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1 On: 9 May 7 Access Details: Free Access Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 79 Registered office: Mortimer House, 7- Mortimer Street, London WT JH, UK Journal of Earthquake Engineering Publication details, including instructions for authors and subscription information: Site Effects Analyses in the Old City Center of Trieste (NE Italy) Using Accelerometric Data To cite this Article: Fitzko, Francesca, Costa, Giovanni, Delise, Andrea and Suhadolc, Peter, 'Site Effects Analyses in the Old City Center of Trieste (NE Italy) Using Accelerometric Data', Journal of Earthquake Engineering, :, - To link to this article: DOI:./666 URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Taylor and Francis 7

2 Journal of Earthquake Engineering, :, 7 Copyright A.S. Elnashai & N.N. Ambraseys ISSN: 6-69 print / 9-X online DOI:./666 Downloaded At: 9: 9 May 7 UEQE X Journal of Earthquake Engineering, Vol., No., December 6: pp. Site F. Fitzko Effects et al. Analyses in the Old City Center of Trieste (NE Italy) Site Effects Analyses in the Old City Center of Trieste (NE Italy) Using Accelerometric Data FRANCESCA FITZKO, GIOVANNI COSTA, ANDREA DELISE, and PETER SUHADOLC Department of Earth Sciences, University of Trieste, via Weiss, Trieste, Italy The influence of local geology and soil conditions on the intensity and the amplification of ground shaking are well known. Part of the old city center of Trieste is built on the site of a former salina, placed at a river mouth and is characterized by soft sediments several tens of meters thick. A new accelerometric station has been recently installed in a historical building, in order to analyse earthquake-induced site amplifications. This station has recorded five regional earthquakes and the related records are compared to those obtained at a nearby bedrock-installed accelerometeric station. Fourier and response spectra for all components are computed and both the H/V ratio and the reference station techniques are used to assess site effects. Noise measurements performed in the historical building, where the accelerometer is located, confirm these results. Relevant amplifications are detected in the frequency range of Hz, particularly important for the type of buildings present in this part of the city. Keywords Site Effects Estimation; Weak Ground Motion; Seismic Ambient Noise (Microtremors); Historical Building. Introduction Shallow soft soil deposits can considerably amplify earthquake ground motion and consequently increase the damage suffered during earthquakes, which is often referred to as site effects (e.g. Aki, 99). The identification of the fundamental frequency and of the local amplifications of earthquake ground motion is often carried out in urban areas through site response analysis. One of the most common techniques to estimate site effects is the use of spectral ratios of earthquake records, both using a reference station (when available) or a single station [e.g., Borcherdt and Gibbs, 97; Lermo and Chavez-Garcia, 99; Field and Jacob, 99; Theodulis et al., 996]. Such studies are often performed using weak motion data, which are more easily available, especially in moderate-seismicity areas. Weak motion data do not include information on the possible inelastic soil behavior under strong ground motion, which is, however, to be expected only for very loose soils and high sustained accelerations. Beside earthquake records, site effects analyses are performed employing seismic noise measurements [e.g., Nogoshi and Igarashi, 97; Nakamura 99], which proved to be very helpful in the identification of the fundamental frequency of alluvial sites [e.g., Duval et al., ]. Received January ; accepted October. Address correspondence to Peter Suhadolc, Department of Earth Sciences, University of Trieste, via Weiss, 7, Triest, Italy. suhadolc@units.it

