Evaluation of site effects in the Aterno river valley (Central Italy) from aftershocks of the 2009 L'Aquila earthquake

Bergamaschi F. (1), G. Cultrera (1), L. Luzi (1), Azzara R.M. (1), Ameri G. (1), Augliera P. (1), Bordoni P.

(1), Cara F. (1), Cogliano R. (1), D’Alema E. (1), Di Giacomo D. (2), Di Giulio G. (1), Fodarella A. (1),

Franceschina G. (1), Galadini F. (1), Gallipoli M.R. (3), Gori S. (1), Harabaglia P. (4), Ladina C. (1), Lovati

S. (1), Marzorati S. (1), Massa M. (1), Milana G. (1), Mucciarelli M. (4), Pacor F. (1), Parolai S. (2), Picozzi

M. (2), Pilz M. (2), Pucillo S. (1), Puglia R. (1), Riccio G. (1), Sobiesiak M. (2)

(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 – Contrada da Ciavolone, 83035

Grottaminarda (AV), Italia

(2) Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Telegrafenberg, 14473

Potsdam, Germania

(3) IMAA-CNR, Contrada S.ta Loja Tito Scalo (PZ)

(4) Università della Basilicata, dell'Ateneo Lucano 10 -85100, Potenza, Italia

AbstractA temporary network of 33 seismic stations was deployed in the area struck by the 6 th April 2009, Mw 6.3,

L’Aquila earthquake (central Italy), with the aim to investigate the site amplification within the Aterno river

Valley. The seismograms of 18 earthquakes recorded by 14 of the 33 stations were used to evaluate the

average horizontal to vertical spectral ratio (HVSR) for each site and the standard horizontal spectral ratio

(SSR) between a site and a reference station. The obtained results have been compared to the geological and

geophysical information in order to explain the resonance frequencies and the amplification levels with

respect to surface geology of the valley.

The result indicate that there is no uniform pattern of amplification, due to the complex geologic setting, as

the thickness and degree of cementation of the deposits is highly variable. As consequence, a large number

of the local site response is observed, therefore it is very difficult to elaborate a unique model that can

explain such a variability of the amplification.

IntroductionOn April 6th 2009, a Mw 6.3 earthquake struck the Abruzzo region (central Italy; Figure 1), in an area with

the highest hazard in Italy (Gruppo di Lavoro MPS, 2004; Akinci et al., 2009). The hypocenter was located

at 6 km SE from L’Aquila town (about 70,000 inhabitants) and at a depth of 9.5 km (Chiarabba et al., 2009).

The strongest aftershocks occurred on April 7th (Mw 5.6) to the South-East and on April 9th to the North-

1

West (Mw=5.4) in the Campotosto area, where the seismicity migrated on a NW-SE normal fault bordering

the Laga Mountains (Figure 1).

The mainshock and most of the aftershocks are associated to extensional normal faulting, striking along the

Apennine direction (Galadini and Galli, 2000) and dipping at nearly 50° to the SW. The fault segment of the

mainshock extends for 15-18 km (Chiarabba et al., 2009; Cirella et al., 2009), consistently with the surface

trace of the Paganica-San Demetrio fault and with the alignment of the mapped surface breaks (Falcucci et

al., 2009).The total length of the area affected by the sequence extends to more than 35 km (Figure 1;

Chiaraluce et al., 2010).

The fault parameters and coseismic slip distribution of the mainshock have been estimated by Atzori et al.

(2009) using a joint inversion of SAR interferometric and GPS data, and by Cirella et al. (2009) using strong

motion and GPS data. Both find a maximum slip of about 1 m and a high value of slip concentrated on the

SE patch of the fault model. Moreover, the area of maximum coseismic subsidence is located SE of

L’Aquila town in the close vicinity of the Onna village (Atzori et al., 2009; Cheloni et al., 2010). Peak

ground acceleration in the near-fault region ranges from 3.47 to 6.46 m/sec2, the maximum representing one

of the highest values recorded in Italy (Ameri et al., 2009).

