Geomorphoogical evidences of Asti - Alessandria earthquakes 2000 - 2003

1. THE SEISMICITY OF THE ASTI - ALESSANDRIATERRITORY:LONG-TERM GEOLOGICAL AND TO
VE
GEOMORPHOLOGICAL EVIDENCES, PRESENT-DAY SUPERFICIAL EFFECTS AND DAMAGES WESTERN ALPS
ROME
M. Giardino(1), G. Lampone(1), M. Lanfranco(2)
(1)
Dipartimento Scienze della Terra - Università degli Studi di Torino - Italy ITALY
(2)
Regione Piemonte - Direzione Opere Pubbliche
The August 21st, 2000 earthquake is the major seismic event that struck the southern-central part of Pie-
monte (North Italy) during recent times. The most affected area is located between Asti and Alessandria.
The present study is focused on the earthquake effects of the possible regional context of local seismicity.
From a geomorphological point of view, the study area includes the smooth hills south of the Tanaro River,
mainly engraved by the Belbo and Tiglione streams. Geologically, it is set in the sedimentary Piemontese Ter-
tiary Basin; particularly it deals with transgressive terrigenous deposits of the Astian marine formation.and
Quaternary alluvial deposits terraced units.
The 2000 seismic sequence has been particularly resentful, for its intensity, reaching magnitude 4.9, as well as
for the persistence of the seismic activity, initiated suddenly August 20th 2000 and extended continuously until
November with some scarce events during 2001. The possible seismogenetic sources are two: 1) a pure strike-
slip high angle seism-genetic source orientated N-NW/S-SE; 2) a release mechanism with inclination of around
45° directed E-NE/W-SW.
The earthquake of August 21st 2000,has also been studied under by means of macroseimic questionnaires
(over 500) distributed, analysed and filed at the University of Turin. This information has also been integrated by
damage analysis reports by the local Civil Protection service. Similar dat have been also produced for minor
sesmic events of 2003.
Despite of the low magnitude, the major event caused a considerable damages to the buildings. Other consi-
derations: 1) the macroseismic epicentre coincides with the instrumental one, that is, it is located between the
communes of Incisa Scapaccino, Bergamasco, Bruno and Castelnuovo Belbo; 2) the isoseimal paths, specially
the greater ones, have a NNW-SSE preferential extension direction.
The geological-geomorphological mapping of the area, the study of the superficial formations
with the help of the geognostic surveys and a morphostructural photo-interpretation of the dia-
gnostic elements allowed to recognize the actual and past seismic surface effects. These data
show the substantial simplicity in the geometrical distribution of the geological bodies in the
area. Nevertheless some structural discontinuities orientated sub-vertically NNW-SSE have been
noticed. Also some geomorphological aspects of the hydrographic network along the Asti syn-
cline are remarkable:
- the western sector introduces geological and geomorphological evidences of a tilting from
South to North;
- on the other hand, the eastern sector is underlined by a differential lifting from North to South.
The two sectors seem to be separated, in correspondence of Incisa Scapaccino ridge, by a NNW -
SSE buried structure. Even though there are not clear geomorphological and structural evidences,
it could hypothetically be geometrically correlated with the structure that appears on the surface
near Castle of Annone (Terzano, 1984).
The reconstruction of a seismotectonic model in an area like the present one is not immediate. The presence
of a deep structure or a deformation zone transversal to the Asti syncline appears evident. The extraordinary
coincidence among the seismic, macro-seismic and geomorphological data supposes a NNW-SSE orientation
for this structure, according to which the Rio Freddo Deformation Zone would be the prosecution of the
Villarvernia-Varzi Line, that marks the limit between the alpine crust and the apenninic units, and where was
located the 2003 earthquake. In this case, the examination area would be subjected to a transpressive stress
activity produced by the interference between different thrust structures.