Evaluation of the local site effects in the upper and middle Aterno valley
coordinators: G. Cultrera (1) e L. Luzi (1)
Authors: Ameri G. (1), Augliera P. (1), Azzara R.M. (1), Bergamaschi F. (1), Bertrand E.(2),
Bordoni P. (1), Cara F. (1), Cogliano R. (1), Cultrera G. (1), D’Alema E. (1), Di Giacomo D. (3), Di
Giulio G. (1), Duval A.-M.(2), Fodarella A. (1), Franceschina G. (1), Gallipoli M.R. (4), Harabaglia
P. (5), Ladina C. (1), Lovati. S. (1), Luzi L. (1), Marzorati S. (1), Massa M. (1), Milana G. (1),
Mucciarelli M. (5), Pacor F. (1), Parolai S. (3), Picozzi M. (3), Pilz M. (3), Puglia R. (1), Pucillo S.
(1), Régnier J. (2), Riccio G. (1), Salichon J. (6), Sobiesiak M. (3)
(1) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma - Via
Bassini 15, 20133 Milano - Via Uguccione della Faggiuola 3, 52100 Arezzo, Italia
(2) LRPC de Nice, CETE Mediterranee, Nice Cedex 4, Francia.
(3) Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Telegrafenberg,
14473 Potsdam, Germania
(4) IMAA-CNR, Contrada S.ta Loja Tito Scalo (PZ)
(5) Università della Basilicata, dell'Ateneo Lucano 10 -85100, Potenza, Italia
(6) UMR GEOAZUR, CNRS-UNSA, Sophia-Antipolis, 06560 Valbonne, Francia.
Abstract
In the months following the April 6th, 2009, L’Aquila earthquake, several Italian and foreign
research institutions installed dozens of seismic stations to monitor more than 100 localities with
the aim of studying the local site effects in the epicentral area (upper and middle Aterno valley).
The stations (accelerometers and velocimeters) have been deployed inside or very close to the
inhabited areas. Among the investigated sites there are Onna, where almost the totality of the
buildings collapsed, and the historic centre of L’Aquila, both towns suffering many casualties. The
preliminary results for the examined sites show an extreme variability of ground motion and
significant amplification for the most damaged localities.
1. Introduction
On Aprile 6th 2009 an earthquake of moment magnitude 6.3 struck the Abruzzo region in the
neighbourhood of the L’Aquila city. The day after the main-shock and for several months, more
than 100 sites (Figure 1) have been monitored using accelerometers and velocimeters with the aim
of studying the local site effects in the most damaged centres. The installation was carried out by
several research institutions, including Istituto Nazionale di Geofisica e Vulcanologia (Italy),
Università della Basilicata (Italy), GFZ-Potsdam (Potsdam, Germany) and CETE-Mediterranee
(Nice, France).
The activities consisted of two steps: an emergency phase, soon after the main event, and a second
phase in the framework of two projects promoted by the Protezione Civile Italiana, the Italian civil
defence (C.A.S.E. project and seismic microzonation).
The field surveys have been carried out with the aim of evaluating the local site response through
ambient noise and/or earthquake recordings in the middle Aterno valley, in the municipal territory
of L'Aquila and in the neighbouring municipalities (upper Aterno valley). The stations have been
installed nearby or inside the most damaged centres, where the macroseismic intensity reached the
IX/X MCS degree (Galli and Camassi eds., 2009) and where coseismic surface effects have been
observed (Emergeo Working Group, 2009).
The accelerometric and velocimetric seismic stations have been installed in free-field or inside
buildings. In particular, the accelerometric sensors have been installed in order to record the largest
aftershocks without the signal saturation and to evaluate the possible presence of nonlinear effects.
The relevant number of installed stations, together with the huge amount of collected data, allows
the estimation of the local amplification phenomena through empirical techniques, such as the
horizontal to vertical spectral ratios for a single station (HVSR; Lermo and Chavez-Garcia, 1993) or
the spectral ratio between the horizontal (or vertical) components of the selected site and a reference
site (SSR; Tucker and King, 1984). The large number of ambient noise recordings (microtremors)
allows the use of empirical HVSR techniques applied to microtremors (HVNSR; Bard, 2002) and
the comparison among results obtained with different data and techniques.
