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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS004 Oral Presentation 1901
Numerical modelling of strain rates in Italy
Dr. Salvatore Barba
Seismology and Tectonophisics Istituto Nazionale di Geofisica e Vulcanologia IASPEI
Carafa Michele Mc
Italy is earthquake country, but investigating active faults is difficult as many if not most of them are
hidden or positively blind. Geological strain rates in can be confidently assessed only in very few
seismogenic areas, and seismic hazard studies generally rely only on earthquake rates from catalogues.
GPS networks are still rather sparse and their resolving power is limited by the peculiar configuration of
the country. Our work attempts to bridge this knowledge gap by constructing a geodynamic model that
may be used to assess strain-rates in most of known fault zones and possibly to identify as yet unknown
seismogenic areas. We computed the strain-rate in integrating several datasets: seismic events, active
faults, GPS measurements, stress and strain indicators, and tectonic regime. Different ways to assess
strain rates have obvious advantages and disadvantages, due to the different spatial coverage of data,
different temporal scale, and probably different physical significance with respect to the future strain
rates. Using different classes of observations, we increase the spatial coverage of the separate datasets
and show the inconsistencies where the datasets overlap. Although we do not face how the different
observations relate to the future strain rate, being the physical relationships not yet clear, the
integration guarantees a more robust result than the datasets taken separately. By means of a trial-anderror
procedure, we build a series of 3D dynamical models of and surrounding regions, compute the
velocities, stress, and strain in the thin-shell approximation by using the finite element code SHELLS
(Bird, 1999, Computers & Geosciences, 25(4), 383-394), and compare the model predictions with the
available observations. We test many free parameters in acceptable ranges, including boundary
conditions, fault friction, rheology, etc., and study the models or family of models whose predictions
deviate less from the data. We build the structure of the models based on the most recent information
on the crustal structure, faults, and rheology. The range of boundary conditions and the ideas of basal
shear tractions are adopted from the literature. To compare the predictions with the observations, we
use regional published datasets and compute the deviation of each model with each dataset. Last, we
combine the single deviations assigning a weight to each dataset based on a possible estimate of the
signal that cannot be modelled by our approach. We find that our model predictions satisfactorily agree
with data (we do not reproduce 17-33% of tectonic regime observations, the predicted azimuth deviates
of 26-30 degrees, and the RMS of velocities is 1.2-1.4 mm/y). In the most active areas, we find a strainrate
of 10-16-10-14 s-1, compatible with the estimates of other authors but with somewhat a different
pattern that accounts for the different datasets. Most importantly, we show that using only one class of
observations leads to very different results depending on the choice of the data, whereas our results
appear to be more robust.
Keywords: strain rates, italy, numerical models