Download comparison between shallow geology, resonance frequency and

GNGTS 2016
Sessione 2.2
Comparison between shallow geology, resonance frequency
and buildings damage due to the April 6, 2009 Mw 6.1
and the February 2, 1703 Mw 6.7 earthquakes at L’Aquila
historical downtown
M. Tallini1, F. Durante1, L. Macerola1, M. Nocentini2
1
2
Dipartimento di Ingegneria Civile, Edile-Architettura e Ambientale, Università degli Studi dell’Aquila, Italy
Dipartimento di Scienze, Università degli Studi Roma Tre, Italy
Introduction. For L’Aquila historical downtown, the comparison between the areal
distribution of the shallow geology, the resonance frequency (f1) and the buildings damage
distribution caused by the recent April 6, 2009 (Mw 6.1) and the more severe February 2, 1703
(Mw 6.7) earthquakes is presented. This comparison allows to understand deeply the seismic
site characterization of a high seismic hazard urban area of Central Apennines, such as L’Aquila
historical downtown (i.e. the urban area encircled by the mediaeval walls).
Geological setting and seismostratigraphy. The L’Aquila historical downtown stratigraphy
is composed downward by: i) 2-20 m thick of Holocene-Upper Pleistocene soft soil (Red silt
Fm.); ii) 20-100 m thick of calcareous breccia (Middle Pleistocene) which represents the relief
on which the L’Aquila historical downtown lays; iii) 200 m thick of pelite and sand (Lower
Pleistocene); iv) Meso-Cenozoic carbonate bedrock (Del Monaco et al., 2013; Gruppo di
Lavoro MS–AQ, 2010; Nocentini, 2016; Tallini et al., 2016a, 2016b).The HVNSR analysis
of about 300 microtremor recordings identifies two resonance frequencies, f0 (0.4-0.7 Hz) and
f1 (3-10 Hz). 1D and 2D numerical simulations evidences that the origin of f0 and f1 is caused
by two main geophysical impedance contrasts. f0 is due to the superposition of pelite and sand
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GNGTS 2016
Sessione 2.2
(Madonna della Strada Synthem, Nocentini, 2016) onto the Meso-Cenozoic carbonate bedrock
at 200-300 m below ground level. f1 is caused by shallower impedance contrast due to 3-20
m thick of Red silt onto calcareous L’Aquila breccia (Colle Macchione-L’Aquila Synthem of
Upper Middle Pleistocene, Marine Isotope Stages – MIS - 6-8) (Nocentini, 2016; Tallini et al.,
2016a).
The Red silt corresponds to a colluvium and karst deposits made of reddish to dark brown
clayey silts with sparse sub-angular clasts. These deposits show thickness up to 20 m and are
generally interpreted as re-worked and colluviated paleosol (Alfisol) mantling the epikarst
developed onto the L’Aquila breccia in a wet and warm interglacial stage (Last Interglacial,
Eemian, MIS 5e) (Nocentini et al., 2016).
The L’Aquila breccia is composed by clast- to matrix-supported, massive or stratified, both
well cemented and incoherent debris flow and rock avalanche breccias and megabreccias, with
highly heterometric (up to m3), poorly sorted, angular to sub-angular carbonate blocks in a
whitish calcareous sandy silty matrix. Whitish to greyish calcareous clayey silts levels and
lenses are interlayered at different elevations (Nocentini, 2016; Nocentini et al., 2016).
While the seismic significance of f0 was already analyzed in detail (Bordoni et al., 2014;
Del Monaco et al., 2013; Gruppo di Lavoro MS–AQ, 2010), then our study was focused on the
relationship between the shallow subsoil model, the medium resonance frequency (f1) and the
buildings damage distribution due to the recent April 6, 2009 (Mw: 6.1) and the February 2,
1703 (Mw: 6.7) earthquakes.
Results and discussion. We elaborated via ArcGis the map of the bottom surface of the
Red silt, the maps of the f1 contour lines and the building damage distribution due to the above
mentioned earthquakes (Clementi and Piroddi, 1986; Tertulliani et al., 2011).
The contour line pattern of Red silt matches enough with that of f1 (Figs. 1 and 2). To analyze
deeply the relationship between the Red silt thickness and f1 we select about ten sites which the
subsoil model is well known (Tab. 1). We apply for the sites reported in Fig. 1 and Tab. 1 the
following equation:
f1=Vs/4Hcal
Fig. 1 – Comparison of the Red silt thickness and the damaged buildings due to the earthquake of February 2, 1703,
Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to the heavy buildings damage or buildings
collapse, respectively; the letters refer to the sites of Tab. 1.
