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Local variability of the ground shaking during - Mw

6.3): the case study of Onna and Monticchio villages Di Giulio G.1, Marzorati S.2, Bergamaschi F.3, Bordoni P.1, Cara F.1 2,Ladina C.2, Massa M.2 a

3, Cogliano R.4, Cultrera G.1,Fodarella A.4, Luzi L.2, Milana G.1, Pucillo S.4, Riccio G.4

Istituto Nazionale di Geofisica e Vulcanologia (INGV) (1) Via di Vigna Murata 605, 00143 Roma (2) Via Bassini 15, 20133 Milano (3) Via Uguccione della Faggiuola 3, 52100 Arezzo (4) Sede Irpinia, 83035 Grottaminarda, Avellino

Corresponding author:email: giuseppe.digiulio@ingv.it, Tel. +39/0651860613, Fax +39/0651860507

AbstractThe 2009 Mw 6.3 Aquila event caused extensive damage in the quila and in some small towns in its vicinity. The most severe damage was recognized quila town along the Aterno river valley. Although building vulnerability and near-source effects are strongly responsible for the high level of destruction, site effects have been invoked to explain the damage heterogeneities and the similarities between the 2009 macroseismic field with the intensities of historical earthquakes. The small village of Onna is settled on quaternary alluvium and suffered an extremely heavy damage in the masonry structures with intensity IX-X on the Mercalli-Cancani-Sieberg (MCS) scale. The village of Monticchio, far less than 1.3 km from Onna, is mostly situated on Mesozoic limestone and suffered a smaller level of damaging (VI MCS).In the present paper, we analyze the aftershock recordings at seismic stations deployed in a small area of the middle-Aterno valley including Onna and Monticchio. The aim is to investigate local amplification effects caused by the near-surface geology. Because the seismological stations are close together, vulnerability and near-source effects are assumed to be constant. The waveform analysis shows that the ground motion at Onna is systematically characterized by large high-frequency content. The frequency resonance is varying from 2 to 3 Hz and it is related to alluvial sediments with a thickness of about 40 meters that overlay a stiffer Pleistocene substrate. The ground motion recordings of Onna are well reproduced by the predictive equation for the Italian territory.

- site effects Onna seismic microzoning - ground motion prediction equations

1 Introduction

On 6 April 2009 an earthquake of Mw 6.3 occurred at 01:32 UTC in the central Apennines (Abruzzo region, Italy), (about 70000 inhabitants). The event (Fig. 1) is associated to the normal faults system documented in the area (Galadini and Galli, 2000; Chiarabba et al., 2009); the surface co-seismic ruptures were consistently aligned along the Apennine direction (Falcucci et al., 2009; Emergeo Working Group, 2010). The main up-dip rupture successively proceeded

*Manuscript

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towards SE (Pino and Di Luccio, 2009). The seismic activity also affected an echelon secondary normal fault on the north-west, for a whole aftershock area extending more than 35 km (Fig. 1). The permanent static displacement measured by GPS receivers basin showed a vertical maximum subduction of about 15 cm, observed SE of the epicentre (Anzidei et al., 2009; Cheloni et al., 2009). Avallone et al. (2010) estimated the co-seismic dynamic displacement on the causative fault using very high-rate GPS data. The maximum peak-to-peak dynamic displacement was of about 20 and 40 cm for the vertical and horizontal component, respectively. The fault parameters and co-seismic slip distribution have been estimated by Atzori et al. (2009) and Cirella et al. (2009) using a joint inversion of SAR interferometric and GPS data, and strong motion and GPS data, respectively. These authors assess a maximum slip of about 1 m that Regarding the maximum horizontal peak ground acceleration (PGA), it was 0.66 g measured by a near-field station (AQV) located approximately (see Fig. 1). AQV is belonging to the National Strong-motion Network (RAN) owned by Italian Department of Civil Protection (DPC) and it is installed on recent alluvium in the middle of Aterno valley (Ameri et al., 2009). Using an available cross-hole data and following the Eurocode 8 (EC8) building code (CEN, 2004), AQV has been classified as soil class B (see http://itaca.mi.ingv.it/ItacaNet and http://esse4.mi.ingv.it). The Mw 6.3 earthquake was additionally recorded at two stations both of soil class B bi et al., 2010). The first station (AQU) is installed in the Spanish castle of the city and belongs to the MedNet network (http://mednet.rm.ingv.it/data.php) and the horizontal PGA was of 0.31 g. The second station (AQK managed by RAN) recorded a PGA of 0.35 g. The available time series of the mainshock provided a vertical static displacement of about 15 cm (subsidence) for both AQU and AQK (Avallone et al., 2010). The 2003 national seismic code (OPCM 3274/2003) for seismic design classified the Italian territory in four zones with zone 1 indicating the highest seismic hazard. The

reference PGA between 0.15 and 0.25 g on rock. The O.P.C.M 3519 delivered on 20 April 2006, established the seismic hazard map of Italy produced by INGV (Working Group 2004) as the official reference map of the actual design code (NTC 2008). In this reference map, the L'Aquila area is characterized by a 10% probability in a time-interval of 50 years (return period of 475 years) of exceed PGA values (maximum horizontal acceleration on rock) between 0.250 and 0.275 g. The Mw 6.3 event caused nearly 300 causalities, more than 39000 homeless and strong damage (located about 1.5 km NE to the epicentre; Fig. 1) and in the surrounding villages. a Mercalli-Cancani-Sieberg (MCS; Sieberg 1930) intensity of VIII-IX. The maximum assessed intensity was IX-X MCS occurring in the small villages of Onna and Castelnuovo. The damage pattern is mainly distributed in a NW-SE direction according to the orientation of the Aterno River Valley (Galli and Camassi, 2009; GEER, 2009). After the macroseismic surveys, a role of site effects in the damage distribution has been supposed, especially for villages that are settled on soft alluvial deposits of the Aterno River Valley. Tertulliani et al. (2009) discussed the high similarities between the historical damage patterns from available catalogues (e.g. CPTI Working Group, 2004) and the 2009 macroseismic field, with the strongest intensities documented at the villages displaced

In particular the damage of Onna was also extremely heavy during the 1461 and 1762 earthquakes suggesting at this site local amplification phenomena.

