Evidence of active tectonic shortening in the Lake Garda area (Northern Italy) has been very well described in the geological literature for at least 40 years, mainly due to the extensive research conducted during the siting of nuclear power plants in Lombardia in the 80’s. This area includes in fact the causative faults for some of the largest seismic events ever occurred in Northern Italy, i.e. the two medieval earthquakes of January 3rd, 1117 (Verona, Io = IX-X MCS) and the 25th of December, 1222 (Brescia, Io = IX-X MCS). More recently, two “twin” moderate events occurred at Salò on October 30th, 1901 (Io = VIII MCS) and November 24th, 2004
(Ml =5.4). This is therefore a critical area for understanding the relationship between tectonic structures and earthquake source parameters in the Po Plain. We address this issue through new field data and airphoto interpretation, morphobathymetric investigations in South Lake Garda, and a systematic revision of published data. A new map of Quaternary faults is presented, that shows a good correlation between Quaternary deformations, major regional tectonic structures, and macroseismic information derived from local strong seismic events. Style and rates of activity of the proposed capable faults allow us to infer the typical seismogenic features of the characteristic seismic event for the Lake Garda region, which can be regarded as the controlling earthquake for the seismic hazard of the central part of the Po Plain.
seismic hazard
ITHACA PROJECT AND CAPABLE FAULTS IN THE PO PLAIN (NORTHERN ITALY)
The use of the term “capable fault” instead of the more generic “active fault”, is to be preferred when dealing with faults, that, more than being simply active, have the capability to cut or deform the ground surface, generally in association, but not exclusively, with seismic events. Such faults are the most relevant from the societal hazard viewpoint, for the damage they can cause directly or indirectly (e.g., interaction between capable fault and high risk facilities) by offsetting the ground and by shaking (surface ruptures commonly accompany earthquakes above magnitude 6).
Recognition of presence of capable faults and assessment of their potential for surface rupturing is fundamental in the siting process of highly hazardous and strategic structures and, more generally, in land use planning and management of infrastructures, especially during emergencies.
So, being the Italian territory crossed by a large number of potentially capable faults, more than a decade ago the ITHACA (ITaly HAzard from CApable faults) project was initiated, to map and describe all faults recognized or suspected of being capable. The database, that contains more than 1500 faults (with a wide range of variability of the level of reliability of the input data), is managed by the Geological Survey of Italy, department of ISPRA.
In the Po Plain territory, ITHACA displays many of such structures. Despite the modest expected displacement (tens of centimeters at most) for each event along these faults, the high, and still growing, grade of human occupancy of this land makes the risk associated to seismicity and capable faulting a growing factor of anxiety. To begin a process of risk attenuation, specific norms in urban planning are needed to regulate development along capable faults, with special care when dealing with hazardous plants.
INDUCED SEISMICITY AND RELATED RISK IN ITALY
The seismicity induced by human activities is a subject almost unknown in Italy, with an average of published works on the subject at a fraction of a percentage of what is being published abroad. The first part of this paper discusses the historical reasons for this lack of interest, while the second will examine the implications in terms of seismic risk and possible regulations following the example of other European countries.
Simple Probabilistic Seismic Hazard Assessment
Conventional probabilistic seismic hazard assessments (PSHA) are complex and frequently lead to different results because of the interpretations and approximations used by different experts. Simple PSHA is based on deterministically defined peak ground accelerations (PGA) with additional definition of probabilities of their exceedences. This way there is no duality between deterministic and probabilistic approaches used. The use of the simple PSHA is demonstrated for Abu Dhabi and the results are compared with a conventional PSHA. One comparative case is not sufficient to decide about the correlation between the conventional and the simple method of PSHA. It is obvious, however, that the simple method is more transparent and easy to use then more complex conventional method of PSHA. The simple procedure can be used for response spectra and other ground motion parameters with defined attenuation relationships.
GPS Measurements in Central-Northern Italy for Deterministic Estimate of Seismic Hazard
The recognition of the Italian seismic zones most prone to next strong earthquakes could make eco- nomically feasible a gradual restoration of the building heritage in our country. Recent advancements of research suggest that achieving such information is not anymore impossible. The proposed approach is based on the study of the strain perturbation triggered by strong earthquakes. The hypothesis that such phenomenon can produce sig- nificant increase of earthquake probability in seismic zones undergoing particular conditions is suggested by the peculiar distribution of major historical earthquakes in some periAdriatic zones and by the quantification of post seismic visco-elastic relaxation in that area. The possibility of recognizing the propagation of the above phenomenon and its possible implications on earthquake probability in some zones can considerably be strengthened by the use of geodetic measurements. In this work, we describe the information that has been obtained by the analysis of the geodetic data acquired by a fairly dense network of 202 permanent GPS stations in Central and Northern Italy.
For this paper is available an extended abstract after the text in Italian.