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Sessione 2.2
GNGTS 2016
Tab. 1 – Comparison between the Red silt thickness obtained by borehole investigation (Hbor) and the Red silt thickness
calculated with the equation f1=Vs/4H (Hcal). The site location is reported in Fig. 1.
site
f1 (Hz)
Hbor (m)
Hcal (m)
Hbor-Hcal (m)
TE
5.8
13.2
14.1
-0.9
DU
5.7
12.4
14.4
-2.0
BE
7.7
11.8
10.5
1.3
CA
6.6
13.0
12.3
0.7
BI
14.3
5.0
5.7
-0.7
SB
6.8
13.5
11.9
1.6
CE
4.6
18.7
17.7
1.0
-0.5
CS
6.5
12.0
12.5
DA
6.3
14.0
13.0
1.0
CF
11.6
6.5
7.0
-0.5
RO
4.8
17.0
16.9
0.1
where, for each site, f1 is the above mentioned resonance frequency, Vs is the shear wave
velocity of the Red silt (around 325 m/s as obtained from down hole investigations, Del Monaco
et al., 2013) and Hcal is the thickness of the Red silt. For each site, we note a good agreement
between Hcal and the red soil thickness found in the borehole (Hbor). So f1 is caused by the
seismic impedance contrast due to the superposition of the Red silt onto L’Aquila breccia which
has a Vs around 800 m/s.
Moreover, to understand in-depth if the seismostratigraphy had a role on the buildings
damage due to the two mentioned earthquakes, we compare the areal distribution of f1 with that
of buildings damage (very heavy damage and building collapse) (Figs. 1 and 2). We note that
the 80% of the heavy damaged buildings are placed in sites characterized by f1 in the range of
3-10 Hz (Fig. 3). But, considering the April 6, 2009 earthquake, the majority of the buildings
of L’Aquila historical downtown have a resonance frequency also in the range of 3-10 Hz (F.
Di Fabio, pers. com.). The sites of L’Aquila historical downtown, where the subsoil resonance
Fig. 2 – Comparison of the subsoil resonance frequency (f1) and the damaged buildings due to the earthquake of
February 2, 1703, Mw 6.7 (A) and April 6, 2009, Mw 6.1 (B). Blue and magenta colors refer to severe buildings
damage or collapse, respectively.
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GNGTS 2016
Sessione 2.2
Fig. 3 – L’Aquila historical
downtown
buildings
damage
caused by 1703 (A) and 2009
(B) earthquake vs f1 resonance
frequency. Data of buildings
damage due to the 1703 and 2009
earthquakes are from Clementi and
Piroddi (1986) and Tertulliani et
al. (2011), respectively. Blue and
red colors refer to severe buildings
damage or collapse, respectively.
frequency (f1) is comparable in the range of 3-10 Hz to the resonance frequency of the building
we note the presence of heavy damaged buildings (Fig. 2 and Fig. 3). So we can hypothesize
the coupling between the resonance frequency of subsoil (f1) and that of the damaged building
because they have the same range of values (3-10 Hz). In the case of the February 2, 1703
earthquake, we note a similar areal distribution of the heavy damaged buildings of the April 6,
2009 earthquake, hypothesizing a quite similar cause of the building damaging for the February
2, 1703 earthquake.
Conclusions. The main conclusions are summarized as follows:
the resonance frequency f1 is caused by the seismic impedance contrast due to the
superposition of the Red silt onto L’Aquila breccia (Tab. 1);
the damaged/collapsed buildings caused by 1703 and 2009 earthquakes are scattered in
areas with subsoil resonance frequencies in the range of 3-10 Hz (Figs. 1 and 2);
taking into account the April 6, 2009 earthquake, the 80% of the heavy damaged buildings
are placed in sites characterized by f1 in the range of 3-10 Hz (Fig. 3);
the majority of the buildings of L’Aquila historical downtown have a resonance frequency also
in the range of 3-10 Hz, so the coupling between the resonance frequency (f1) of subsoil and that of
the buildings could be hypothesized because they have the same range of values (3-10 Hz).
The ongoing study will investigate this proposed possible coupling phenomena between
the resonance frequency of the shallow subsoil model and that of the buildings of L’Aquila
historical downtown through GIS zonal statistics analysis and the indirect evaluation of the
resonance frequency of the buildings damaged by the February 2, 1703 earthquake.
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GNGTS 2016
Sessione 2.2
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