Page 261 - Numerical Analysis and Modelling in Geomechanics

242 SEISMIC MICROZONING USING NUMERICAL MODELLING
of several interconnected faults rather than a single segment of a main seismogenic
structure.
The epicentral distribution of the first two sequences shows a NW-SE trend
for a total length of about 30 km. Fault plane solutions, computed by the
Centroid Moment Tensor (CMT) method, indicate a dip-slip mechanism along a
primary NW-SE plane, with a T-axis oriented NE-SW. The depth of the foci
shows a concentration between 4 and 8 km, increasing westward.
These directions are in good agreement with the structural framework of the
area, represented by a conjugate system of normal faults oriented along the axis
of the Apennines. The repeated earthquakes gave cumulative effects: the final
estimated maximum intensity was as high as IX–X on the MCS scale. This
peculiar seismic sequence caused the collapse of several buildings and severe
damage to many of them, also because of the high vulnerability of old masonry
buildings. Despite the amount of damage, fortunately only 11 people died, 126
were injured, but the homeless numbered more than 25,000. The estimated
monetary losses were more than 2 billion dollars.

Geologic and geomorphologic overview and sample area
selection
The investigated area is located in the central Apennines, across the Umbria-
Marche regional boundary. The Umbria-Marche sedimentary sequence,
composed of limestones, marly limestones, marls and flysch sequences,
represents the stratigraphy of the study area.
The central Apennines are made up of several tectonic units formed since the
Oligocene as a result of convergence and collision between the continental
margins of the Corsica-Sardinia block and the Adriatic block (Cello et al., 1995).
The main compressive phase started in the Tortonian and the lack of Pliocene-
Pleistocene marine deposits proves that after the Miocene the area was
definitively uplifted. The compressive structures were dissected by normal faults
during the Quaternary, and, according to the most recent studies (Calamita et al.,
1994; Lavecchia et al., 1994), these are related to the crustal thinning processes
occurring in the Tyrrhenian Tuscan area. The Quaternary normal faults led to the
formation of intra-mountain basins, of which the Colfiorito plain is a clear
example, and the seismicity of the area is mainly related to the activity of these
faults.
The geomorphologic setting is characterized by a general conformity between
structural-lithologic elements and morphologies. High relief zones are found in
correspondence with the calcareous ridges, and hilly and smooth areas
correspond to the flysch deposits in the zone of Nocera Umbra and Camerino.
Even the drainage network is influenced by the structural pattern and the main
drainage lines are located along the trace of the main faults and fractures.
Climatic factors, especially the last glacial and interglacial period, influenced the
landscape evolution and the deposition type. Stratified periglacial slope waste