Estimating crustal deformation from geodetic data
Creators: name: Cheloni, Daniele <1979>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: GEO/10 Geofisica della terra solida

description: By the end of the 19th century, geodesy has contributed greatly to the knowledge of regional
tectonics and fault movement through its ability to measure, at sub-centimetre precision, the
relative positions of points on the Earth’s surface. Nowadays the systematic analysis of
geodetic measurements in active deformation regions represents therefore one of the most
important tool in the study of crustal deformation over different temporal scales [e.g., Dixon,
1991]. This dissertation focuses on motion that can be observed geodetically with classical
terrestrial position measurements, particularly triangulation and leveling observations. The
work is divided into two sections: an overview of the principal methods for estimating longterm
accumulation of elastic strain from terrestrial observations, and an overview of the
principal methods for rigorously inverting surface coseismic deformation fields for source
geometry with tests on synthetic deformation data sets and applications in two different
tectonically active regions of the Italian peninsula. For the long-term accumulation of elastic
strain analysis, triangulation data were available from a geodetic network across the Messina
Straits area (southern Italy) for the period 1971 – 2004. From resulting angle changes, the
shear strain rates as well as the orientation of the principal axes of the strain rate tensor were
estimated. The computed average annual shear strain rates for the time period between 1971
and 2004 are γ˙1  = 113.89 ± 54.96 nanostrain/yr and γ˙2 = -23.38 ± 48.71 nanostrain/yr, with
the orientation of the most extensional strain (θ) at N140.80° ± 19.55°E. These results
suggests that the first-order strain field of the area is dominated by extension in the direction
perpendicular to the trend of the Straits, sustaining the hypothesis that the Messina Straits
could represents an area of active concentrated deformation. The orientation of θ agree well
with GPS deformation estimates, calculated over shorter time interval, and is consistent with
previous preliminary GPS estimates [D’Agostino and Selvaggi, 2004; Serpelloni et al., 2005]
and is also similar to the direction of the 1908 (MW 7.1) earthquake slip vector [e.g., Boschi et
al., 1989; Valensise and Pantosti, 1992; Pino et al., 2000; Amoruso et al., 2002]. Thus, the
measured strain rate can be attributed to an active extension across the Messina Straits,
corresponding to a relative extension rate ranges between &lt; 1mm/yr and up to ~ 2 mm/yr,
within the portion of the Straits covered by the triangulation network. These results are
consistent with the hypothesis that the Messina Straits is an important active geological
boundary between the Sicilian and the Calabrian domains and support previous preliminary
GPS-based estimates of strain rates across the Straits, which show that the active deformation
is distributed along a greater area. Finally, the preliminary dislocation modelling has shown
that, although the current geodetic measurements do not resolve the geometry of the
dislocation models, they solve well the rate of interseismic strain accumulation across the
Messina Straits and give useful information about the locking the depth of the shear zone.
Geodetic data, triangulation and leveling measurements of the 1976 Friuli (NE Italy)
earthquake, were available for the inversion of coseismic source parameters. From observed
angle and elevation changes, the source parameters of the seismic sequence were estimated in
a join inversion using an algorithm called “simulated annealing”. The computed optimal
uniform–slip elastic dislocation model consists of a 30° north-dipping shallow (depth 1.30 ±
0.75 km) fault plane with azimuth of 273° and accommodating reverse dextral slip of about
1.8 m. The hypocentral location and inferred fault plane of the main event are then consistent
with the activation of Periadriatic overthrusts or other related thrust faults as the Gemona-
Kobarid thrust. Then, the geodetic data set exclude the source solution of Aoudia et al. [2000],
Peruzza et al. [2002] and Poli et al. [2002] that considers the Susans-Tricesimo thrust as the
May 6 event. The best-fit source model is then more consistent with the solution of Pondrelli
et al. [2001], which proposed the activation of other thrusts located more to the North of the
Susans-Tricesimo thrust, probably on Periadriatic related thrust faults. The main
characteristics of the leveling and triangulation data are then fit by the optimal single fault
model, that is, these results are consistent with a first-order rupture process characterized by a
progressive rupture of a single fault system. A single uniform-slip fault model seems to not
reproduce some minor complexities of the observations, and some residual signals that are not
modelled by the optimal single-fault plane solution, were observed. In fact, the single fault
plane model does not reproduce some minor features of the leveling deformation field along
the route 36 south of the main uplift peak, that is, a second fault seems to be necessary to
reproduce these residual signals. By assuming movements along some mapped thrust located
southward of the inferred optimal single-plane solution, the residual signal has been
successfully modelled. In summary, the inversion results presented in this Thesis, are
consistent with the activation of some Periadriatic related thrust for the main events of the
sequence, and with a minor importance of the southward thrust systems of the middle
Tagliamento plain.