Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Fig. 2: Assemblage of landforms associated with active strike‐slip
faulting (modified from Wesson et al., 1975)
PALEOSEISMOLOGY AND SEISMIC HAZARD
It is not uncommon to identify the specific dates of strong
earthquakes (the ones that are relevant for their impact
on the society) along a fault over the past few thousand
of years, permitting a fare better quantitative
understanding of the local earthquake hazard than has
ever been possible before.
Figure 3: Sketch diagrams of cross sections of geologic
relations that might result from individual paleo‐earthquakes.
A tremendous advantage of paleoseismic studies is that
they offer the opportunity to extend the seismicity record
to an appropriate time window (i.e. the Holocene for
interplate areas and the whole Quaternary for intraplate
areas). Objectives of Paleoseismology are:
To recognise “Seismites” by comprehensive
geological investigations, including trenching (Fig. 3).
To identify the seismic source producing a specific
seismite through tectonics and geophysics.
To quantify the “energy” (intensity or magnitude) by
using displacement vs magnitude relationships (Fig.
4).
A Case Study
The magnitude vs displacement relationship has been
used for seismic hazard measurement from paleoseismic
evidence in the Rieti region, central Italy.
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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Fig. 4: Diagram showing our model of surface displacement vs.
magnitude relationships for a region in crustal extension like the
Central Apeninnes in Italy
Rieti region is a part of the central Apeninnes, a mountain
belt characterized by active extentional tectonics. This
area has three paleoseismic sites along the normal fault
system at the eastern boundary of Rieti graben which is
the main Quaternary structures of this region. Evidence of
late Pleistocene and Holocene earthquake rupture is well
preserved.
One of these sites called Caporio site was personally
visited by the author (Photo). At Caporio site a recent
Photo: CAPORIO site in Rieti Region of Central Apennines
quarry excavation along the fault provides a cross section
of a graben in the limestone bedrock. In excess of 20m of
upper Quaternary slope deposits fill this graben. The
accumulation of such a thick colluvial sequence is the
result of a small valley trending perpendicularly to the
graben in the mountain slope. Near the base a volcanic
layer provides a marker suitable for evaluating the
displacement inside the graben. This layer is offset 5m at
one fault and dragged against the second fault. Located
down relative to the graben and in the non faulted
coluvial sequence the same layer is present near the
bedrock at the bottom of the slope deposits, suggesting
that the 20m of displacement occurred after its
deposition. Several channel fill deposits are preserved in
another fault in the upper 10m of the graben, which
indicate deflection in the original course of such channels.
This suggests that recurrent surface faulting events
occurred at this site.