The event caused more than 300 casualties and about 60,000 persons were displaced. The strongest damage

was observed in L’Aquila (Figure 1) and in the villages to the SE, in agreement with the rupture propagation

pattern, toward up-dip and SE (Cirella et al., 2009; Ameri et al., 2009; Pino and Di Luccio, 2009; Akinci et

al., 2010). The macroseismic survey carried out in the days after the mainshock (Galli and Camassi, 2009)

indicates that the largest damage is mainly distributed in a NW-SE direction according to the orientation of

the Aterno River Valley. In particular, sixteen localities including L’Aquila downtown, on the hanging-wall

side of the causative fault, suffered damage greater than VIII degree of the Mercalli-Cancani-Sieberg

intensity scale (MCS). The maximum observed intensity was IX-X MCS in the small villages of Onna and

Castelnuovo (Figure 1).

The building typology in the epicentral area varies within the same village, where, in general, an historical

part with old masonry buildings is typically surrounded by a more modern part, built often after the ‘60,

consists of good quality masonry and/or reinforced concrete buildings. The old and badly-maintained stone

or poor-masonry buildings without tie-beams suffered the most severe destruction, while the more recent

buildings in general suffered from slight to moderate damage. Only in very few cases, reinforced concrete

buildings totally collapsed or resulted heavily damaged, due to the poor quality of the construction

(Tertulliani et al., 2009; Tertulliani et al., 2010; Aydan et al., 2009; EERI, 2009). However, the

macroseismic observations reported by Galli and Camassi (2009) as well as the Geoengineering Extreme

Events Reconnaissance Association (GEER, 2009) induce to hypothesize the influence of site effects to

explain the heterogeneities occurred in the damage pattern, especially for the villages settled on the

Holocene alluvial deposits of the Aterno River Valley (Figure 2).

2

Site effects can play an important role in the damage distribution, since the variation of local geology setting

can produce significant discrepancies in ground motion at sites very close one to the other. The day after the

mainshock a temporary seismic network was deployed in the middle Aterno Valley by the Istituto Nazionale

di Geofisica e Vulcanologia (Italy), Università della Basilicata (Italy) and GeoFOrschungsZentrum (GFZ;

Potsdam, Germany) in order to investigate the site effects distribution. The 33 seismic stations

(seismometers and/or accelerometers) were installed nearby or inside the damaged centres, where the

macroseismic intensity reached the IX/X MCS (Galli and Camassi eds., 2009) and where coseismic surface

effects have been observed (EMERGEO Working Group, 2010).

The relevant number of installed stations, together with the huge amount of collected seismic recordings,

represents an important dataset for the estimation of the local amplification phenomena and/or of nonlinear

effects in the epicentral area. Moreover, they can be used to integrate the temporary network data for

tomographic studies and for the analysis of the seismic sequence.

In this paper we first describe the geological setting of the area where the experiment was performed, and

we present the dataset of collected seismograms. We then evaluate the amplification of a subset of

investigated sites using two widely used empirical techniques. The results are compared with the geological

and geotechnical information available from previous studies and from the seismic microzonation promoted

by the Italian Department of Civil Protection soon after the L’Aquila mainshock (Gruppo di Lavoro MS-

AQ, 2010).

Description of the experiment

During the days after the mainshock about 33 sites were selected (Figure 2) close to the most damaged

villages, where the macroseismic intensity reached the IX/X MCS degree (Galli and Camassi eds., 2009)

and coseismic surface effects have been observed (EMERGEO Working Group, 2010). The sites were

monitored by a temporary network equipped with seismometers and accelerometers (Le3d-5s, Le3d-1s,

MarkL4c3d and Episensor) coupled to 24-bit digitizers (Reftek R72A and R130, EDL), or by integrated

systems digitizer/accelerometer (Kinemetrics K2 or ETNA).