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Indeed it is known within the seismology-engineering community that site effects can amplify the ground motion. Variation of local geology can produce significant dissimilarity of the damage level at sites that are very close one to the other, with amplification phenomena expected in soft-soil areas. The other two effects potentially affecting the damage distribution are source effects and vulnerability. In general, a very short spatial distance between near sites implies that source effects are reasonably uniform. During the 2009 sequence a significant role of seismic vulnerability has been observed in many historical centres, with heavy damage and collapses concentrated in the unreinforced masonry buildings including ancient churches (Aydan et al., 2009; EERI, 2009). The more recent buildings, developed during the last few decades and with the presence of reinforced concrete (RC) and good masonry, in general suffered from slight to moderate damage (Celebi et al., 2010).In this paper we investigate site effects in a small area including three very close villages (Monticchio, Bazzano, and Onna) of the middle-Aterno River valley (Fig. 2). The seismic stations considered in this paper were installed for the evaluation of local amplification phenomena (Bergamaschi et al., this issue) as well as in the framework of a national project (C.A.S.E project, where C.A.S.E stands for Anti-seismic, Sustainable and Environment-Friendly Buildings) aimed at the construction in the epicentral area of seismic-isolated buildings. Although the three villages are within a distance of 2 km from each other and the historical masonry buildings are supposed to share the same quality and characteristics, they suffered a different degree of damage (Galli and Camassi, 2009). The highest destructions were observed in Onna, whereas the lower damage level occurred in the neighbouring village of Monticchio (Fig. 2). The village of Onna suffered IX-X MCS intensity. Onna was composed mainly of 2-3 story unreinforced masonry houses with a collapse rate of 80% whereas very few reinforced concrete structures at the margins of the village showed minor damage (GEER, 2009). Heavy damage was also observed for two RC bridges near Onna. The MCS intensity at Monticchio, about 1.3 km southwest of Onna, was VI. Monticchio suffered almost no damage in its unreinforced 2-3 story masonry houses concentrated in the historical part of the village and no heavy damage to RC residential structures. Bazzano, about 2 km northwest of Onna, suffered intermediate damage (VIII MCS). The industrial structures in the newer part of Bazzano, about 1 km west of Onna, reported many collapses of wall panels and roofs. We analyse the aftershocks recorded by stations shown on Fig. 2 by means of spectral techniques. We compare additionally the observed ground motion at Onna with the predictive equation (GMPE) valid for the Italian territory.

2 The study area

Aterno river valley (Figs. 1 and 2) is worthy of note in terms of site effects

2005) whereas the near-surface geology of the Aterno valley is characterized by lacustrine sediments. The basement of the area is a limestone formation, deposited from Jurassic to Miocene age, which largely outcrops along the valley flanks.

associated to quaternary paleolandslides and is bounded southwest to the Aterno River that flows 50 m below. The Pleistocene deposits of the fluvial terrace are composed of breccias with clasts reaching up to

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several meters in size with varying degree of cementation. Breccias include limestone s of Megabrecce is some

tens of meters, lying on lacustrine sediments composed mainly of silty and sandy layers. The Aterno river valley is a tectonic basin elongated in NW-SE direction (Figs. 1 and 2) and bounded by predominantly NW-SE-striking and SW-dipping active normal faults (Bagnaia et al., 1992). This basin consists mostly of Pleistocene lacustrine deposits (Bosi and Bertini, 1970) that are usually covered by Holocene soils; the result is a complex mixture of clay, silt, sand and gravel (Monaco et al., 2009). Near the valley flanks, large alluvial fan and debris material from the hills are commonly

basin by a geological and morphological analysis along with a gravity survey (Blumetti et al., 2002). The lacustrine sediments likely reach a thickness of around 250 m in the centre

frequency at about 0.6 Hz (De Luca et al., 2005; Milana et al., this issue) whereas in the Aterno river valley the fundamental frequencies can increase up to 5 Hz (Bertrand et al., 2009; Bergamaschi et al., this issue). Although geological, geophysical, and geotechnical investigations are still in progress, some preliminary indications can be inferred for the Onna area. Studies devoted to seismic microzonation have been financed by Italian Department of Civil Protection (DPC) after the Mw 6.3 mainshock ( Microzonazione Sismica

a , 2010). The epicentral area has been organized in different macroaree and one of the tasks was focused on the geotechnical/geological characterization of the main units within the Aterno river valley. The are within the macroaree 3 and 5 (scientific coordinators of these macroaree P. Boncio, and G. Cavuoto & M. Mancini, respectively). Sections the Aterno valley sampling from NE of Onna to Monticchio (Fig. 2). Fig. 3 shows the results inferred from the seismic microzonation (DPC, 2010). The cross-sections indicate in proximity of Onna -from top to bottom- a very thin layer of Holocene deposit (unit 3Paganica fan delta (Compagnoni et al., 2010). It is superimposed to a fluvial-lacustrine sediment (unit 2) about 50 meters thick that overlies a lower Pleistocene conglomerates substrate (unit 1) with a supposed thickness of about 100 m (Fig. 3). The deep basement is assumed to be the limestone/flysch that outcrops in the historical part of Monticchio that is built on a gentle slope of a mountain. The stratigraphyc column extracted from s (DPC, 2010) evidences that the quaternary filling of Aterno valley is a complex mixture of gravels, sands and silts of lacustrine deposit. The complexity of the shear-velocities profile in the lacustrine formation (Fig. 3) is documented by a downhole survey within Onna (Working Group MS-AQ 2010). In such environment it is also possible the presence of very thin layers showing velocity inversions at various depths. Unfortunately the bedrock depth was not resolved by the downhole survey. An array experiment collecting ambient vibrations has been additionally performed very close to the downhole position (Agreement INGV-DPC 2007-2009, Project S4, responsible Stefano Parolai). The inversion of surface waves applying frequency-wavenumber and spatial autocorrelation techniques provides a depth of the soft-to-bedrock interface at about 40 m, and a shear-velocity of the uppermost 30 m (Vs30) of 380 m/s (soil class B) for the best fitting model (see the MI03 station monograph in

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http://itaca.mi.ingv.it/ItacaNet). The downhole data provide a consistent value of Vs30.

