakes, nor a large aperture seismic network, but a dense array of 3-component sensors with a station spacing similar to the expected crustal thickness. This new technique is first applied to the Central Alps, a tectonically complex area where imaging in 3-D is of pivotal interest. The new method is composed of the following main elements. (1) An accurate ray prop- agator, which respects Snell’s law in 3-D at any interface geometry, and allows P-to-S con- verted ray-paths to reach the recording station at <0.1 km accuracy. (2) A new model parameterization, with horizontally fixed but vertically flexible-position nodes, and layer- wise two velocity points defined to accommodate mapping both sharp discontinuities and gradients across layers. (3) A stochastic inversion procedure, combining simulated annealing and a pattern search algorithm, to find discontinuity depths and velocities across the crust by fitting grouped converted waves with synthetics. This inversion is performed locally for each point and its neighbours; it covers the entire study area step-wise with an overlap and at least two iterations. The application to the Central Alps uses 20 years of high-quality data from permanent broad-band stations and from the temporary AlpArray Seismic Network. The initial model includes a Moho depth map and a 3-D P wave velocity model derived from past investigations. The 3-D inversion results at 25 km horizontal resolution provide a series of maps and cross- sections. The crustal thickness generally reflects well the roots of the Alpine orogen, and its jump between the European and Adriatic plates, including the Ivrea Geophysical Body. The lower crustal thickness is less well resolved, yet appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen. A low-Vp/Vs area in the European foreland correlates with lower crustal earthquakes, which we interpret as mechanical differences in rock properties, most likely inherited. Our results are generally similar to those found by 3-D ambient noise tomography in the area. The new method inherently performs better at localizing discontinuities, and less well at imaging bulk anomalies. Thanks to sub-vertically propagating rays, our method maps the full crustal structure across the entire area of a seismic network. Future developments can incorporate joint inversions with gravity or other seismological tomography methods. -- La sismologie passive image la croûte terrestre typiquement par des ondes P directes ou des ondes S basées sur la corrélation du bruit ambiant. Si la première méthode requiert des séismes locaux pour une imagerie haute résolution, la deuxième méthode est limitée dans sa pénétration en profondeur par l’ouverture du réseau enregistrant des signaux. Dans cette thèse je développe une nouvelle méthode d’inversion et j’implémente le logiciel correspondant pour exploiter des ondes converties P-en-S (fonctions récepteurs) pour con- struire un modèle 3-D structural et de vitesse d’onde S de la croûte. Cette méthode requiert ni séisme local, ni une grande ouverture du réseau, mais un déploiement dense de capteurs 3-composantes à un espacement comparable à l’épaisseur attendue de la croûte. La pre- mière application de cette nouvelle technique se focalise sur les Alpes Centrales, une région tectonique complexe où l’imagerie 3-D est un but important. La nouvelle méthode se compose des éléments principaux suivants. (1) Un propagateur de rai exact, qui respecte la loi de Snell en 3-D à une géométrie d’interface quelconque, et permet aux rais convertis P-en-S d’arriver à <0.1 km de la station. (2) Un nouveau paramétrage de modèle, avec des nœuds horizontalement fixes mais verticalement flexibles, et deux points de définition des vitesses par couche pour permettre à la fois l’imagerie des discontinuités et celle des gradients dans les couches. (3) Une procédure d’inversion stochastique, combinant recuit simulé et un recherche de motifs, pour trouver la profondeur des discontinuités et des vitesses à travers la croûte en ajustant des synthétiques à des groupes d’ondes converties. Cette inversion est appliquée localement à chaque point et ses voisins, la procédure couvre toute la zone d’étude pas-à-pas avec un recouvrement et au moins deux itérations. L’application aux Alpes Centrales utilise des données de haute qualité enregistrées sur plus de 20 ans par des stations large-bandes permanentes et par le du réseau temporaire du projet AlpArray. Le modèle initial inclut une carte de profondeur du Moho et un modèle 3-D en vitesse d’onde P d’études précédentes. Le résultat de l’inversion 3-D, à une résolution horizontale de 25 km, inclut une série de cartes et de profils. L’épaisseur de la croûte reflète bien la racine de l’orogène alpin, et le saut entre les plaques européenne et adriatique, y compris le corps d’Ivrée. L’épaisseur de la croûte inférieure est moins bien résolue mais paraît relativement constante. Le rapport Vp/Vs moyenne de la croûte est relativement plus élevé sous la chaîne. Une zone de Vp/Vs faible dans l’avant-pays européen coïncide avec des séismes dans la croûte inférieure, ce que nous interprétons comme une différence dans les propriétés mécaniques des roches, probablement héritée. Nos résultats sont généralement similaires à ceux trouvés par tomographie 3-D du bruit ambiant dans la région. La nouvelle méthode est plus performante à localiser des discontinu- ités, et moins bien pour l’imagerie des anomalies volumétriques. Grâce aux rais sub-verticaux, notre méthode image la structure de toute la croûte sous l’ensemble du réseau sismologique. Des développements futurs peuvent inclure des inversions conjointes avec la gravimétrie ou d’autres types de tomographies sismologiques