The instrumentation has been supplied with batteries and solar panels, and almost all the stations have been

connected to a GPS antenna for time synchronization. The seismic stations have been installed inside

buildings or in free-field. Four sites were installed on stiff soil: Pescomaggiore to the North (MI01,

limestones), Civita di Bagno (GFZ5, calcarenites), Fossa (MI04, cemented breccias of the AP formation)

and Stiffe (AQ07, limestones) to the South (Figure 2). Only three of them were chosen to be used as

reference in the spectral ratio analysis, AQ07 in fact has been excluded due to the short recording interval

caused by technical problems.

The stations (Table 1) were continuously operating for a period of few days to more than a month after the

mainshock occurred, allowing the recording of hundreds of seismic events with magnitudes from about 1 to

3

more than 5. More than 100 seismic events of magnitude greater than 3.0 occurred in within a radius of 30

km around L'Aquila town (Figure 2).

The epicenters are distributed along the NW-SE Apennines trend, following the direction of the system of

direct faults that characterizes the Aterno Valley. During the days after the mainshock, the seismicity

concentrated near L’Aquila in SE direction, where the highest damage occurred. Subsequently, the seismic

activity migrated toward the NW area of Capitignano-Campotosto.

Geologic setting and geophysical characterization of the Aterno river Valley

The NW-SE trending Aterno river valley is located in the innermost portion of the central Apennines. It is

bordered by the Gran Sasso Range towards NE and by the Velino-Sirente Massif towards SW (Figure 1).

From a structural point of view, the setting of this portion of the Apennine chain is closely related to the

Quaternary activity of NW-SE trending normal faults (Bosi and Bertini, 1970) some of which are presently

active, such as the Upper Aterno and the Middle Aterno Valley fault systems (e.g. Galadini and Galli, 2000).

Since the Pliocene, the extensional tectonic regime caused the formation of intermountain basins (e.g.

Cavinato and De Celles, 1999; Galadini et al., 2003) characterized by Plio-Quaternary continental sediments

reaching a thikness up to about 1 km (roughly 1,200 m in the case of the Fucino basin; Galadini and

Messina, 1994).

Homogeneous geologic information that are useful for the purpose of the present study have been derived

from the Italian Geologic map at 1:50,000 scale, no. 359 L’Aquila (APAT, 2006) (Figure 3). The available

geologic information defines that the bedrock outcropping in this area prevalently consists in successions of

limestones, marly limestones, and carbonate marls Triassic-to–Miocene in age and pertaining to different

paleogeographic domains.

The Aterno river tectonic depression hosted continental deposits since the Early Pleistocene, mainly

lacustrine, alluvial and colluvial in origin, as well as landslide accumulations (Bosi and Bertini, 1970). Four

main morpho-stratigraphic units have been distinguished in APAT (2006):

1) Holocene deposits (Holocene; Ol in Figure 3): coarse deposits (gravels and, more sparsely, sands) of

alluvial origin associated to the hydrographical system of the Aterno river. These sediments are exposed

along the whole Aterno river valley, from the area of L’Aquila to Monticchio – Onna and Sant’Eusanio

Forconese.

2) Valle Majelama Synthem (Late Pleistocene; AVM in Figure 3): alluvial plain and alluvial fan sediments,

with different grain size (gravels, sands), generally terraced along the western flank of the Aterno river

valley.

3) Catignano Synthem (Middle – Late Pleistocene; ACT in Figure 3): cobbles, gravels and sands related to

alluvial fans, or characterized by lacustrine and/or glacial deposits, the latter related to the Last Glacial

Maximum. They have limited extension and crop out in the area of S. Demetrio ne’ Vestini. 4

4) Aielli-Pescina Supersynthem (Pliocene – Middle Pleistocene; AP in figure 3): lacustrine and fluvial

deposits, alluvial fan sediments and slope debris, outcropping along the whole Aterno river valley, having

been detected in the Roio and L’Aquila depressions, and along the eastern flank of the Aterno river valley,

from Paganica to S. Demetrio ne’ Vestini and Castelnuovo.

