Download different damage distribution observed in the villages of pescara del

GNGTS 2016
Sessione Amatrice
Different damage distribution observed in the villages of Pescara
del Tronto and Vezzano after the 2016 Central Italy Earthquake:
some remarks
A. Masi1, G. Santarsiero1, L. Chiauzzi1, M.R. Gallipoli2, S. Piscitelli2, L. Vignola3, J. Bellanova2,
G. Calamita2, A. Perrone2, C. Lizza3, S. Grimaz4
School of Engineering, University of Basilicata, Potenza, Italy
National Research Council - IMAA, Tito Scalo (PZ), Italy
3
National Association for Public Assistance (ANPAS), Marsicovetere (PZ), Italy
4
University of Udine, Italy
1
2
Abstract. In the aftermath of the ML=6.0 central Italy earthquake occurred on August 24,
2016 the authors surveyed many damaged villages located in the epicentral area. Among them,
some unexpected situations were found like, for example, the very different damage occurred
to the villages of Vezzano and Pescara del Tronto (both districts of Arquata del Tronto).
Despite these two villages are spaced just about 1300 m from each other, Pescara del Tronto
undergoes very heavy damage with many collapses of masonry buildings and 48 fatalities,
while Vezzano was subjected to very light damage to few buildings. In this paper possible
causes of the different damage distribution are analysed from the engineering and geophysics
point of view. Site geophysical investigations have been performed and their results are used to
detect possible local effects responsible of increase of damage in Pescara del Tronto respect to
Vezzano. Further, speditive visual inspections have been carried out in the two villages in order
to detect possible significant differences in the vulnerability of their building stocks.
Introduction. In the aftermath of the main event, the authors surveyed many damaged
villages located in the epicentral area and some unexpected situations were found like, for
example, the very different damage occurred to the villages of Vezzano and Pescara del Tronto
(both districts of Arquata del Tronto). Despite these two villages are spaced just about 1300 m
from each other, Pescara del Tronto undergoes to very heavy damage with many collapses of
masonry buildings and 48 fatalities, whereas Vezzano was subjected to very light damage to
few buildings. Observing the seismic history of the two villages, it is interesting to note that,
during an earthquake occurred in the 1941 with the epicentre very close the village Vezzano, the
two surveyed intensities were different for one degree of MCS scale, particularly VII MCS for
Pescara del Tronto and VI MCS for Vezzano. The paper reports the results of site geophysical
investigations, performed in order to detect possible local effects, and speditive visual
inspections carried out in the two villages in order to detect possible significant differences in
the vulnerability of their building stocks.
Damage occurred in the villages of Pescara del Tronto and Vezzano and vulnerability
comparison. In the first hours after the event, in the framework of the Copernicus Project
(Copernicus, 2016) aerial images were taken in order to quickly evaluate the extent of damage
in the villages prone to the epicentral area. The map related to Pescara del Tronto shows that
85% of the buildings suffered heavy damage or partial/total collapse, 10% suffered moderate
damage, whereas just 5% of the buildings undergone no or slight damage. Damage observed
in Vezzano is very different and no fatalities or injuries have been reported. Copernicus maps
(Copernicus, 2016) refer that only one building out of about 70 reported heavy damage, while
six buildings (less than 10%) suffered negligible to slight damage (Fig. 1).
In Pescara del Tronto most of the structures are unreinforced masonry buildings of 2 to 3
storeys with some of them having 4 storeys. Few building are reinforced concrete. For what
concern masonry buildings, quality of construction in Pescara del Tronto appears to be very
poor with high inhomogeneity of stone elements. Moreover, it has been always possible to
check the mortar quality that has very poor quality. The final judgment on the Pescara del
Tronto masonry building’s vulnerability is very high. The vulnerability estimates for the village
of Vezzano is harder due to the absence of damage and must be based only on what visible
from outside the buildings. The survey revealed the absence of reinforced concrete buildings.
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GNGTS 2016
Sessione Amatrice
Fig. 1 – a) Buildings with collapsed masonry walls in Pescara del Tronto; b) undamaged buildings in the main square
of Vezzano village; c) building with two added floors made of hollow bricks in Pescara del Tronto; d) Buildings with
good quality corners in Vezzano.
For what concern masonry buildings, generally speaking, better quality of the walls’ corners
is visible allowing a more effective connection, less cases of addition of storeys are found and
tie rods to restrain the walls are not so widespread. It is worth noting that masonry walls seems
to be made of limestone elements without travertine elements meaning a higher compressive
strength, although this latter is also related to the quality of the mortar that cannot be checked.
