SITE EFFECTS AS PROXY FOR BUILDING BEHAVIOR: (, ) STUDY CASE

Antonio COSTANZO1, Arrigo CASERTA2, Fawzi DOUMAZ3

ABSTRACT

The moderate-magnitude aftershocks of the August 24th 2016 M 6.0 (Italy) mainshock have been used for assessing the site effects in Acquasanta Terme (Ascoli Piceno) town. This latter is located at around 20 Km far from the epicenter in the south-west direction. The earthquake struck a sector of the central Apennine that is considered as the most potentially dangerous seismogenic zone in Central Italy, magnitudes as high as 7 are expected in that region, indeed. The town has experienced severe damages, cultural heritage included. The aim of our study is to assess the soil shaking features in order to better characterize the input seismic radiation to the main cultural-religious building of the area: the San Giovanni Battista Church (A.D. 1039). For such purpose we deployed an array of seismic stations across the Tronto valley through Acquasanta Terme town. At the same time three stations have been installed inside the church in order both to assess the energy transfer process from the soil to the structure and to evaluate the contribution, if any, of the bell tower in causing the observed church damages. Preliminary results concerning both site effects and the building response are shown and discussed.

Keywords: Central Italy seismic sequence; Local Seismic Response; Cultural Heritage; Structural behavior; Seismic effects

1. INTRODUCTION

Since 2016 August 24th, following the Mw 6.0 Amatrice earthquake, a seismic sequence produced another Mw 5.4 earthquake (UTC 2016-08-24 02:33:34) and hundreds of moderate and weak earthquakes per day. The frequency of earthquakes has decreased until October 26th, when other two earthquakes Mw 5.4 and Mw 5.9 occurred near Visso village, preceding the largest Mw 6.5 event occurred on October 30th at 6km North of Norcia (see Figure 1). Some days after the Amatrice earthquake, we deployed an array of five seismic stations along the Tronto valley through the Acquasanta Terme town at about 20 Km from the epicenter. In addition, we installed three seismic stations inside the main historic-religious building represented by San Giovanni Battista Church (A.D. 1039). The aims of this work are both to assess site effects and the interaction between the incoming seismic wave-field and the historic buildings. The paper show the results, based on moderate- magnitude aftershocks recorded between September and October, 2016, related both to the local seismic response at Acquasanta Terme town centre and structural response of the historic building.

2. GEOLOGICAL SETTING AND SEISMIC SITE RESPONSE

2.1 Geological aspects Acquasanta Terme town is located on the eastern side of the Tronto river, moreover the whole municipality is made up of many hamlets settled on both sides of the valley. Following Boni and

1Researcher, Istituto Nazionale di Geofisica e VulcanologiaItaly, [email protected] 2Researcher, Istituto Nazionale di Geofisica e Vulcanologia, Italy, [email protected] 3Senior Researcher, Istituto Nazionale di Geofisica e Vulcanologia, Italy, [email protected]

Colacicchi (1996), in Figure 2 a schematic geological section was reproduced, also, by projecting on it settlements and position of the seismic array. The temporary seismic stations were deployed along the WNW-ESE direction (cf. Figure 2), where the section crosses the Tronto Valley encountering: San Vito hamlet (station ACS07 at WNW), the thermal area at the bottom of valley (ACS02), Acquasanta (ACS01) town centre, the lower (ACS08) and upper part (ACS03 at ESE) of Cagnano village. The array was designed to investigate the role played by near-surface geology in amplifying the soil shaking and to check the presence of any topographic effects.

Figure 1. Epicenters of the 2016 August-October seismic sequence in the Central Apennine mountain chain (blue circles). Five earthquakes with Mw between 5.4 and 6.5 (red stars) are located between 16 and 28 km from Acquasanta Terme (yellow circle).

Figure 2. Schematic geological section of the Tronto River valley that crosses town centre of Acquasanta Terme. In addition, on the section the main settlements and the seismic array are projecting (red triangles).

