Publication Date:
2017-04-04
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
Keywords:
tomography
;
surface waves
;
inverse problem
;
04. Solid Earth::04.06. Seismology::04.06.07. Tomography and anisotropy
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
thesis
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