3 Data Acquisition and Methods of Analysis

Based on aftershocks analysis, we investigate the seismic amplification in the area of Fig. 2. We focus on seven seismological stations of Table 1 installed starting from the 7 April 2009 in the vicinity of Onna (Fig. 2). All the stations of table 1 recorded earthquakes with the exception of NO03 that was deployed for a short time (few hours) in order to collect ambient seismic noise. Sampling rate was 100 samples per sec. The instruments have been supplied by batteries or solar panels and connected to a GPS antenna for time synchronization. Station MI03 was deployed in the western part of Onna (Fig. 2) on fluvial-lacustrine deposits of the Aterno valley. Station PC04 was installed on Mesozoic limestone outcropping in the uppermost Monticchio. Stations PC05, NO03 and AQ02 were spread in the recent lacustrine deposits in proximity of Monticchio. The remaining stations of Table 1 (Fig. 2) were installed in proximity of Bazzano (AQ01 and MI07) characterized probably by surface gravel deposits of the Paganica alluvial fan with unknown thickness over a sandstone/marly-siltstone formation. PC04 was used as reference site to estimate the relative amplitude at close sites using classical reference methods (Tucker and King, 1984). We integrate the horizontal spectral ratios using a bedrock site (HHSR) with single-station methods computing the spectral ratio of horizontal component over the vertical one (HVSR; Lermo and Chavez-Garcia, 1993). In order to derive the HHSR, we selected 48 local seismic events in the period April 29th- May 7th 2009 with magnitude ranging from 2.3 to 3.8 (Table 2) and with a good signal-to-noise ratio in the analyzed frequency band. This selection allows having contemporary records at the reference site PC04 and at the remaining stations. We computed the Fourier spectra at the three components of the selected events on a 10 s time-window starting from the P arrival including the most energetic part of the signal. The time-windows were first de-trended and tapered with a 5% Hanning window. The resulting Fourier spectra were processed using an arithmetic smoothing algorithm through a moving window. Then the horizontal spectral ratios for the NS and EW components were divided by the horizontal reference spectrum (HHSR) or by the vertical component spectrum (HVSR). Finally the spectral ratios were geometrically averaged over the resulting ensemble of the selected events. We make also use of seismic ambient noise to compute the H/V spectral ratios (HVNSR; Nogoshi and Igarashi 1971; Nakamura, 1989) between the horizontal components and the vertical component of ambient vibration recordings. HVNSR is able to map the resonance frequencies when a strong impedance contrast is present between soft soil and basement. In general HVNSR, similarly to the HVSR based on earthquakes, fails in estimating possible amplification at frequency higher than the fundamental resonance. However, under the assumption of 1D wave propagation, the resonance frequency f0 is linked through the simple relation f0=Vs /4H to the average shear-wave velocity (Vs) and to the sediment thickness (H) of the soft soil. We calculated HVNSR dividing the hourly recordings in 60 s time-windows and removing the time windows contaminated by transients often associated with human activity using an anti-trigger algorithm. We calculated HVNSR by means of geopsysoftware (www.geopsy.org) implemented in the framework of a research European project (SESAME, 2004). For every selected time window, the Fourier amplitude

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spectra are smoothed following Konno and Ohmachi filter (1998), with a bandwidth equals to 40. The mean representing the horizontal amplitude spectra is then computed combining the spectra relative to the two horizontal components. The final HVNSR and the associated standard deviations are obtained by geometrically averaging the H/V spectral ratios from all the selected time-windows.

4 Results and Discussion

During the experiment three strong aftershocks (magnitude 5.4, 5.1 and 5) were recorded by our stations. The Ml 5.4 aftershock occurred the 7th of April with an epicentre localized near Onna whereas the remaining two aftershocks (Ml 5.1 and 5) occurred the 9th of April in the Campotosto north-west area at larger epicentral distance (see stars symbols in Fig. 1; http://iside.rm.ingv.it ).Both the Ml 5.1 and Ml 5 aftershocks of the 9th of April were recorded simultaneously by MI03, AQ01 and AQ02. The recordings of the Ml 5.1 earthquake were slightly biased at AQ01 and AQ02 by digital data-saturation. However, the comparison of time-series indicates that MI03 is more energetic at high frequencies than the sites AQ01 and AQ02 (Fig. 4). Furthermore the seismograms of the horizontal components suggest a role of long-period waves at AQ02 that is settled on lacustrine sediments near Monticchio. It is worthy of note that the response spectra of MI03 show a large spectral content in the horizontal components at T < 0.3 s; energy in this period range is missing at AQ01 and AQ02 sites (Fig. 4). The large ground motion of MI03 at high frequencies observed from the strongest aftershocks is confirmed by weak-motion seismograms (see aftershocks in Fig. 2). The smallest amplitude is systematically shown by PC04 being a stiff site deployed on limestone outcropping in historical part of Monticchio; whereas PC05 on alluvial deposits nearby Monticchio shows intermediate amplification.

The results of the conventional spectral ratio approach (HHSR) using PC04 as reference site are illustrated in Figure 5. MI03 shows a spectral peak around 2.5 Hz with a relative amplification level of 5 on the average. The shapes of spectral ratios indicate that a magnification is present in a considerable frequency band (up to 10 Hz).The HHSRs of AQ01 indicate a spectral peak around 3 Hz with a relatively weak amplitude (about 2 on the average). The HHSR curves of MI07, although more ambiguous to interpret, do not exhibit clear spectral peaks. PC05, about 300 m far from PC04, shows a resonance frequency between 1-2 Hz and the amplified frequency band is up to 6 Hz with a relative factor of about 3. Because AQ02 do not have simultaneous recordings with PC04, the HHSR of AQ02 is shown in Fig. 5 using three reference sites within the Aterno valley already adopted by Bordoni et al. (2009). The stiff sites are VGFZ05, MI01 and MI04 (see Figure 2) and they were expected to share the same geological unit (limestone) of PC04. Apart the discrepancies among the spectral shapes related to the choice of the reference site, the HHSRs of AQ02 are consistently characterized by a low-frequency (~1 Hz) peak with an average amplification level of 3.

The spectral ratios of the vertical component (VVSR; see Figure 5) show also a relative amplification at sites within the Aterno valley (AQ01, AQ02 and MI03) between 1 and 2 Hz. Interestingly the only site with no relative amplification on

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vertical component is MI07 located on a marly-siltstone formation. This evidence of amplification phenomena at soft sites on vertical ground motion could be connected directly to a P-velocities contrast between the quaternary sediments of the basin and the stiffer conglomerate outcrops. For AQ01 and MI03 sites, the spectral ratio of the vertical components show a peak at about 1 Hz whereas the resonance frequency derived from the horizontal components is at higher values (i.e. around 2-3 Hz, see Figure 5). This observation contrasts with the fact that for the soft resonant layer the average shear-wave velocity (Vs) must be lower than the average P-wave velocity (Vp). It could be explained considering a different thickness of the resonant layer in the two cases; with the Vp contrast occurring deeper than the Vs contrast. In Figure 6, the spectral ratios are estimated through the single-station methods using both earthquakes and ambient seismic noise (HVSR and HVNSR, respectively). The HVSR and HVNSR of bedrock site PC04 are almost flat without spectral trough or peak. The H/V ratios of MI03 indicate clearly the resonance frequency at about 2.5 Hz consistently with the one estimated by HHSR. A good agreement between the shape of the HHSR and H/V curves is also found near Bazzano at AQ01 and MI07. AQ01 and MI07 are characterized by similar spectral curves, without the narrow spectral peak depicted at the close MI03 site. The results of Fig. 6 are integrated by the station NO03 deployed for a short time (see Table 1) with the purpose to record seismic ambient noise. The resonance frequencies yielded within the Monticchio plain at PC05, NO03 and AQ02 decrease from ~2 Hz to 1 Hz, respectively (see Fig. 6). This variation between PC05 and NO03-AQ02 is consistent with a thickening of lacustrine deposits proceeding towards north in the alluvial river valley.