2

In the area around Acquasanta Terme, a large plate of travertine dominates the right flank of the Tronto River related to rise of sulfidic water (Boni and Colacicchi 1996) or to revegetation at the beginning of interglacials (Farabollini et al. 2011). Continental deposits form thin cover of colluvial and detrital slope and, along the valley, some terraced deposits composed by sand and gravel (Boni and Colacicchi 1996). These formations stay on a Miocene marly unit (Bisciaro, Marne con Cerrogna, Marne a Pteropodi) with thickness that can exceed 500 m, this and the underlying Oligocene Scaglia Cinerea unit (up to 200m thick), constitute the most important regional aquiclude (Galdenzi et al. 2010). On the right flank of the Tronto River valley, the settlements are located on terraces composed by travertine with an overlaying layer of alluvial deposit, generally it is composed by silty and clayey fine sands, thick from few meters, as for town centre, up to tens of meters. The terraces rest on the Bisciaro unit, that is considered a seismic bedrock (Vs > 1800m/s following the geophysical surveys carried out in the area for civil designs). Instead, On the left flank of the valley, the San Vito hamlet stay on the alluvial deposit directly overlaying the same Miocene formation.

2.2 Dataset Since early September, 2016, right after the Amatrice earthquake (August 24th), a seismic array was deployed along the section of the Tronto Valley that crosses some settlements of the Acquasanta Terme municipality. Each seismic station is composed by Centaur seismic digitizer (Nanometrics 2016a) equipped with a triaxial Trillium broadband seismometer (Nanometrics 2016b) and a triaxial Titan accelerometric transducer (Nanometrics 2016c).

Table 1. Information on aftershocks following the Amatrice Mw 6.0 mainshock.

2 Date UTC Time Latitude Longitude Depth Mag. Azimuth depi (km) 1 (°) (km) 2016-10-16 09:32:35 42.7542 13.1767 9.6 4.0* 274.99 19.08 2016-10-08 18:11:10 42.7433 13.1927 10.9 3.9+ 279.21 17.93 2016-10-09 04:42:42 42.7437 13.1953 9.5 3.6+ 279.18 17.71 2016-10-08 12:19:03 42.7502 13.1823 8.7 3.5+ 276.47 18.67 2016-10-04 12:41:35 42.8545 13.121 7.8 3.4+ 248.02 25.38 2016-10-02 23:47:07 42.7897 13.2303 9.4 3.4+ 261.10 14.80 2016-09-30 19:38:38 42.8948 13.2468 9.3 3.4+ 223.50 19.27 2016-09-30 19:22:28 42.896 13.2482 9.3 3.3+ 317.02 19.29 2016-09-22 20:04:55 42.7597 13.1882 11.1 3.3+ 266.68 18.10 2016-10-11 21:24:10 42.8672 13.0747 8.2 3.2+ 291.78 29.41 2016-10-08 12:40:59 42.7508 13.1932 10.7 3.2+ 263.42 17.78 2016-10-08 08:15:02 42.7802 13.2003 10.2 3.2+ 274.13 17.12 2016-10-06 05:34:18 42.7935 13.2173 9.7 3.2+ 279.81 15.92 2016-09-20 01:20:53 42.6772 13.2902 9.3 3.2+ 223.62 14.12 2016-10-09 02:47:29 42.7442 13.1948 10.2 3.1+ 261.02 17.75 2016-10-11 07:32:51 42.8562 13.2533 8.5 3.0+ 307.25 16.00 2016-10-06 16:09:44 42.8793 13.1408 9.4 3.0+ 299.22 25.11 1 * represent the moment magnitude (Mw) and + the local magnitude (Ml) 2 site-to epicenter azimuth, clockwise from north.

3

The horizontal components were oriented along the north-south and east-west direction. Data were sampled at 250 Hz using 24-bit analog-to-digital converters, and time synchronism was provided by the embedded GPS system at each station. A local Wireless (WiFi) network was installed for data transmission thanks also to a set of antennas connected to each station and to a server hosted in the municipal seat of the town. The data are continuously recorded at the surface. In this paper, in order to study the seismic site response, we analyze data related to 17 aftershocks with magnitude between 3.0 and 4.0, which occurred after the Amatrice earthquake at epicentral distance from Acquasanta Terme between 14km and 30km and site-to-epicenter azimuth between 223° and 307°. Table 1 gives information for each event.