The oldest deposits of the Aielli-Pescina Supersynthem are consisted of slope-derived carbonate breccias

and partly cemented carbonate landslide accumulations (maximum thickness of about 100 m) made of

differently sized cemented carbonate clasts. These deposits represent the foundations soils of L’Aquila. In

some areas (e.g. between Barisciano and Castelnuovo), the breccias are laterally passing to carbonate

lacustrine silts known as “Limi di S. Nicandro” (Bertini and Bosi 1993). The deposits of this supersynthem

are severely displaced by the Plio-Quaternary fault activity along segments of the Aterno valley fault

systems.

The lithological and geophysical characteristics of the recent continental deposits of the area have been

investigated in the last five years in the framework of projects funded by the Italian department of civil

protection (compilation of the Italian strong-motion data base, Luzi et al., 2008; implementation of shake-

maps, Michelini et al., 2008). The Italian civil protection also promoted additional geotechnical and

geophysical investigations for the microzonation study of the area struck by the 2009 earthquake (Gruppo di

lavoro MS-AQ, 2010). Figure 4 displays the shear wave velocity profiles derived from eight cross-hole and

down-hole tests (AQA, AQV and AQK, Working Group ITACA, 2010; BRS, Cercato et al., 2010) and two

microtremor array surveys (BZZ and MI03; Working Group ITACA, 2010) whose location is shown in

Figure 3. Two tests have been performed in the upper Aterno valley (AQA and AQV, Figure 4a), in the

Holocene deposits of alluvial origin; the other two (AQK and BRS, Figure 4b) sampled the Aielli-Pescina

Supersynthem, in particular the landslide carbonate breccias exposed in L’Aquila (AQK) and the cemented

conglomerates in the vicinity of Barisciano. The two microtremor arrays (Working Group ITACA, 2010)

were set up to investigate the subsoil of the Aterno river alluvial plain in the vicinity of Bazzano and Onna

(Figure 4c).

The uppermost portion (about 50 m) of the alluvial sedimentary sequence, investigated by all the boreholes

in the area (Gruppo di lavoro MS-AQ, 2010) but not shown in this work, is mainly made of gravels with

different degree of cementation, alternating to thin layers of finer deposits (sands and/or silts) that often

include carbonate boulders. In the upper Aterno river valley (AQA and AQV sites), the Holocene deposits

are superposed to the carbonate bedrock, intercepted at a depth of 50 m by borehole AQV, performed in the

central sector of the valley (Figure 3). The depth of the continental deposits increases towards south: about

40 m of carbonate breccias overlay the lacustrine silts and clays at AQK (Figure 4b), while the carbonate

bedrock can be only hypothesized as laying at a depth of several hundred metres from Bazzano to

Castelnuovo (Figure 3).

Three geologic cross sections in Figure 3, NW-SE oriented, have been drawn across the Aterno river valley

in order to define stratigraphic setting at depth (Figure 5).5

Section 1 (Figure 5a) has been drawn from Paganica to Civita di Bagno (from station MI02 to GFZ05,

Figure 3), where the continental deposits are displaced by a normal fault affecting the central sector of the

valley. The Aielli–Pescina Supersynthem (AP in the map and sections), in the area between L’Aquila and

Onna, is prevalently composed of clays, which lay below the calcareous breccias in the city of L’Aquila, as

evident from the available boreholes (not shown here) and the velocity inversion of Figure 4b (AQK). The

shear wave velocity measured in the more recent deposits of the alluvial fan of Paganica (station MI02),

prevalently composed of coarse grained deposits, is about 500 m/s in the uppermost 35 m (Figure 4d,

PAGA3), while at Bazzano (stations AQ01 and GFZ06) the velocities increase to about 600 m/s in the first

20 m and then to more than 800 m/s (Figure 4c, BZZ). We hypothesize that station AQ02 is installed on a

layer, about 50-m-thick, of Holocene deposits overlying the AP clays.