2. Geology
The area struck by the sequence is located in the Lazio-Abruzzo Apennines, which has been
subjected to an extensional tectonic regime, with prevalent SW-NE direction, starting from the
middle Pliocene (Pace et al., 2006). The extensional regime caused the aperture of the intramountain basin corresponding to the Aterno river valley, oriented NW-SE and bordered by the Gran
Sasso massif to the north-east and by the Velino-Sirente chain to the south-west. The deposits
filling the valley are dated from Middle Pleistocene to present and are formed by alternations of
calcareous silts, gravels and conglomerate layers. They have been buried by coarse debris deposits,
which ended the depositional phase (alluvial fans and slope debris), dissected by the recent
tectonics at different heights (Bosi e Bertini, 1970). The aforementioned geologic and
geomorphologic characteristics make the area prone to possible ground motion amplification.
3. Local site response of the middle Aterno valley
The evaluation of the local site effects in the middle Aterno valley have been performed through
earthquake recordings, with particular attention to the sites located nearby or inside the damaged
villages. The seismic stations have been equipped by velocimeters and/or accelerometers (Le3d-5s,
Le3d-1s, MarkL4c3d and Episensor) and digitizers (Reftek R72A e R130, EDL), or with integrated
systems digitizer/accelerometer (Kinemetrics K2 or ETNA). The instruments have been supplied by
batteries or solar panels, and almost all stations have been connected to a GPS antenna for time
synchronization. Four sites among the monitored locations are considered reference stations:
Pescomaggiore to the north (limestone), Civita di Bagno, Fossa and Stiffe to the south (cemented
conglomerates).
The preliminary results show an extreme variability of the seismic response of the examined sites
(Figure 2), with amplification in the frequency band 1 - 5 Hz at several villages.
We will describe the results obtained for the sites of Onna and Paganica, two among the most
damaged villages with a high number of casualties (Galli and Camassi eds., 2009). In particular, at
Onna almost the totality of the buildings collapsed and a significant role of the site amplification
has been hypothesized.
Both sites have been equipped with an Episensor accelerometer and a Reftek130 24 bit digitizer
(fullscale set to 1 g). More than 100 earthquakes have been recorded in the magnitude range 3.0 ≤
M ≤ 5.3. For each event, the portion corresponding to 10 seconds starting from the S-phase has been
selected to compute the HVSR at single station; this window entirely includes the energetic part of
the signal for the considered magnitudes and distances.
The seismic station at Onna has been installed nearby the historic centre (Mi03 in Figure 2), built
on fluvial-lacustrine and lacustrine deposits of unknown depth. The Paganica station (Mi02 in
Figure 2) has been installed in the historic centre, built on an alluvial fan.
In Figure 3 the average HVSRs for the two sites are shown, evaluated for a subset of events. Onna
presents a spectral peak at about 2.5 Hz with average amplitude reaching a factor of 5 (Figure 3a),
while Paganica has a frequency peak of about 4.5 Hz, with an average amplification value of about
6.5 (Figure 3b).
These two examples are representative of the majority of the centres built on the lacustrine and
fluvial-lacustrine deposits of the middle Aterno valley and, as a preliminary estimate, we can
hypothesize that the site effects contributed substantially to the damage observed in the historic
centres. The large amount of data collected during the sequence will be used to study in details the
seismic response of the valley.
4. The local site response of the L'Aquila city centre
Since May 27th 2009, 37 sites have been monitored inside and outside the Middle-Age walls of the
L’Aquila city. About 300 earthquakes, located on the seismic sources activated during the seismic
sequence, have been selected in the period May 27th – June 24th 2009.
The Fourier spectra of the selected events have been calculated on a 10 s window starting from the
P phase. The windows include the P phase as well as the S phase: because of the short source-to-site
distance, the S-P interval has a duration lower than 1 s and it is impossible to separate the S from
the P phases.
The selected reference site is located at Poggio di Roio (limestone), about 2.5 km south of
L’Aquila. A second reference site was installed in the San Giuliano monastery (limestone), 2.5-3
km to the north. The latter was also used as a reference site by De Luca et al. (2005) to study the
seismic response of the historic centre of L’Aquila.
The preliminary analysis for the examined sites confirm the results obtained by De Luca et al.
(2005), that is the presence of a persistent low frequency amplification (0.5-0.7 Hz) in the historic
centre of the city. In particular, Figure 4 shows the results of a site in proximity of the Casa dello
Studente (address Via XX Settembre), where the total collapse of the building occurred causing
many casualties among the resident students.
