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| Titel |
Latest Pleistocene to Holocene thrust faulting paleoearthquakes at Monte Netto (Brescia, Italy): lessons learned from the Middle Ages seismic events in the Po Plain |
| VerfasserIn |
Alessandro Maria Michetti, Andrea Berlusconi, Franz Livio, Giancanio Sileo, Andrea Zerboni, Leonello Serva, Eutizio Vittori, Helena Rodnight, Christoph Spötl |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043079
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| Zusammenfassung |
| The seismicity of the Po Plain in Northern Italy is characterized by two strong Middle Ages
earthquakes, the 1117, IË X MCS Verona, and the December 25, 1222, IË IX-X
Brescia, events. Historical reports from these events describe relevant coseismic
environmental effects, such as drainage changes, ground rupture and landslides.
Due to the difficult interpretation of intensity data from such old seismic events,
considerable uncertainty exists about their source parameters, and therefore about their
causative tectonic structures. In a recent review, Stucchi et al. (2008) concluded
that “the historical data do not significantly help to constrain the assessment of
the seismogenic potential of the area, which remains one of the most unknown,
although potentially dangerous, seismic areas of the Italian region”. This issue needs
therefore to be addressed by using the archaeological and geological evidence of past
earthquakes, that is, archeoseismology and paleoseismology. Earthquake damage to
archaeological sites in the study area has been the subject of several recent papers.
Here we focus on new paleoseismological evidence, and in particular on the first
observation of Holocene paleoseismic surface faulting in the Po Plain identified at the
Monte Netto site, located ca. 10 km S of Brescia, in the area where the highest
damage from the Christmas 1222 earthquake have been recorded. Monte Netto is a
small hill, ca. 30 m higher than the surrounding piedmont plain, which represent the
top of a growing fault-related fold belonging to the Quaternary frontal sector of
the Southern Alps; the causative deep structure is a N-verging back thrust, well
imaged in the industrial seismic reflection profiles kindly made available by ENI
E&P.
New trenching investigations have been conducted at the Cava Danesi of Monte Netto in
October 2009, focused on the 1:10 scale analysis of the upper part of the 7 m high
mid-Pleistocene to Holocene stratigraphic section exposed along the quarry walls. In
particular, we excavated a 3 m deep trench across the graben that affects the crest of a
decametric anticline due to paleoseismic bending-moment faulting; evidence of
paleoseismicity is also provided by the observation of paleoliquefaction features near the
graben. The trench walls allowed to identify 3 discrete events of graben reactivation,
interpreted as generated by 3 strong paleoearthquakes. These paleoearthquakes occurred
between ca. 45 kyr BP and ca. 5.5 kyr BP, based on OSL and AMS dating of stratigraphic
units sampled on the main quarry walls, and mapped also in the October 2009 trench.
The post 5.5 kyr BP stratigraphy is missing at the trench site, due to plowing and
reworking.
The paleoseismic deformation observed at Monte Netto is suitable to constrain the
magnitude of the causative earthquakes. At a local scale, the November 24, 2004, Ml 5.2
Salò, earthquake, occurred ca. 40 km NE of Monte Netto, produced rockfalls and
fractures in the Lake Garda shores, but no fault displacement at the surface. The
only well known surface faulting earthquakes in the Southern Alps is the May 6,
1976, Ms 6.5 Friuli event; secondary surface ruptures were observed for a length of
few km and with maximum offset of 20 cm. Using the global database of surface
faulting events, the smallest thrust faulting earthquake known to be associated with
secondary tectonic ruptures at the ground surface is the October 29, 1989, Mw 6.0
Mount Chenoua, Algeria, event. On the other hand, shallow crustal compressional
earthquakes with M>7.0 such as the 1980 El Asnam, 1988 Spitak, and 1999 Taiwan
events are invariably associated with tens of km of primary tectonic scarps, with
maximum surface displacement of several meters. In the Monte Netto area there is
no cumulative geomorphic evidence for repeated surface faulting events of this
size.
Therefore, our best estimate of the Monte Netto paleoseismic magnitudes is in the range
of M6.0 to 6.8, in good agreement with the macroseismically estimated magnitude of the
Christmas 1222 event.
If this seismogenic potential is confirmed in the Brescia area, similar earthquake hazard
should be associated to several Quaternary thrust and backthrust that have been mapped in the
Lombardia and Ticino Southern Alps. In particular, Holocene reverse displacement recently
observed near Como along the Gonfolite backthrust strongly suggests that tectonic structures
capable of producing M6 to 6.8 earthquake may exist also in the region between Lake Como
and Lake Maggiore. |
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