2.3 Seismic site response To assess the seismic site response, in Figure 3 the Horizontal-to-Vertical-Spectral-Ratio (HVSR) obtained by ambient vibrations and as mean of the 17 earthquakes are compared, also the confidence ranges are shown as shadow area for noise and dashed lines for earthquakes. For ambient vibration 2h- recorded signals were manually windowed in time-series of 30s length. The Fourier spectra were smoothed with a Konno and Omachi (1998) algorithm, hence, spectral ratios were calculated (Figure 3). HVSRs by aftershocks are computed on the horizontal motions (recorded along EW and NS direction) rotated into radial (R) and transverse (T) direction, using the site-to-epicenter azimuths measured clockwise from north (cf. Table 1).

Figure 3. Ratios of horizontal to vertical smoothed Fourier spectra at the ACS01 site: comparison between noise and mean of 17 earthquakes (4.0 ≥ M ≥ 3.0). For the noise, spectral ratios are geometrical mean of the two horizontal components. For the aftershocks, once the horizontal motions have been rotated through the site-to- epicenter azimuths measured clockwise from north (cf. Table 1), the HVSR were calculated for transverse (a) and radial components (b).

Instead, for the ambient vibrations HVSR is the geometrical mean of the two horizontal EW and NS components. The data obtained by noise and earthquakes show a general good agreement, with 4

exception for low frequencies (< 1 Hz) where the noise-based HVSR is characterized by high standard deviation. Taking into account the frequencies with HVSR greater than 2, as required by the clarity criterion specified by SESAME (2004), the frequency range between 2.8 Hz and 4.7 Hz is identified. In addition, the HVSR curves show a peak at 4.2 Hz from noise, whereas, it results lightly shifted towards the lower frequencies analyzing the earthquakes: 3.5Hz and 3.8Hz for radial and transverse component, respectively. HVSR measurements is a good factor to estimate the first fundamental frequency, however, it fails to detect the higher resonance modes (e.g. Bonnefoy-Claudet et al. 2008). Therefore, to detect the frequency ranges where ground motion is amplified and to quantify its values, we calculate the Standard Spectral Ratio (SSR, Borcherdt 1970). The SSR is defined as the ratio of the Fourier amplitude spectrum at the site and that at a reference site obtained from the same earthquake and the same component. In this case, the recordings at the bottom valley (ACS02) were used as reference. In fact, in this site is detectable the contact between travertine and Bisciaro, that we assumed as seismic bedrock because VS >1800 m/s. Moreover, the detected fundamental frequency at ACS02 site is about 19Hz. In Figure 4 the SSRs by aftershocks are shown for radial and transverse component of the ground motion. These curves show a significant amplification factor (greater than 2) for the frequency range between 3.5Hz and 7.2Hz, with a substantial agreement for the two components. The peak of the mean SSR curves show a value of 3.7 at about 4.6Hz.

Figure 4. Ratios of smoothed Fourier spectra record at the town centre and that at the reference site (ACS02 in Figure 2): comparison between noise and mean of 17 earthquakes (4.0 ≥ M ≥ 3.0). Once the horizontal motions have been rotated through the site-to-epicenter azimuths measured clockwise from north (cf. Table 1), the SSR were calculated for transverse (a) and radial components (b).

3. HISTORIC BUILDING RESPONSE

To study the dynamic behavior of the monumental structure under the action of the incoming wave- field, we decided to monitor the most important religious building of the Acquasanta Terme: the San Giovanni Battista Church. The structure is composed by both church, that is composed by only central nave with a series of small chapels, and tower bell built above the church block. The church dates back to 1039 AD, whereas, some elements as the buttresses and the bell tower are more recent. In fact, the

5

building was restructured several times since its construction because it suffered damages due to different historic earthquakes.

3.1 Damage framework In order to reconstruct the damage framework of the monument due to the Amatrice earthquake, we carried out a terrestrial laser scanning of the structure (cf. Figure 5).

Figure 5. Some damages detected on the San Giovanni Battista Church through the laser scanning survey with high resolution. Defects on the vault of the central nave: cracks and detachment of plaster on frescoed vault (a) and crack along the contact between vault and frontal wall (b).

In addition, the laser scanning survey has been coupled with some indoor and outdoor drone

6

photogrammetric flights, mainly to cover the non-visible parts. The data processing following the procedure proposed by Costanzo et al. (2015) allowed to reproduce a detailed tridimensional model, that is representative of the state after the first strong earthquake of the Central Italy seismic sequence. In Figure 5, some damages are shown on the base of the 3D model: cracks and detachments of plaster on the frescoed vault (Fig. 5.a), the crack along the contact between vault and frontal wall (Fig. 5.b). Cracks are evident in different parts of the structure, especially, in correspondence of some vertical contacts between walls, which seem poorly joined.

