Shallow and deep deformation in northern Apennines region using seismological data
Creators: name: Salimbeni, Simone <1974>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: GEO/10 Geofisica della terra solida

description: For its particular position and the complex geological history, the Northern Apennines has
been considered as a natural laboratory to apply several kinds of investigations. By the
way, it is complicated to joint all the knowledge about the Northern Apennines in a unique
picture that explains the structural and geological emplacement that produced it. The main
goal of this thesis is to put together all information on the deformation - in the crust and at
depth - of this region and to describe a geodynamical model that takes account of it.
To do so, we have analyzed the pattern of deformation in the crust and in the mantle. In
both cases the deformation has been studied using always information recovered from
earthquakes, although using different techniques. In particular the shallower deformation
has been studied using seismic moment tensors information. For our purpose we used the
methods described in Arvidsson and Ekstrom (1998) that allowing the use in the inversion
of surface waves [and not only of the body waves as the Centroid Moment Tensor
(Dziewonski et al., 1981) one] allow to determine seismic source parameters for
earthquakes with magnitude as small as 4.0. We applied this tool in the Northern
Apennines and through this activity we have built up the Italian CMT dataset (Pondrelli et
al., 2006) and the pattern of seismic deformation using the Kostrov (1974) method on a
regular grid of 0.25 degree cells. We obtained a map of lateral variations of the pattern of
seismic deformation on different layers of depth, taking into account the fact that shallow
earthquakes (within 15 km of depth) in the region occur everywhere while most of events
with a deeper hypocenter (15-40 km) occur only in the outer part of the belt, on the Adriatic
side. For the analysis of the deep deformation, i.e. that occurred in the mantle, we used
the anisotropy information characterizing the structure below the Northern Apennines. The
anisotropy is an earth properties that in the crust is due to the presence of aligned fluid
filled cracks or alternating isotropic layers with different elastic properties while in the
mantle the most important cause of seismic anisotropy is the lattice preferred orientation
(LPO) of the mantle minerals as the olivine. This last is a highly anisotropic mineral and
tends to align its fast crystallographic axes (a-axis) parallel to the astenospheric flow as a
response to finite strain induced by geodynamic processes. The seismic anisotropy pattern
of a region is measured utilizing the shear wave splitting phenomenon (that is the
seismological analogue to optical birefringence). Here, to do so, we apply on teleseismic
earthquakes recorded on stations located in the study region, the Sileny and Plomerova
(1996) approach. The results are analyzed on the basis of their lateral and vertical
variations to better define the earth structure beneath Northern Apennines. We find
different anisotropic domains, a Tuscany and an Adria one, with a pattern of seismic
anisotropy which laterally varies in a similar way respect to the seismic deformation.
Moreover, beneath the Adriatic region the distribution of the splitting parameters is so
complex to request an appropriate analysis. Therefore we applied on our data the code of
Menke and Levin (2003) which allows to look for different models of structures with
multilayer anisotropy. We obtained that the structure beneath the Po Plain is probably
even more complicated than expected. On the basis of the results obtained for this thesis,
added with those from previous works, we suggest that slab roll-back, which created the
Apennines and opened the Tyrrhenian Sea, evolved in the north boundary of Northern
Apennines in a different way from its southern part. In particular, the trench retreat
developed primarily south of our study region, with an eastward roll-back. In the northern
portion of the orogen, after a first stage during which the retreat was perpendicular to the
trench, it became oblique with respect to the structure.