Section 2 (Figure 5b) trends parallel to the S. Gregorio–Onna–Monticchio alignment, where the valley

becomes narrower. At the valley border (station GFZ07), the terraced deposits of the Valle Majelama

Synthem (AVM in the map and section) are superimposed on the calcareous bedrock and are characterised

by a thickness ranging between 20 and about 100 m (Gruppo di Lavoro MS-AQ, 2010). The shear wave

velocity profiles from the downhole tests indicate velocities of about 400 m/s for these deposits, before the

calcareous bedrock is reached (Figure 4d, SG5). At Onna (station MI03), the recent Holocene deposits also

have a velocity of about 400 m/s in the uppermost 40 m; then the velocity increases to 800 m/s (Figure 4c,

MI03). We hypothesize that station AQ03 is installed on the AVM and AP units characterised by a large

thickness.

The southernmost cross section (Figure 5c), from Barisciano (station GFZ02) to station AQ06, has been

modified from Bosi and Bertini (1970). It indicates an increase in the structural complexity of the area, as

the oldest continental deposits (AP) are displaced by sets of almost parallel, NW-SE trending normal faults

to form sort of terraces suspended at various height. The cross-hole test performed at Barisciano (Cercato et

al. 2010, Figure 4b), close to station GFZ02, clearly evidences that the Pleistocene continental deposits are

strongly cemented and may have shear wave velocities comparable to those of the carbonate bedrock

measured at AQV (Figure 4a). We hypothesize that station AQ06, GFZ04 and AQ05 are installed on

continental sediments having large thickness (i.e. > 200m), while stations GFZ02 and GFZ03 are located on

highly cemented deposits.

The information derived from the geophysical tests highlights that the shear wave velocities of the

continental deposits are generally high, as shown by the Vs30 values reported in Table 2. The recent and

poorly cemented deposits have Vs30 values in the range 300-400 m/s, while the cemented deposits have

velocities larger than 700 m/s. Moreover, shear wave velocity inversions are very frequent because of the

alternation of layers characterised by different grain size and degree of cementation.

Spectral ratio analysis and correlation with geology

6

In order to evaluate the amplification at the investigated sites, we computed the spectral ratio between the

horizontal (and vertical) components of the selected sites and a reference site (SSR; Tucker and King, 1984)

and the horizontal to vertical spectral ratios for a single station (HVSR; Lermo and Chavez-Garcia, 1993).

Not all the installed seismic stations have been included in the analysis. Some of them were excluded due to

a short operating interval or to some technical defaults and, in general, the velocity sensor stations were

preferred.

The selected 14 seismic stations (Table 1) are located in the epicentral area of the mainshock, therefore the

hypocentral distances are comparable with the inter-stations distances and source and propagation

differences could be not negligible. In order to avoid these effects we analyzed a subset of 18 earthquakes

located near the Campotosto lake, about 20 km N-NW from the city of L’Aquila, in the magnitude range 3.3

- 5.1 (Table3; Fig. 2).

The waveforms of the selected events have been extracted from the continuous stream of each station,

starting from the origin time, and P and S waves arrival times were manually detected. Moreover, the

instrumental correction and the reduction of the sampling rate were necessary, in order to homogenize the

data before computing the spectral ratios. The spectral analysis was performed on a 10s window from the S-

wave arrival, which enable us to include the most energetic part of the seismogram corresponding to the

shear wave phase. After a linear trend removal and a 5% cosine tapering, the Fourier spectra for each

component have been computed. The obtained Fourier spectra were smoothed by using a Konno-Ohmachi

(1998) algorithm with b value of 40.