4.1 Onna

In a second step of the analysis, we focus on MI03 (Onna) to investigate i) the reliability of the resonance obtained by standard spectral methods, ii) a possible role played by directional resonances, and iii) the agreement between ground motion recordings at Onna and the predictive models of the Italian territory.

To answer to issue (i) we investigate the variations (over distance and time) of the resonance frequency derived from seismic noise. Figure 7 compares the HVNSR of MI03 with additional four short-time measurements collected about 10 months later. MI03 located southwest Onna shows a resonance of 2.3 Hz. The resonance frequency in the northern part of the village increases over 3 Hz suggesting a thinning of the fluvial-lacustrine deposit towards north, consistently with the geological setting. We additionally considered three days of continuous recordings of seismic noise for MI03 (from 26 May 01:00 UTC to 29 May 00:00 UTC). We processed time-series computing the probability density function (PDF) for a set of power spectral densities (PSD), following McNamara and Buland (2004). The time-series is divided by means of running time-window where each window has a length of 120 s allowing an overlapping of 50%. PSD was computed on a length of 4.5 minutes obtaining 496 estimates for each component of motion. Figure 8 on the left shows the probability density function (PDF) of the power spectral density (PSD) computed within the three days and plotted using a colour scale proportional to the spectral energy. The PDF shows clearly a bimodal pattern in the low- and high-frequency range; the boundary between the two distributions is at about 0.6 Hz (see Fig. 8). As discussed in Marzorati and Bindi (2006) for Italian seismic stations, the energy variation of the PDF spectrum at low frequency is related to the occurrence of climatic changing (i.e.

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marine storms), whereas at high frequencies the noise level is affected by anthropic activity. In spite of the variability of the noise levels considering the spectrum of each single component (not shown here), the ratios of the spectrogram of the horizontal components with the spectrogram of the vertical component (NS/V and EW/V) show that the resonance frequency at about 2.5 Hz is very stable with time.

In principle, preferential directions of the ground-motion (issue ii) can be explained in different ways invoking the topography (Pischiutta et al., 2010; Massa et al., 2010), or a focusing mechanism caused by faults (Cultrera et al., 2003) or basin structure (Davis et al., 2000), or simply in terms of man-made activities (Cara et al., 2010). In order to highlight site-specific directional effects at MI03, we compute the horizontal-to-vertical spectral ratio (HVSR) as a function of frequency and direction of motion as introduced by Spudich et al. (1996). We divided the horizontal plane into a set of directions uniformly spaced at intervals of 5°, from 0° (north) to 180° (south). The H/V for the ensemble of rotated horizontal components is computed searching for azimuth variations assuming a time-window of 10 s after the S arrival. We consider as dataset the 48 small-magnitude earthquakes of Table 2. The azimuth pattern of HVSR in Fig. 9 shows a very stable behaviour, with no dependence on the rotation angle and therefore with no evidence of directional resonance on weak motion. In contrast, directional effects emerge using the three strong aftershocks recorded at MI03 (Fig. 10). The shapes of HVSR of the Ml 5.4, 5.1 and 5 events are quite consistent although the HVSR of the Ml 5.4 shows the spectral peak occurring at a lower frequency (~1.8 Hz). An additional peculiarity of the Ml 5.4 is the great content at long periods (f < 1 Hz) on the EW component, consistently observed on the EW/V ratio and on the azimuthal pattern. It is caused by a great pulse recognized on this component by the low-pass filtered seismogram (see inset of Fig 15). Similar strong pulses on the horizontal components were already recognized at strong motion stations available in the Aterno valley by Pacor et al. (2009), Bindi et al. (2009a) an(2010). The pulse-like signals at the near-source stations document rupture directivity effects (Chioccarelli and Iervolino, 2010). A further directional effect for the Ml 5.4 event is recognized at about 2 Hz in a NW-SE direction; such direction is well confirmed by the Ml 5.1 and Ml 5 events within 2-3 Hz (Fig. 10). A focusing of energy of the horizontal ground motion approaching the f0 and along the NW-SE orientation could be consistent with the elongation of the Aterno River valley (on the average 140° from north). Although this could suggest a propagation effect related to the geometry of the valley in case of strong events, we cannot exclude a role played by the source, i.e. of directivity related to the rupture on the fault.

The last issue (iii) regards the comparison between the ground motion recorded in Onna and the available predictive models. The maximum horizontal peak ground accelerations (PGHA) and the response spectra ordinates for MI03 were compared to recent ground motion prediction equations (GMPEs) developed for Italy (hereinafter ITA08; Bindi et al., 2009b). ITA08 equations were calibrated considering Italian earthquakes (4.0 Mw 6.9) recorded by RAN strong-motion sites in the period 1972-2004. For the aim of the paper, ITA08 coefficients were slightly modified (ITA08-mod) introducing in the dataset the recordings of the (Mw down to 3.5 and distance up to 300 km). Regarding the site classification, ITA08 follows the scheme proposed by Sabetta and Pugliese (1996) that includes three classes: C0 (rock), C1 (shallow alluvium with thickness < 20 m) and C2 (deep alluvium, thickness > 20 m). Considering the discussion of the previous paragraphs,