Summarizing, the better material (limestone), better corners’ connection and less presence of
RC floors lead to state that Vezzano masonry building are less vulnerable than Pescara del
Tronto ones.
Geophysical survey in Pescara del Tronto and Vezzano. A geophysical survey was carried
out with the aim to distinguish sites where enhanced local damages were due to lower quality
buildings and/or to local geo-structural settings responsible for ground motion amplifications.
In particular, five geophysical techniques were performed (Fig. 2):
1.���������������������������������������������������������������
n.1 Electrical Resistivity Tomography (ERT) (Loke, 2001; Galli et al., 2006; Giocoli et
al., 2011);
2.��������������������������������������������������������������������������������������
n. 14 single station noise measurements (Horizontal-to-Vertical Noise Spectral Ratio,
HVNSR) (Bonnefoy-Claudet et al., 2006; EVG1-CT-2000-00026 SESAME, NATOSfP980857) with 20 minutes of recordings sampled at 128 Hz by Tromino equipment
(Moho srl); single and multi station noise measurements (Bonnefoy-Claudet et al., 2006;
EVG1-CT-2000-00026 SESAME, NATO-SfP980857);
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GNGTS 2016
Sessione Amatrice
3.�������������������������������������������������������������������������������
n.1 bi-dimensional passive seismic arrays processed with Extended Spatial AutoCorrelation (ESAC) (Ohori, 2002; Okada, 2003; Parolai et al., 2006);
4.��������������������������������������������������������������������������������
n. 1 active seismic arrays (Multistation Analysis of Surface Waves, MASW) (Park et al.,
1999);
5.������������������������������������������������������������������������������������������
foreshocks recordings by accelerometers in four site (three in Pescara del Tronto and one
in Vezzano).
Fig. 2 – Sketch map of the survey area with location of the geophysical surveys.
Below, some results coming from geophysical surveys were summarized. Fig. 3 shows the
ERT carried out in Pescara del Tronto. The electrical image shows a relatively low resistivity
range, from 10 to over 320 Ωm, and a resistivity pattern characterised by relatively strong
lateral and vertical gradients. The shallow relatively high resistivity layer could be associated to
slope deposits (pelitic-arenaceous lithofacies-LAG1e), overlying the substratum constituted by
the Laga Formation (arenaceous-pelitic lithofacies - LAG1b), according to the geological map
of the area (Marche Region Geological Maps at http://www.ambiente.marche.it).
The Vs velocity profile estimated by ESAC analysis shows two impendence contrasts at
about 10 m (VS from 300 to 600 m/s), probably due to the discontinuity between slope deposits
and the underlying Laga Formation, and at about 30 m (VS from 600 to 1000 m/s), probably
due to the different lithostratigrafic members, from pelitic to arenaceous, in the frame of the
Laga Formation.
All sites reveal amplification in frequency range between 3-7 Hz (range of interest of built
environmental).
The preliminary Horizontal-to-Vertical Spectral Ratio (HVSR) functions using 80
earthquakes with 2.4 ≤ ML ≤ 4.3 in AS1, 60 earthquakes with 1.2 ≤ ML ≤ 3.3 in AS2 and
88 with 2.5 ≤ ML ≤ 3.9 in Vezzano site (Fig. 3) show that the rocky site in Vezzano shows
flat amplification function, whereas Pescara del Tronto evidences a significant amplification
variability. Although both stations in Pescara del Tronto are located on the same geological
formation (Laga Formation), they show different amplification functions: AS1 site has a clear
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GNGTS 2016
Sessione Amatrice
Fig. 3 – ERT 01 carried out at Pescara del Tronto.
resonance peak at about 6.5 Hz, whereas AS2 site is characterised by a slight amplification in a
broad frequency range (at about 2-4 Hz and 7-10Hz, this behaviour is more evident in the WE
component).
Last remarks. The results of geophysical investigations show that a very variable local
effects are responsible of the seismic intensity amplification occurred in Pescara del Tronto.
The building vulnerability surveyed carried out on the two villages defines possible significant
differences between the building stocks but not suitable to determine the high difference in
the observed damage levels. Then, although engineering, geological and geophysical studies
are currently on-going, a high variability has been found in the site amplification of seismic
intensity on the two villages.
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