The spectral ratio between the horizontal components of the selected and reference sites (SSR) and the ratio

between the horizontal and vertical spectra of each station (HVSR) have been computed. For each event the

single spectral ratios for NS and EW components have been geometrically averaged to obtain a horizontal

SSR and a HVSR. Finally, in order to evaluate horizontal average SSR's and HVSR's representative for each

site, the geometrical mean of all the spectral ratios have been computed for each station.

The SSR method implies the selection of one or more reference sites, possibly located on bedrock in free

field and not subjected to amplification effects (Borcherdt and Gibbs, 1970; Field and Jacob, 1995; Steidl et

al., 1996). A preliminary analysis for the selection of the reference site has been performed for the three sites

(MI01, MI04 and GFZ5) installed either on stiff soil or rock (Figure 3).

Average HVSR's indicate that MI01, to the North, shows an amplification of 4 at 5 Hz (Figure 6a), probably

due to the fractured calcareous bedrock characterized by a very high fault density. MI04, to the South, is

moderately amplified (up to a factor 3) in the range 1-5 Hz for the presence of weathered breccias (Figure

6b). Finally, GFZ05, although installed on the same geologic formation as MI04, exhibits an amplification

less than 2 over the entire frequency band of interest, so it can represent the most reliable reference site

(Figure 6c). We calculated the average SSR ratios for all the sites using the three different references (MI01,

MI04 and GFZ05). Figure 6 also shows the comparison between SSR and HVSR spectral ratios at two

stations (AQ03 and AQ04). When MI04 is used as reference, the amplification of SSR decreases over the 7

entire frequency band at the two sites. The SSR respect to MI01 show different behaviour: at AQ03, whose

fundamental frequency is around 1 Hz, SSR and HVSR give similar results (Figure 6d); the SSR computed

at AQ04 instead, having a resonance frequency of about 5 Hz, shows an amplitude decrease from about 4 of

the HVSR to 1, as the fundamental frequencies of both stations coincide (Figure 6e). Finally, there is a good

agreement between SSR's with respect to GFZ5 and the HVSR's, therefore GFZ5 can be reasonably used as

reference site. Discrepancies can be found for some sites, such as MI05 (Figure 7). In this case the

differences between SSR (Figure 7a) and HVSR (Figure 7c) are due to a large amplification on the vertical

component (Figure 7b) at around 5 Hz, leading to an apparent double resonance peak on HVSR. See Ameri

et al. (this issue) for details.

In the following section we will use the SSR with respect to GFZ5 and HVSR curves to correlate the site

response with the geological information (average S-wave velocity and depth). Figure 8 shows the spectral

ratios of the recording sites, which are plotted in Figure 9 on the geologic map, together with the symbols

indicating the resonant frequency and the corresponding amplitude range.

In general, all the stations show peaks or bands of amplification at different frequencies, ranging from 0.8

Hz to more than 10 Hz (Figure 8). The northern part of the investigated area, where the town of L’Aquila

and the villages of Onna, Monticchio and Paganica are located, was sampled with 8 stations (Figure 9). They

were installed in the south-western flank of the valley (GFZ05, AQ02, AQ03 and MI04), in its central part

(AQ01, MI03) and close to the northern edge (MI01, MI02). Following the direction perpendicular to the

major axis of the valley (section 1 and 2 of Figure 5), the peaked frequencies range from about 5 Hz in the

NNW edge (MI01 and MI02) to 2-3 Hz in the central part (AQ01 and MI03), down to 0.9 Hz to SSW

(AQ02) (Figure 8-1a and 8-1b; Figure 9). This behaviour is consistent with the deepening of the fluvial

deposit toward SW (section 1 and 2 of Figure 5; Di Giulio et al., this issue). The level of amplification for

these stations varies from 2 to 6, whose variability is most probably due to the very heterogeneous

geological setting of the area, which implies the presence of consistent changes of impedance contrast

between the superficial and the deep layers in nearby sites.

