Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

properties. The mean value of specific mass ( of the
chimneys is 2309.66 Kg/m 3 . The Young’s modulus (E)
average is about 23.5 GPa obtained on basis of S and P
waves velocities. The variation around these values is 4.6
GPa. The resistance in simple compression is between 29
and 88 MPa, while the tensile resistance evaluated by
static bending tests on three samples is ranging between
3 and 6.1 MPa. The mean value and the standard
deviation of these failure stresses are 4.6 and 1.55 MPa,
respectively, with a minimum value of 3 MPa.
HOW BIG WAS THIS EARTHQUAKE?
The chimneys are located preferentially on top of
topographic relieves and are founded on a thick layer of
very soft mud. Both conditions are particularly prone to
ground motion amplification. Topographic amplification
effects are well known to happen on top of ridges and on
steep slopes, which are the conditions for most of the
chimneys’ fields. Many building seismic codes account for
the topographic effect; for instance, Eurocode‐8 evaluates
this effect as a factor of 1.2 to 1.4. In addition,
amplification due to soft soil conditions is also a very well
known effect. Clay or silt thick deposits with high water
content are particularly significant for this effect and, in
fact, these are the soil conditions of the chimneys’ fields.
Mean shear velocity for the first 30 m (Vs 30) is used in
seismic codes to evaluate the soil amplification factor. In
our case, we estimate a Vs30 of 140 m/s from mean P‐
wave velocity (Vp) measured in a 25 m depth well drilled
in the vicinity of the chimneys’ fields and using the
correlation of Castagna et al. (1985). For this kind of soils
with such a low Vs30, seismic code provisions account for
the highest amplification factors, which are of the order
of 2.0 to 2.5 (e.g. Eurocode‐8, NEHRP). Furthermore,
considering that the maximum thickness of the soft layer
varies between 70 and 110 m, its natural resonance
period can be estimated in between 2 and 3 seconds,
approximately (cf. Kramer, 1996).
The chimneys can be regarded as very rigid objects. They
are in general much shorter than 2 meters and narrower
than 20 cm. The most frequent Height/Diameter ratio
(H/D) is 4 and the modal chimney can be regarded as 30
cm tall and 8 cm wide. Assuming them as cantilever
beams their natural period of vibration can be estimated
as low as 0.003 to 0.050 seconds (Cadorin et al., 2000).
Such a low vibration period means that the chimneys
were particularly sensitive to the higher frequencies of
the seismic ground motion. For this range of frequencies a
very convenient parameter to evaluate the amplitude of
strong ground motion is the horizontal peak ground
acceleration (PGA) (i.e, the largest horizontal acceleration
value recorded by an accelerometer). Hence, comparing
the minimum acceleration needed to break down the
chimneys to PGA drawn from ground motion prediction
equations (GMPEs) can be used as a first order
approximation to the most likely seismic scenario (i.e. an
earthquake defined by a certain magnitude and distance).
The minimum acceleration needed to break the chimneys
has been calculated simplifying the seismic ground
motion as a harmonic wave. Although this is a
simplification, it is very convenient to represent the
moment when PGA was reached during the shaking. The
acceleration values obtained exhibit a wide dispersion
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
85
(0.21‐0.69 g), depending strongly on H/D parameter.
Considering H/D ratios of 4, which represent the modal
chimney, minimum breaking acceleration values range
from 0.43 to 0.51 g. We shall focus on the upper limit of
this range to compare with PGA drawn from GMPEs.
WHERE WAS THE EARTHQUAKE GENERATED?: LOCATION
THE CAUSATIVE FAULT
Even though there are many ground motion prediction
equations (GMPE) derived from the most important
seismic regions of the world (cf. Douglas, 2003), there is
no one specifically performed for the south Iberian
Atlantic margin. We have then selected a few from the
specialized literature (Boore et al.,1997; Ambraseys and
Douglas, 2003; and Ambraseys et al., 2005), which share
the following characteristics: they are derive from
extensive databases that comprise large regions of the
world; measure the same horizontal component of
acceleration; account for shallow crustal earthquakes
(h300 km).
As in the 1985 Mexico City earthquake, the predominant
vibration period of such an earthquake would be closer to
the natural period of the soft layer where the chimneys
are founded, so facilitating the occurrence of a resonance
effect.