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MI03 can be classified as deep alluvium (class C2). In terms of EC8 (CEN, 2004) site classification and considering the Vs30 obtained in the framework of microzoning activity (Vs30 of 380 m/s, see Fig. 3), the site under study is associated to soil class B (360 Vs30 800). The goodness of fit considering 21 events (3.5 Mw 5.6) recorded by MI03 (deep alluvium site) and ITA08-mod predictions (for class C2) was evaluated in term of PGHA using the approach of Spudich et al. (1999). This method defines the residuals as difference between the logarithms of observed and predicted values in a likelihood sense. The distribution of PGHA residuals is reported in the top panel of figure 11. In the middle and bottom panels, the best-fitting functions obtained by the residuals distribution with respect to distance R (middle) and magnitude M (bottom) are also shown. The total bias, defined as the mean value of the residual distribution, was found equal to -0.08 indicating a negligible overestimation of the predictions. The good agreement between MI03 data and ITA08-mod evidences as the values recorded at Onna are not surprising, at least in term of PGA. The dependence of the fit between MI03 and ITA08-mod with increasing periods was further evaluated through the inter-station error (Figure 12), a parameter that measures the correlation for different earthquakes data recorded at the same site (Bindi et al., 2009c). The inter-station error is computed comparing real and predicted acceleration response spectra (5% damping) for periods from 0.04 s to 4 s. The random effect model (Abrahamson and Youngs, 1992) was applied for calculation; more details about the methodology can be found in Bindi et al., 2009b and 2009c; and Massa et al., 2008. The inter-station values (Figure 12), ever included in the interval 0.1,appear negligible for the considered periods. ITA08-mod tends to slight overestimate (negative error) up to 0.3 s, whereas the highest underestimation (positive error) is detected at 0.35 s (2.8 Hz) and this can be easily explained in terms of resonance amplification of the site. We additionally investigated the regional attenuation of the studied area by comparing the Italian average decay (ITA08-mod) to that obtained by considering only the MI03 recordings (3.5 Mw 5.6). The functional form used in Massa et al. (2008) was adopted for the regression. Figure 13, where an example for a Mw 5.0 is reported, shows a similar attenuation pattern between the earthquakes collected at MI03 and the predictive models.

In conclusion, the seismogram analysis of the station in Onna (MI03) shows systematically high-frequency amplification. The resonance frequency (f0) at MI03 is at 2.3 Hz and is very stable with time. Aftershocks of MI03 are characterized by a general agreement with the Italian predictive equation assuming a deep alluvium site; a thickness of soft deposits greater than 20 m is consistent with the geotechnical information provided by the microzonation activity for Onna (Fig. 3). The role of directional resonance is more questionable. No directivity evidences are observed by the weak motion analysis (Ml 3.5; Fig. 9), whereas on the strong aftershocks we note preferential direction of horizontal motion at low frequencies (~ 1 Hz) and near the f0 (2-3 Hz; see Fig. 10). The directivity effect is approximately EW at low frequencies, whereas is along the NW-SE orientation approaching the f0 (Fig. 10).

4.2 Response spectra (MI03 and AQV)

Both near-source effects and building vulnerability have likely contributed with site effects to the destruction of Onna during the mainshock. The aftershocks analysis documents ground-motion amplification for MI03 at high frequency (see Fig. 5) and

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this could suggest large PGA experienced by Onna. Applying the GMPE of Bindi et al (2009b), the estimated PGA at Onna, classified as deep-alluvium site, is 0.26 g for the Mw 6.3 event with an epicentral distance of 8 km. Including a standard deviation the PGA range is very large spanning from 0.12 to 0.6 g. In order to try a better assessment of PGA, it is interesting to compare the seismic response of Onna with the behaviour of strong-motion site that recorded the maximum PGA during the mainshock (0.66 g at AQV). MI03 and AQV are situated approximately at the same

AQV is about 12 km NW far from MI03. Both sites are characterized by similar values of Vs30 and bedrock depth, consequently the resonance frequencies is within the same frequency range (2-3 Hz; Fig. 14). The available cross-hole at AQV (Fig. 14) shows the bedrock at a depth of 50 m, and a Vs30 of 460 m/s according to the soil class B. The three strongest aftershocks (Ml 5.4, Ml 5.1 and Ml 5) were recorded simultaneously at MI03 (Onna) and AQV. We then compare the acceleration response spectra with 5% damping for the three aftershocks normalized for an hypocentral distance of 20 km. Figure 15 indicates for MI03 and AQV that i) the horizontal components have larger energies than the vertical motion up to 1 s; ii) the magnification is mostly at low periods (T < 1 s). The persistent EW energy of MI03 observed at long periods (T > 1 s) for the Ml 5.4 event is caused by the strong pulse-like signal within few seconds after the S arrival (see inset in Fig. 15). Impulsive signals on the horizontal components recorded at near-source station are related to rupture directivity effects and can increase the seismic action on structures (Chioccarelli and Iervolino, 2010). However, the acceleration response spectra of Figure 15 indicate a role of high-frequency energy at both sites during the three strongest aftershocks. The largest spectral peaks at low periods are observed at AQV, independently from the source azimuth as well as the epicentral distance of the events. Indeed the Ml 5.4 aftershock, with an epicentre situated SE to MI03 (see Fig. 3), still shows the largest spectral values at AQV. Spectral energy of AQV is especially prominent at very low periods (about 0.1 s). Such amplification is likely related to the presence of the very uppermost low-velocity layer (silty clay) documented by the cross-hole data (Fig. 14; note also the secondary H/V peak at 10 Hz). The high-frequency amplification due to the uppermost geology of AQV is consistent with the maximum PGA recorded at this site during the Mw 6. uake. The comparison between AQV and MI03 (Fig. 15) suggests that PGA experienced by Onna did not exceed the value recorded at the strong-motion site of AQV (i.e. 0.66 g). Even though the PGA at MI03 was lower than the one recorded at AQV, we believe that it was an important factor for the destruction of unreinforced masonry houses in

. Different investigations, such as analysis based on , are needed to clarify the PGA level and the possible peculiarities of ground-motion in Onna during the Mw 6.3 earthquake.

5 Conclusions

The purpose of this work was to assess the effect of local geology on ground motion Aquila earthquake. We investigated through an aftershock data

analysis a small area of the middle-Aterno river valley where damage heterogeneities were observed. earthquake, Onna experienced extensive destruction whereas the close villages of Bazzano and Monticchio suffered intermediate and low