The average SSR and HVSR spectral ratios are in good agreement (Figures 4a and 4b) showing a
spectral amplification in the range 3 - 5 at about 0.5 Hz. The resonance frequencies are confirmed
by the HVNSR (ambient noise) with average amplification of about 7 (Figure 4c). Taking into
account the standard deviations, the level of amplification estimated by the three techniques vary
between 2 and 10 at 0.5 Hz. De Luca et al. (2005) correlate the same amplification with the
presence of a deep sedimentary basin made of Quaternary deposits. These sediments do not crop out
in the historic centre of L’Aquila, as they are buried by a thick layer of calcareous breccias with
different degree of cementation. The thickness of the described units and the corresponding S-wave
velocities are inferred from Blumetti et al. (2002) and from direct measurements (De Luca et al.,
2005): the quaternary deposits have thickness higher than 200 meters and have Vs values in the
range 500 - 650 m/s, while the breccias have a thickness varying between 20 and 50 m and Vs of
about 900 m/s. The hypothesis on the basin geometry have been verified by De Luca et al. (2005)
through a finite elements numerical modelling, which allowed to reproduce the characteristics of the
site transfer function characterized by low frequency content.
The vertical component of motion measured in proximity of the Casa dello Studente (Figure 4d)
has a remarkable amplification when compared to the reference site, with an average value of 6 at a
frequency of 1.2 Hz, slightly higher than the horizontal resonance frequency.
5. Discussion
The seismic sequence which struck the Abruzzo region caused hundreds of casualties and a diffused
damage, seriously affecting L’Aquila city and the localities of the upper and middle Aterno valley.
Local site effects are among the main factors to be investigated when examining the possible causes
of building damage and /or collapse and reproducing the distribution of the felt ground shaking,
together with the distance from the seismic source and the building vulnerability. The timely
intervention of the Italian and international research groups gave a significant contribution to the
collection of the recordings of low-to-moderate magnitude events in a large number of sites. The
monitoring of several reference sites will allow the comparison of the ground motion recorded in
the different localities, in order to have robust estimates of the local site response to an earthquake.
The villages located in the middle Aterno valley have high local amplifications at frequencies
generally higher than 1 Hz, partly explaining the high level of damage. The values of amplification
detected at Onna seem very similar to the amplification resulting for nearby sites, such as Paganica,
suggesting that additional factors, such as high building vulnerability, contributed to the total
destruction of this village. The historic centre of L’Aquila city has a low frequency resonance of the
soil (<1Hz) well correlated with the local geology, as already pointed out by De Luca et al. (2005).
Furthermore it is interesting to note the presence of the amplification of the vertical component of
the motion, for frequencies slightly higher than the horizontal.
The preliminary results discussed here represent a first step to the comprehension of the main
causes of damage occurred in the epicentral area and will be the subject of further investigations.
References
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L’Aquila (Abruzzo — Central Italy): new data from geological, morphotectonic and gravity
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Figure 1. Map of the epicentral area with the location of the seismic stations (triangles) installed for
the evaluation of local site effects since April 6th 2009.
Figure 2. Geologic map (L’Aquila sheet n.359, CARG project) with the location of the stations
installed in the middle Aterno valley (triangles). The deposits sampled by the stations are: recent
alluvial deposits (light blue), cemented breccias (beige), fluvial-lacustrine deposits (light green),
limestones (dark green), Miocene limestones (brown). The seismograms represent the NS
component of velocity (in m/s) recorded at some stations during the aftershock of 09/04/2009,
Ml=3.6 (13:19 GMT, lat 42.34°N - lon 13.26°E, about 12 km west of L’Aquila).
Figura 3. Average and standard deviation of the horizontal-to-vertical spectral ratios (HVSR)
relative to the station at Onna (a) and Paganica (b), evaluated on 15 events in the magnitude range 3
- 5.3.
Figura 4. Average and standard deviation of the SSRs and HVSRs relative to the station installed
nearby the Casa dello Studente, in the L’Aquila city centre; the earthquakes have been recorded
after May 27th: (a) SSR of the horizontal component with Poggio di Roio as reference site; (b)
HVSR, ratio between the horizontal components and the vertical component at the site; (c) HVNSR,
ratio between the horizontal components and the vertical component of an ambient noise recording;
(d) SSR of the vertical components with Poggio di Roio as reference site.