3.2 Church and bell tower response

In the first phase of this study, only three seismic stations were available to study the dynamic response of the historic building to the seismic radiation. Therefore, we focused our attention on the dynamic interaction between church and bell tower. For this purpose, we installed one station at the top of bell tower and the other two on the lateral walls of the church on the top of two opposite internal columns (Figure 6). To determine frequencies and values of the amplification factor, the SSR curves were calculated as ratio between Fourier spectra obtained from recordings of 6 aftershocks into the religious building and the same at the church garden (ACS01 in Figure 2 and Figure 6). The NS and EW horizontal components were rotated along the longitudinal and transverse direction referred to the lateral walls of the church (cf. Figure 6).

Figure 6. 3D model of the San Giovanni Battista Church with location of the seismic stations: in the church garden (picture in the upper right corner), on the wall (picture in the lower left corner) and at the top of the bell tower (picture in the lower right corner).

The Figure 7.a shows the SSR curves obtained on the church wall, three peaks seem detectable: the first one at 2.1 Hz, the second at 2.6 Hz and the third 3.6 Hz for transversal or 3.9 Hz for longitudinal direction. The first two peaks are more evident in longitudinal direction assuming mean SSR values of 3.5 and 4.0, whereas the third peak is the maximum of the curves in both directions with values 7

between 5.9 and 7.2. This peak is about 2 times the other ones, and it is identified by recordings on both walls. Hence probably it represents a dominant frequency of the church block. Analyzing the SSR curves obtained for the bell tower (Figure 7.b), we can observe peaks at about 2.1 Hz and 2.5 Hz for transverse and longitudinal direction, respectively. These peaks are characterized by extremely high values of the motion amplification, in fact the SSR is greater than 16. In addition, the same figure shows a second peak at about 3.8 Hz, only in longitudinal direction. In summary, the SSR curves shown in Figure 7 seem to indicate absolute maxima referred to the dominant mode of the structural elements, and relative maxima linked to the interaction between the two blocks of the religious building. In fact, the secondary peaks detected for recordings on the church walls enough corresponding to the main peaks of the bell tower, and vice versa.

Figure 7. Standard spectral ratios respect to the church garden (assumed as reference site): curves for church wall (a) and for bell tower (b). The blue and red curves are referred to the transverse and longitudinal components respect to the wall (cf. Figure 6).

The energy contents measured into the building have been calculated as Arias intensities normalized respect to that at the ground (Figure 8).

Figure 8. Normalized Arias intensities respect to the church garden (assumed as reference site): yellow boxes for the church wall and purple boxes for the bell tower. The boxes represent the range between 25%le and 75%le, whereas the white circle are the median value. The bold lines are the minimum and maximum values obtained by aftershocks analyzed.

In the figure the median values of the analyzed earthquakes are shown as white circles; whereas the boxes represent the range between 25%le and 75%le and the bold lines indicate maximum and 8

minimum values. The energy content measured on the wall is included between 3 and 4.2 times that recorded on the ground, in both horizontal directions. Instead, the bell tower is characterized by median Arias intensities 2.1 and 6 in transverse and longitudinal direction, therefore it shows a different behavior along the two directions and respect to the church block. The maximum amplification of the energy is shown in vertical direction both for the church walls and the bell tower; in particular, for the latter structure the median value of the Arias intensity is about 25 times respect to the ground.