The geological complexity increases in the south-eastern part of the valley (geologic cross section 3 in

Figure 5c), where other 6 stations have been working in the same period (AQ04, AQ05, AQ06, MI05,

GFZ01 and GFZ04; Figure 8-2a and 8-2b). AQ04, MI05 and GFZ01 are close to limestone outcrops and

stiff sediments, where thin layers of low-velocity deposits produce high frequency resonances (Figure 9).

Moving towards SE (see AQ06 site), the frequency lowering of the first peak can be associated to a

thickening of the recent alluvial deposits. Finally, the stations GFZ04 and AQ05 do not show relevant

amplification being located at the transition between the gravelly Holocene deposits and the older Pliocenic-

Pleistocenic sediments (AP), which probably have very low impedance contrast.

An additional analysis is conducted to discard the presence of earthquake azimuth effects by comparing the

HVSR's from earthquakes located in different areas. We selected station AQ06, as it is located in the centre

of the valley, in free field on the recent alluvial deposits and therefore we do not expect directional site 8

amplification. Figure 10 shows how the average HVSR’s obtained from the events located in the SE area

and the local earthquakes are very similar to the average of the events located in the NE.

Finally, the occurrence of a frequency/amplitude variation in spectral ratio during the S-phase, recovered

after few seconds, has been observed by Puglia et al (present issue) only at the station AQV, located in the

upper Aterno valley (Figure 3, panel 1) on recent gravelly deposits. The same paper discusses if the

observed phenomenon could be attributed to soil non-linearity due to the instantaneous increase of pore

pressure at the P-wave arrivals, or to the different importance in the early converted phases at the station on

soft soil (AQV) and at the one on stiffer material (AQM).

Discussion

The seismic sequence which struck the Abruzzo region caused hundreds of casualties and a diffused damage

pattern, 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 to reproduce the distribution of ground shaking, together with the distance from

the seismic source and the vulnerability of buildings. As discussed in Cultrera et al. (2009), the timely

intervention of the Italian and international research groups significantly contributed to the collection of

low-to-moderate magnitude event recordings in a large number of sites.

The Aterno river valley is a complex geologic structure, where the deposition of the continental deposits has

been driven by tectonics. The displacement of the sediments at different heights and the highly variable

degree of cementation imply a lateral variation of the thickness of the deposits (Figure 3 and Figure 5) and

therefore a large variation of local site response to ground shaking. In addition, the shear wave velocities of

the sediments are generally high (Table 2), with the exception of the first meters of the Holocene deposits,

and the velocity inversions are very frequent due to the presence of cemented layers (Figure 4). The

amplification frequencies from the spectral ratio analysis presented in this study range between 0.8 and more

than 10 Hz (Figure 9). Despite of the complexity of the results, we can subdivide the area in two sectors: the

NW part of the valley, where the peaked frequencies range from about 5 Hz down to 0.9 Hz, in agreement

with the deepening of the fluvial deposit toward SW, and the south-eastern sector, characterized by higher

geological and site amplification variability.

The high variability of the site response and the complex geologic setting of the valley indicate that further

investigations are required for the comprehension of the causes of damage occurred in the epicentral area.

Acknowledgments

We acknowledge the Italian Civil Protection Department and Dr. Giuseppe Naso for providing the S-wave

velocity profiles.

We also are grateful to the population of the villages struck by the 6 th of April, 2009, L’Aquila earthquake

for their kind collaboration, despite of the devastating calamity they were facing.9

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Tables comments

Table 1 - Temporary networks of accelerometric and velocimetric stations. * indicates the stations used in this study.

Table 2. Average shear wave velocity in the upper 30 m for the sites located in Figure 3.

Table 3. Seismic events whose recordings have been used in this study (occurred * inside the Aterno valley;

** south of the Aterno valley). Locations and Magnitudes (Ml) are from Italian Seismic Bulletin (Istituto

Nazionale di Geofisica e Vulcanologia, http://iside.rm.ingv.it/).