A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alps

We develop a new inversion approach to construct a 3-D structural and shear-wave velocity model of the crust based on teleseismic P-to-S converted waves. The proposed approach does not require local earthquakes such as body wave tomography, nor a large aperture seismic network such as ambient noise tomography, but a three-component station network with spacing similar to the expected crustal thickness. The main features of the new method are: (1) a novel model parametrization with 3-D mesh nodes that are fixed in the horizontal directions but can flexibly vary vertically; (2) the implementation of both sharp velocity changes across discontinuities and smooth gradients; (3) an accurate ray propagator that respects Snell’s law in 3-D at any interface geometry. Model parameters are inverted using a stochastic method composed of simulated annealing followed by a pattern search algorithm. The first application is carried out over the Central Alps, where long-standing permanent and the temporary AlpArray Seismic Network stations provide an ideal coverage. For this study we invert 4 independent parameters, which are the Moho discontinuity depth, the Conrad discontinuity depth, the P-velocity change at the Conrad and the average Vp/Vs of the crust. The 3-D inversion results clearly image the roots of the Alpine orogen, including the Ivrea Geophysical Body. The lower crust's thickness appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen, and a low-Vp/Vs area in the northern foreland seems to correlate with lower crustal earthquakes, which can be related to mechanical differences in rock properties, probably inherited. Our results are in agreement with those found by 3-D ambient noise tomography, though our method inherently performs better at localizing discontinuities. Future developments of this technique can incorporate joint inversions, as well as more efficient parameter space exploration

Local earthquake tomography in the junction between south-eastern Alps and External Dinarides using the seismic data of the CE3RN network

Introduction. The main force responsible of tectonic activity in south-eastern Alps is given by the collision between Adriatic microplate and Eurasian plate. This motion has a fundamental role in geodynamic evolution due to the convergence rate between Adria and Eurasian plate estimated greater than 2 mm/yr (Platt et al., 1989). The largest historical earthquakes occurred in the region between Italy and Slovenia are reported in Burrato et al. (2008) and Galadini et al. (2005). Several large earthquakes hit the studied region in historical times, most of them were distributed in the Friuli Venezia Giulia region, as two events recorded in Tramonti in 1776 and 1794, but the most important earthquake that spread its effects in wide region was the 1511 Idrija event that represents the most destructive event that occurred so far in the region of the junction between south-eastern Alps and the External Dinarides, as reported in the catologues (Ribari\u10d, 1982). In general, the area of Friuli Venezia Giulia region is characterized by a moderate seismicity, in fact the most important instrumental seismic event recorded in the area was the destructive 1976 Friuli earthquake with Ms=6.5 (Aoudia et al., 2000), that was widely studied thanks to the large amount of collected seismometric data. In Veneto region, seismicity normally decreases and the only destructive earthquake instrumentally recorded was the Cansiglio event with Mw=5.9 (Sirovich and Pettenati, 2004). Evidences that the collision is still active nowadays and the main seismogenic structures are located in the proximity of the political boundaries between Italy and Slovenia is also shown by the main shock of Bovec in April 1998 with Ms=5.7 (Bajc et al., 2001) and, later, in July 2004, with the event of Kobarid (Ms=4.9), which generated a sequence that lasted until the end of November 2004 (Bressan et al., 2009). It is evident that, in an area with complex seismogenic structures, the use of 1-D velocity model might not be sufficient, so defining a 3-D velocity model represents a better solution to understand the inner structure of the Earth. For this reason, we used the travel-time tomography technique to obtain an accurate 3-D velocity model: in this work, we used a Cat 3-D software to perform the travel time tomography and a non-linear location tool to define the position of the events