11

damage. The differences in the damage distribution within the investigated area are explained in terms of near-surface geology: Onna is built on fluvial-lacustrine sediments of the Aterno valley where Monticchio on Mesozoic limestone and Pleistocene silts.Seismic data indicate that the Onna area is exposed to high-frequency ground-motion amplification and this can partially explain the severe damage during the 2009 mainshock. In detail, the seismic station PC04 is settled on Jurassic limestone outcropping in the historical part of Monticchio and can be regarded as a stiff site. Stations within Monticchio plain (PC05, NO03 and AQ02) show a variation of f0 related to the thickening of alluvial deposits proceeding towards the NW direction. The largest low-frequency (f < 1 Hz) amplification has been observed at AQ02. Sites near Bazzano (AQ01 and MI07) are settled on a sandstone and marly-siltstone formation and show weak amplitudes of the spectral ratios. The resonance frequency at MI03 (Onna) is 2.3 Hz. At this frequency, the HHSR approach using PC04 as reference site shows a relative ground-motion amplification for Onna of about 5 (Fig. 5); the magnification of horizontal components extends up to 10 Hz in a frequency range of engineering interest. Although building vulnerability, near-source effects, and possible phenomena of soil-structure interaction must be taken into account in relating damage to ground motion, the amplification at Onna inferred by weak-motion data analysis is consistent with the observed damage, in particular if compared to the low level of damage observed at Monticchio during the mainshock (IX-X and VI MCS intensity, respectively). Further seismic stations deployed in a second time step by other authors (Tortora, 2010) and sparse noise measurements within Onna confirm a slight variability of the f0 up to about 3 Hz (see Fig. 7). On-going studies are in progress in order to give a more complete 2D interpretation of the seismic response of Onna. Consistently with the available down-hole data and with the results of surface waves analysis (Fig. 3), we can simplify the soil model assuming an average Vs of 400 m/s for the near-surface sediments of Onna. The thickness of the soft filling is estimated in 40 m through the quarter of wavelength 1D approximation (f0=Vs/4H) and using a f0 of 2.5 Hz. Therefore the lower-Pleistocene formation composed of gravels and conglomerates in silty lacustrine matrix can be considered as the seismic bedrock (i.e. unit 1 or equivalently LACa2 deposit of Fig. 3), with the f0 that is related to the velocity contrast between this formation and the relatively young alluvial deposits.

ACKNOWLEDGMENTS We are grateful to Associate Editor and two anonymous reviewers for their constructive comments. The article benefited from comments by Antonio Rovelli. This research was partially conducted with funds Microzonazione Sismica

and C.A.S.E DPC projects.

12

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Table 1. List of seismological stations analyzed in the present paper.

Station code

Latitude (degree)

Longitude (degree)

Position Equipment (digitizer and

sensor)

Recording period Start End

AQ01 42.331 13.468 Industrial area of Bazzano

Reftek72A with Le3d-5s

8 april 2009

13 may 2009

AQ02 42.320 13.458 Monticchio (on alluvial deposits)

Reftek72A with Le3d-5s

8 april 2009

20 april 2009

MI03 42.327 13.476 Onna Reftek130 with Le3d-5s and episensor

7 april 2009

9 June 2009 (not working from 13 to 27 April)

PC04 42.317 13.466 Monticchio (on limestone)

Quanterra330 with Le3d-5s and episensor

29 april 2009

7 may 2009

PC05 42.317 13.462 Monticchio (on alluvial deposits)

Quanterra330 with Le3d-5s

29 april 2009

7 may 2009

MI07 42.336 13.446 Bazzano (sandstone/marly

limestone)

Reftek130 with Le3d-5s and episensor

27 April 2009

9 June 2009

NO03 42.319 13.460 Monticchio (on alluvial deposits)

Reftek130 with Le3d-5s

Noise recording of about 1 hour the 29 april 2009 and of about 2 hours the 1 may 2009

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Table 2. List of earthquakes collected contemporary at the reference site PC04 and at the remaining stations of Table 1. These events were used in the HHSR computation.

# Origin Time (UTC) Latitude (°)

Longitude (°)

Depth (km)

Magnitude ML

1 29/04/2009 22:09:32.000 42.495 13.387 10 2.4 2 29/04/2009 22:41:04.000 42.358 13.342 10 2.5 3 29/04/2009 23:24:07.000 42.374 13.329 11 2.4 4 30/04/2009 04:34:33.000 42.491 13.376 9 2.5 5 30/04/2009 06:22:31.000 42.307 13.449 10 2.3 6 30/04/2009 07:18:57.000 42.457 13.335 16 2.5 7 30/04/2009 09:13:55.000 42.365 13.416 10 38 30/04/2009 09:35:13.000 42.403 13.338 11 2.3 9 30/04/2009 11:59:39.000 42.465 13.313 13 2.4 10 30/04/2009 13:01:01.000 42.361 13.364 11 3.5 11 30/04/2009 16:41:47.000 42.358 13.342 9 2.5 12 30/04/2009 17:12:11.000 42.459 13.33 12 2.4 13 01/05/2009 05:12:51.000 42.28 13.47 9 3.8 14 01/05/2009 08:08:24.000 42.265 13.487 10 2.5 15 01/05/2009 09:26:00.000 42.261 13.481 10 2.6 16 01/05/2009 13:30:32.000 42.407 13.399 10 2.3 17 01/05/2009 20:16:36.000 42.417 13.262 10 2.6 18 01/05/2009 20:34:44.000 42.329 13.495 10 3.3 19 01/05/2009 20:42:31.000 42.396 13.399 9 2.3 20 02/05/2009 02:21:02.000 42.496 13.358 10 3.2 21 02/05/2009 02:23:12.000 42.283 13.459 10 2.4 22 02/05/2009 05:21:23.000 42.338 13.384 11 2.7 23 02/05/2009 10:36:38.000 42.514 13.392 9 2.3 24 02/05/2009 10:58:29.000 42.289 13.471 10 2.3 25 02/05/2009 10:59:25.000 42.295 13.614 10 2.4 26 02/05/2009 15:43:07.000 42.319 13.561 6 2.6 27 02/05/2009 20:15:00.000 42.296 13.495 9 3.3 28 03/05/2009 00:23:48.000 42.345 13.429 17 2.3 29 03/05/2009 02:25:14.000 42.258 13.548 9 2.5 30 03/05/2009 02:47:26.000 42.29 13.488 10 2.3 31 03/05/2009 04:57:15.000 42.256 13.482 10 2.3 32 03/05/2009 05:14:43.000 42.365 13.39 10 3.2 33 03/05/2009 06:33:10.000 42.254 13.489 10 2.3 34 03/05/2009 13:07:02.000 42.281 13.435 8 335 04/05/2009 02:03:46.000 42.315 13.429 9 2.3 36 04/05/2009 02:22:46.000 42.501 13.399 10 2.6 37 04/05/2009 05:19:43.000 42.363 13.379 11 2.3 38 04/05/2009 05:47:05.000 42.449 13.449 10 2.4 39 04/05/2009 15:56:46.000 42.323 13.532 10 2.3 40 05/05/2009 02:20:20.000 42.299 13.427 11 2.7 41 05/05/2009 10:44:03.000 42.282 13.498 10 342 05/05/2009 18:03:41.000 42.27 13.509 10 3.1 43 05/05/2009 18:16:13.000 42.295 13.496 9 2.3 44 06/05/2009 17:52:10.000 42.458 13.376 13 2.6 45 06/05/2009 21:54:27.000 42.335 13.395 10 2.6 46 07/05/2009 05:38:14.000 42.259 13.518 9 2.9 47 07/05/2009 08:29:55.000 42.322 13.535 10 2.3 48 07/05/2009 09:40:59.000 42.356 13.352 11 2.3

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Figure Captions

Figure 1triangles show the seismic stations. The yellow stars indicated the 6 April mainshock and the three aftershocks considered in this paper (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 Ml 5). The white circles indicate the earthquakes with M 2.5 from April to December 2009. The filled orange circles show the events of Table 2 used for spectral analysis. The red lines and the green squares show the main faults of the Aterno River Valley and the historical earthquakes (from CPTI Working Group, 2004), respectively. The white square bounds the area shown in Fig. 2.