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Figures comments

Figure 1. Topographic map of the investigated area. The stars indicate the epicenters of the largest

earthquakes occurred during the L’Aquila 2009 seismic sequence. The symbols are proportional to

the magnitude: largest star is for the April 6th 2009, Mw 6.3, mainshock (black), empty stars are for

Mw=4.6-5.6 and Mw=4.1-4.6. The shaded rectangular box corresponds to the surface projection of

fault responsible for the mainshock.

Figure 2. Map of the temporary seismic network installed in the Aterno Valley (black triangles).

Circles are the earthquakes recorded during the experiment, in black are the events selected for this

study; their size is proportional to the magnitude.

Figure 3. Geological sketch of the Aterno river valley (Ol = Alluvial, lacustrine and swamp

deposits, alluvial fan, eluvial-colluvial deposits (Holocene); AVM = Sintema di valle Majelama:

deposits of alluvial, debris, glacial and landslide origin (upper Pleistocene); ACT = Sintema di

Catignano: deposits of alluvial, debris, glacial and landslide origin (middle Pleistocene), AP =

Supersintema di Aielli - Pescina: deposits of alluvial and debris origin alternated to lacustrine and

swamp deposits (Pliocene - middle Pleistocene); Ar = Arenaceous-pelitic unit; Cl = Clay-marl unit;

Calc = Limestones and marly limestones; Co = Conglomerates; Mar = Marls and marly limestones).

The insets in the upper right panel indicate the position of geological map of panel 1 and 2.

Figure 4. Vs velocity profiles of (a) AQA and AQV, (b) AQK and BRS (Barisciano), (c) BZZ

(Bazzano) and MI03 (Onna), (d) PAGA3 (Paganica) and SG5 (S. Gregorio), (e) AQ1 and AQ2

(Working Group ITACA, 2010; Cercato et al. , 2010; Gruppo di Lavoro MS-AQ, 2010). Locations

are in Figure 3.

Figure 5. Geological cross-sections reported in Figure 3: (a) section 1 and (b) section 2 are inferred

from geological and geotechnical studies (Working Group ITACA, 2010; Cercato et al., 2010;

1

Gruppo di Lavoro MS-AQ, 2010); (c) section 3 is redrawn from Bosi and Bettini (1970) using the

same geological units of the other sections. Labels refer to the legend of Figure 2. Toponyms and

location of the seismic stations and the velocity profiles are also shown.

Figure 6. Top panels show HVSR at three different reference sites: (a) MI01, (b) MI04 and (c)

GFZ5. Lower panels are the horizontal SSR computed for (d) AQ03 and (e) AQ04 respect to the

three different reference sites (red for GFZ5, blue for MI01, green for MI04) compared with their

HVSR (black).

Figure 7. Spectral ratios at MI05: SSR on (a) horizontal and (b) vertical components (respect to

GFZ5); (c) HVSR.

Figure 8. (a) HVSR and (b) SSR respect to GFZ5 for selected sites (average and standard

deviation); plots on rows (1) refer to northern sites (section 1 of Figure 5) and rows (2) to southern

sites. In each plot is the number of seismograms used for the spectral ratio average (#).

Figure 9. SSR (mean and one standard deviation) computed for the 13 sites of this study,

superimposed to the geological map (see Fig. 3 for description). For three sites (MI02, MI03 and

GFZ01) HVSR is shown. The scale is the same for all the plots (0 – 10), the frequency range is 0.1-

20 Hz. For each station colored circles correspond to a resonance frequency range (see the legend),

the dimension being proportional to the amplification. The squares inside the circles corresponds to

the secondary resonance frequency, if any.

Figure 10. Azimuthal dependence of the average HVSR's computed at AQ06 for events located in

three distinct areas: 20 km NE from the Aterno valley (18 earthquakes, black circles in Figure 2)

(solid, grey line), 10-15 kilometers SE from L’Aquila (solid, black line) and the entire Aterno

Valley area (dotted, black line).

1