5. CONCLUSIONS

Following the Amatrice earthquake occurred on August 24th, 2016, the authors deployed a seismic array along the Tronto valley crossing the Acquasanta Terme town. In addition, three seismic stations were located into the main historic building of the town, the San Giovanni Battista Church, that resulted strongly damaged by the earthquake. In particular, the structural behavior of both church and bell tower was monitored, and a laser scanning survey allowed reproducing the entire religious complex with the detected damages. In this first phase of the work, authors tried to understand if the different damage framework between church and bell tower could be attributed to the soils-structure interaction. The aftershock-based spectral ratio respect to the reference site allowed to detect the resonance frequencies of 3.6 Hz or 3.9 Hz (for the two horizontal components) for the church wall and 2.1 Hz or 2.5 Hz for the bell tower. Moreover, the horizontal to vertical spectral ratio shown free-field resonance frequency of 3.5Hz and 3.8Hz, hence very close to those of the church. In fact, the soils- structure resonance factor proposed by Gosar (2010) is lower than 10% for church walls, hence a high danger (factor ≤ 15%), and about 40% for bell tower, therefore a low danger (factor > 25%) for this structural block. Such results are in agreement with the observed damage framework of the monument, that was reconstructed through terrestrial laser scanning technique. The church resulted strongly damaged in many of its parts; whereas, the bell tower suffered only the fall of a decorative column from the top. In the next phases of the work, the analysis on the whole dataset coupled with a numerical modeling of the historic structure will explain the structural response in more detail. In fact, afterwards the Norcia and Visso earthquakes, the number of seismic stations into the religious structure was increased, both in the church and bell tower, in order to analyze the structural behavior in more detail. Moreover, the laser scanning of the building was repeated to analyze the effects due to the successive strong events. These analyses will be the topic of a forthcoming work. The complete data analysis of this research will provide a better understanding of the induced-earthquake damage, as well as a starting point for the restoration of the cultural heritage.

6. ACKNOWLEDGMENTS

The authors thank citizens, mayor and public administration of Acquasanta Terme for their support during the activities related to the deployment of the seismic stations. We are grateful to the priests of the S. Giovanni Battista Church for their availability and the firefighters, who provided the technical support for the installation of the seismic stations on the bell tower and the church walls. The authors are grateful to the three anonymous reviewers for their helpful suggestions.

7. REFERENCES

Boni C, Colacicchi R (1966). I travertini della valle del Tronto [The travertines of the Tronto Valley]. Memorie Società Geologica Italiana, v. 5, p. 315–339. Bonnefoy-Claudet S, Baize S, Bonilla LF, Berge-Thierry C, Pasten C, Campos J, Volant P, Verdugo R. (2008). Site effect evaluation in the basin of Santiago de Chile using ambient noise measurements. Geophysical Journal International, 176(3), 925-937.

9

Borcherdt RD (1970). Effects of local geology on ground motion near San Francisco Bay. Bulletin of the Seismological Society of America, 60(1), 29-61. Costanzo A, Minasi M, Casula G, Musacchio M, Buongiorno MF (2015) Combined Use of Terrestrial Laser Scanning and IR Thermography Applied to a Historical Building. Sensors, 15(1), 194-213. doi:10.3390/s150100194. Farabollini P, Gentili B, Materazzi M (2001). Freshwater travertines in the Central Apennine (Italy): Genesis and climatic and neotectonic significance, Proceedings of the 5th International Conference on Geomorphology, Tokyo, Japan. Galdenzi S, Cocchioni F, Filipponi G, Morichetti L, Scuri S, Selvaggio R, Cocchioni M. (2010). The sulfidic thermal caves of Acquasanta Terme (central Italy). Journal of Cave and Karst Studies, 72(1), 43–58. DOI: 10.4311/jcks2008es0056 Gosar A (2010). Site effects and soil-structure resonance study in the Kobarid basin (NW Slovenia) using microtremors. Natural Hazards and Earth System Sciences, 10(4), 761–772. Konno K, Ohmachi T (1998). Ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremor, Bulletin of the Seismological Society of America, 88(1), 228-241. Nanometrics Inc. (2017). CENTAUR technical specifications, Kanata, Ontario, Canada, available on line: www.nanometrics.ca/seismology/products/digitizers/centaur, (accessed on December 29, 2017). Nanometrics Inc. (2017). TRILLIUM COMPACT technical specifications, Nanometrics, Inc., Kanata, Ontario, Canada, available online:www.nanometrics.ca/seismology/products/trillium-compact, (accessed on December 29, 2017). Nanometrics Inc. (2017).TRILLIUM COMPACT technical specifications, Kanata, Ontario, Canada, available on line:www.nanometrics.ca/seismology/products/titan, (accessed on December 29, 2017). SESAME (2004) Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations measurements, processing and interpretation. SESAME European research project. WP12 – Deliverable D23.12. http://sesame-fp5.obs.ujf-grenoble.fr/index.htm

10