Figure 2 Top) Geological map of the Aterno River valley surrounding Onna (white box of Fig. 1). The triangles indicate the seismic stations; PC04 has been used as reference site in our analysis (VGFZ05, MI01 and MI04 are also stations settled on stiff sites). The red lines show the main faults. The yellow star shows the Ml 5.4 aftershock of the 7 April (origin time 17:47 UTC). The colour from white to

provided by the microzonation studies after the Mw 6.3 mainshock (Working Group MS-AQ, 2010). Bottom) As above where the topography is shown in grey tonality. The recordings of Ml 3.2 aftershock (EW component, origin time 2 May 2009, 2:21 UTC) are also shown. The seismograms amplitudes are normalized at the same scale.

Figure 3 Geotechnical characterization of the Onna area after the microzonation studies of DPC

-section of Figure 2 (simplified after DPC, 2010). Bottom) The column shows the stratigraphy inferred for MI03 station (Onna), and it was derived from

-section (DPC, 2010; see also Compagnoni et al., 2010). The shear-wave profile assessed for Onna is also shown. The dashed grey curve is the Vs inferred from a downhole survey carried out in Onna (by Terra Drilling Technology srl; Working Group MS-AQ, 2010). The continuous black curve reports the Vs derived from the inversion of surface waves applying array techniques based on frequency-wavenumber and spatial autocorrelation methods (http://itaca.mi.ingv.it/ItacaNet, see the MI03 station monograph, responsible Stefano Parolai).

Figure 4 Top) Ml 5 aftershock (2009/04/09/19:38 Ml) recorded simultaneously at AQ01, AQ02 and MI03. The seismograms amplitudes are normalized at the same scale. Middle panel) Seismograms of the top panel after a low pass filter (< 1 Hz). Bottom) Acceleration response spectra of the three components of the ground motions. Note the larger energy of MI03 at low periods.

Figure 5 H/H and V/V spectral ratios (HHSR and VVSR, respectively) computed on earthquakes using PC04 as reference site. The mean spectral ratio and the mean 1 standard deviation are plotted. The HHSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). The mean curves are plotted as thick lines. In case of AQ02 the two horizontal components were averaged, and the mean 1 standard deviation are plotted using three different reference sites (VGFZ05, MI01 and MI04 as red, blue and green curves, respectively; see text).

Figure 6 H/V spectral ratios using earthquakes of table 2 and ambient noise (HVSR and HVNSR, respectively). The mean H/V ratio and the mean 1 standard deviation are plotted. The HVSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). The mean curves are plotted as thick lines

Figure 7 The shadow area marks the f0 variation inferred from the HVNSR curves. The four noise measurements in the northern part of Onna (squares symbols) were collected in February 2010 (about 40 minutes of ambient vibration recordings at each site). Note the increase of f0 from 2.3 up to 3.3 Hz proceeding in the north direction. The positions of the down-hole survey (Working Group MS-AQ, 2010) and the array experiment are also shown (white circles; the resulting Vs profiles are reported in Fig. 3).

Figure 8 Left) Probability density function (PDF) of power spectra densities at MI03 station. PDFs are computed on three days of continuous recording for the three components of the ground motion.

19

Colour scale is proportional to the probability. The new high noise and the new low noise -models (black curves) of Peterson (1993) are also shown. Right) Ratio of the spectrogram of the horizontal components over the vertical one (NS/UP e EW/UP) considering three days of data recording. Colour scale is proportional to the amplitude of HVNSR.

Figure 9 HVSR at MI03 using the weak motions listed in Table 2. Left) The mean H/V ratio and the mean 1 standard deviation are plotted. The HVSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). Right) The HVSRs are plotted as function of azimuth with the colour scale that is proportional to the amplitude of the spectral ratio.

Figure 10 Left) HVSR computed at MI03 on the three strong aftershocks (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 M 5). The red and black curves indicate the EW/V and NS/V spectral ratio, respectively. Right) The HVSRs are plotted as function of azimuth with the colour scale that is proportional to the amplitude of the spectral ratio. Note the directional effects in a NW-SE orientation in correspondence of the resonance frequency of the site (2-3 Hz).

Figure 11 Top) PGHA residuals (logarithm of observations minus logarithm of predictions) estimated considering MI03 recordings (21 records with magnitude down to 3.5) versus ITA08-mod predictive model. Middle) Dependence of MI03 residuals versus distance (epicentral distance or Joyner Boore distance if available; Joyner and Boore, 1981) expressed as slope (R) curve (black solid line). Bias and its standard deviation are also indicated (solid and dotted grey lines respectively). Bottom) Dependence of MI03 residuals versus magnitude (Ml or Mw if available) expressed as slope (M) curve (black solid line). Bias and its standard deviation are also indicated (solid and dotted grey lines respectively).

Figure 12 Inter station errors (grey squares) computed by acceleration response spectra (5% damping) calculated from MI03 recordings and those predicted by ITA08-mod. The error for period 0.0 s is relative to PGHA.

Figure 13 Comparison between ITA08-mod and MI03 curves calculated for PGHA and a target event of magnitude 5.0. The MI03 curve is obtained considering only events with magnitude higher (or equal) than 3.5, included in a distance interval ranging from 10 km to 40 km (epicentral or Joyner Boore distance). Mw are available in http://itaca.mi.ingv.it.

Figure 14 Cross-hole and HVNSR obtained at the AQV site (AQV position is shown in Fig. 1). The resulting values of Vs30 and f0 are 460 m/s (soil class B) and 3 Hz, respectively.

Figure 15 Acceleration response spectra of the three strong aftershocks (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 Ml 5) recorded simultaneously at MI03 and AQV (left and right panel, respectively). The hypocentral distance of the events is indicated by Rhypo; the spectra are normalized for a distance of 20 km. The Ml 5.4 event (2009/04/07/17:47) recorded at MI03 is plotted in the inset (on the right the same time-series after a 1 Hz low pass filter).

