ask 6 - Gubbio - Deliverable D21: Geological model of the Gubbio basin (Italy) for the characterization of local seismic response

Progetto INGV-DPC S3 “Scenari di scuotimento in aree di interesse prioritario e/o strategico” (coord. F.Pacor e M. Mucciarelli)

Within the framework of the project S3 “Ground shaking scenarios for some strategic
areas in Italy-Task6” the town of Gubbio has been selected as a test site to compute ground
motion taking into account finite fault and site effects.
Gubbio is located in central Italy, on the northern slopes of one of the many valleys
characterising the central Appennines. The historical settlement is located on a rocky
slope, while new developments extend on the Quaternary fluvio-lacustrine deposits filling
the basin. The Gubbio basin was chosen as a test site for a number of reasons: its
geomorphological setting is very common in central and southern Italy, the area is
characterised by frequent near events of moderate magnitude (Mmax 6) and large
amplifications were observed there for the Colfiorito seismic sequence of the 1997/98.
In order to understand the mechanisms controlling the 3D seismic response of the basin, a
subsurface model has been constructed.
The reconstruction of the model has been done using data coming from different surveys
(figure A). Several active and passive seismic measurements have been carried out in the
plain, also favoured by the intense seismic activity typical of the area. These investigations
mainly consisted of monitoring activities of 4 temporary transects of seismometric
stations, operating between June 2005 and May 2006 and described in detail in PS3-
Deliverables D22-D23. Two transects have been positioned perpendicularly to the valley
axes (GFZ and Ge2 Transects), one parallel (INGV transect) and one near Gubbio (Ge1
transect). More than 300 local and regional earthquakes have been recorded, with
maximum magnitude equal to 4. A 2D array was run as well from June to September 2006,
to better understand the generation of surface waves. All data are collected in a GIS
(Progetto S3 Deliverable D23)
Parallel to these activities, ambient noise data have been collected in the field for 90 sites,
using the single station technique for the estimation of the fundamental resonance
frequencies. Furthermore, 4 noise measurements with a seismic array have been collected
to build an S-wave velocity profile of soft sediments, considered representative for the
whole plain. The geometry of the basin below 500 m of depth has been investigated
through the acquisition of an active seismic line 4.5 km long, in correspondence of one of
the transects.
This has been used to perform a tomography of arrival times which allowed to recognise
the position and shape of the reflecting horizons. As a final step the first 30 metres of
deposits have been characterised over a 30 km2 area, using both stratigraphic
investigations provided by the Gubbio Municipality and data collected from in-situ
surveys and lab tests.
In this deliverable we describe the input data for the 3D model of the Gubbio basin and we
outline the steps performed to define the geometry, the layers and their seismic properties
on the base of the experimental data collected during the project and of the available
geological information.

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