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Figure 1. Map of the epicentral area; is bounded by the dashed white curve. The triangles show the seismic stations. The yellow stars indicated the 6 April mainshock and the three aftershocks considered in this paper (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 Ml 5). The white circles indicate the earthquakes with M 2.5 from April to December 2009. The filled orange circles show the events of Table 2 used for spectral analysis. The red lines and the green squares show the main faults of the Aterno River Valley and the historical earthquakes (from CPTI Working Group, 2004), respectively. The white square bounds the area shown in Fig. 2.

colour figure

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Figure 2. Top) Geological map of the Aterno River valley surrounding Onna (white box of Fig. 1). The triangles indicate the seismic stations; PC04 has been used as reference site in our analysis (VGFZ05, MI01 and MI04 are also stations settled on stiff sites). The red lines show the main faults. The yellow star shows the Ml 5.4 aftershock of the 7 April (origin time 17:47 UTC). The colour from white to brown is from quaternary deposits to limestone, respectively.

shock (Gruppo di Lavoro MS-AQ, 2010). Bottom) As above where the topography is shown in grey tonality. The recordings of Ml 3.2 aftershock (EW component, origin time 2 May 2009, 2:21 UTC) are also shown. The seismograms amplitudes are normalized at the same scale.

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Figure 3. Geotechnical characterization of the Onna area after the microzonation studies of DPC Macroseismic

-section of Figure 2 (simplified after DPC, 2010). Bottom) The column shows -section (DPC, 2010). The shear-wave profile assessed for Onna is also shown. The dashed grey curve is the Vs inferred from a downhole survey carried out in Onna (by Terra Drilling Technology srl; Gruppo di Lavoro MS-AQ, 2010). The continuous black curve reports the Vs derived from the inversion of surface waves applying array techniques based on frequency-wavenumber and spatial autocorrelation methods (http://itaca.mi.ingv.it/ItacaNet, see the MI03 station monograph, responsible Stefano Parolai).

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Figure 4. Top) Ml 5 aftershock (2009/04/09/19:38 Ml) recorded simultaneously at AQ01, AQ02 and MI03. The seismograms amplitudes are normalized at the same scale. Middle panel) Seismograms of the top panel after a low pass filter (< 1 Hz). Bottom) Acceleration response spectra of the three components of the ground motions. Note the larger energy of MI03 at low periods.

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Figure 5. H/H and V/V spectral ratios (HHSR and VVSR, respectively) computed on earthquakes using PC04 as reference site. The mean spectral ratio and the mean 1 standard deviation are plotted. The HHSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). The mean curves are plotted as thick lines. In case of AQ02, the mean 1 standard deviation are plotted using three different reference sites (VGFZ05, MI01 and MI04 as red, blue and green curves, respectively; see text).

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Figure 6. H/V spectral ratios using earthquakes of table 2 and ambient noise (HVSR and HVNSR, respectively). The mean H/V ratio and the mean 1 standard deviation are plotted. The HVSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). The mean curves are plotted as thick lines

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Figure 7. The shadow area marks the f0 variation inferred from the HVNSR curves. The four noise measurements in the northern part of the village (squares symbols) were collected in February 2010 (about 40 minutes of ambient vibration recordings at each site). Note the increase of f0 from 2.3 up to 3.3 Hz proceeding in the north direction. The positions of the down-hole survey (Gruppo di Lavoro MS-AQ, 2010) and the array experiment are also shown (white circles; the resulting Vs profiles are reported in Fig. 3).

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Figure 8. Left) Probability density function (PDF) of power spectra densities at MI03 station. PDFs are computed on three days of continuous recording for the three components of the ground motion. Colour scale is proportional to the probability. The new high noise and the new low noise -models (black curves) of Peterson (1993) are also shown. Right) Ratio of the spectrogram of the horizontal components over the vertical one (NS/UP e EW/UP) considering three days of data recording. Colour scale is proportional to the amplitude of HVNSR.

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Figure 9. HVSR at MI03 using the weak motions listed in Table 2. Left) The mean H/V ratio and the mean 1 standard deviation are plotted. The HVSR is computed separately on the EW and NS horizontal components (red and blue curves, respectively). Right) The HVSRs are plotted as function of azimuth with the colour scale that is proportional to the amplitude of the spectral ratio.

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Figure 10. Left) HVSR computed at MI03 on the three strong aftershocks (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 M 5). The red and black curves indicate the EW/V and NS/V spectral ratio, respectively. Right) The HVSRs are plotted as function of azimuth with the colour scale that is proportional to the amplitude of the spectral ratio. Note the directional effects in a NW-SE orientation in correspondence of the resonance frequency of the site (2-3 Hz).

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Figure 11. Top) PGHA residuals (logarithm of observations minus logarithm of predictions) estimated considering MI03 recordings (21 records with magnitude down to 3.5) versus ITA08-mod predictive model. Middle) Dependence of MI03 residuals versus distance (epicentral distance or Joyner Boore distance if available; Joyner and Boore, 1981) expressed as slope (R) curve (black solid line). Bias and its standard deviation are also indicated (solid and dotted grey lines respectively). Bottom) Dependence of MI03 residuals versus magnitude (Ml or Mw if available) expressed as slope (M) curve (black solid line). Bias and its standard deviation are also indicated (solid and dotted grey lines respectively).

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Figure 12. Inter station errors (grey squares) computed considering acceleration response spectra (5% damping) calculated from MI03 recordings and those predicted by ITA08-mod. The error for period 0.0 s is relative to PGHA.

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Figure 13. Comparison between ITA08-mod and MI03 curves calculated for PGHA and a target event of magnitude 5.0. The MI03 curve is obtained considering only events with magnitude higher (or equal) than 3.5, included in a distance interval ranging from 10 km to 40 km (epicentral or Joyner Boore distance). Mw are available in http://itaca.mi.ingv.it.

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Fig. 14. Cross-hole and HVNSR obtained at the AQV site (AQV position is shown in Fig. 1). The resulting values of Vs30 and f0 are 460 m/s (soil class B) and 3 Hz, respectively.

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Figure 15. Acceleration response spectra of the three strong aftershocks (2009/04/07/17:47 Ml 5.4; 2009/04/09/00:52 Ml 5.1; 2009/04/09/19:38 Ml 5) recorded simultaneously at MI03 and AQV (left and right panel, respectively). Rhypo indicates the hypocentral distance; the spectra are normalized for a distance of 20 km. The Ml 5.4 event (2009/04/07/17:47) recorded at MI03 is plotted in the inset (on the right the same time-series after a 1 Hz low pass filter).