3 F. Fitzko et al. The city of Trieste (NE Italy), though not affected by strong earthquakes in its recent history (Table ), is located near several recognized active faults [Aoudia, 99; Benedetti, 999], belonging to both the Alpine and the Dinaric Domains (Figure ). Downloaded At: 9: 9 May 7 TABLE Historical and recent seismicity felt in Trieste: the largest macroseismic intensity reported in the city (I s ) is related to the Idrija earthquake. I s is the intensity reported in the town; I x is the epicentral intensity SEISMIC HISTORY OF TRIESTE Year Month Day Hour Min I s Epicentral area Appr. Distance, km I x M s 6 VIII Idrija (Slo) X VII Friuli (I) VIII-IX VI Ljubljana (Slo) 7 VIII-IX VI Sneznik (Slo) VI-VII VI Tramonti (I) VII-VIII VI Bellunese (I) X VI Friuli (I) IX-X V Bakar (Cro) 7 VI-VII.7 99 V Bassa padana (I) VI-VII. 9 9 V Carnia (I) VII. 96 V Bosco Cansiglio (I) IX. September, 976 9: Ms = 6. : Ms = 6. May 6, 976 Ms = 6. July, Ml =.9 Bovec-Kobarid April, 99 Ms =.7 Bovec-Krn March 6, M = 6.9 Idrija AUSTRIA 6 6 ITALY km Udine Adriatic Sea Trieste Ljubljana SLOVENIA CROATIA FIGURE Trieste is located at the border between Italy and Slovenia. In this area the Alpine and the Dinaric System join. Recent (977 ) earthquakes and some of the largest events at the Alps-Dinarides Junction (Slovenian catalogue kindly provided by the Agencija Republike Slovenije za Okolje, Ljubljana. For Northeastern Italy: OGS catalog downloaded from

4 Site Effects Analyses in the Old City Center of Trieste (NE Italy) Downloaded At: 9: 9 May 7 The largest reported event ever felt in the city is the Idrija earthquake, which reached MCS intensity VIII. An intensity VII was reported as a consequence of the September 976 Friuli earthquake. Part of the old city center was built on a former salina placed at a river mouth and characterized by thick soft sediments. Such sedimentary cover could considerably amplify the earthquake ground motion and, therefore, reasonably increase the seismic hazard of the town, in case of an earthquake. However, to our knowledge, local site-response analyses in Trieste and its old city center have not been carried out so far. A new accelerometric station (CARC) has been recently set up in an historical building in the old center (Figure ). The choice of installing an accelerometer allows to fully record large ground motions without saturating and the high trigger threshold avoids the recording of urban noise. The purpose of this installation is to collect and analyze earthquake data in order to identify site effects in the area. The station CARC is close to two bedrock stations: DST, inside a building and the broadband station TRI, installed inside a cave and considered a reference station. During the two years of operation of the CARC station, only few earthquakes have been detected mainly because of the high urban noise level. In particular, five regional events have been recorded by at least two of the above-mentioned stations and the related recordings are discussed in this paper. We first apply the H/V technique [Lermo and Chavez-Garcia, 99; Field and Jacob, 99; Theodulis et al., 996] over weak motion data (HVSR) and seismic ambient noise (HVNSR) recorded at the three stations. Later we compare the receiver functions to the transfer functions obtained applying the reference site technique (SSR). In doing so, we employ both Fourier and response spectra of the records [Fäh and Suhadolc, 99]. Additional noise measurements have been performed inside the building in which the station FIGURE Palazzo Carciotti in the Trieste old city center. The station CARC is installed in the building s basement and it s equipped with a CMG-T accelerometer and a Mars Digitizer. The stratigraphy underneath the station in sketched on the right: the bedrock (Flysch unit) is overlain by about m of soft sediments.

5 6 F. Fitzko et al. Downloaded At: 9: 9 May 7 CARC is located and the Nakamura technique is applied on microtremors (HVNSR), in order to discriminate between the natural frequencies of the sedimentary layer and those of the building. In particular, this latter effect has been analyzed using spectral ratios of upper floor vs. basement microtremor measurements. Both weak motion analysis, HVSR and SSR, performed at CARC evidence a clear amplification around Hz, which can be related to the sedimentary deposits underlying the station CARC. Noise measurements performed in the palace highlight frequency amplifications around Hz and in the frequency range of 7 Hz for the horizontal components, plausibly related to the building dimensions.. The Stations and the Data The station CARC is located in the old city center, in the basement of a -story historical building (about -m long, -m wide, and -m high) located just in front of the sea. This palace was built in the early 9 th century on the site of a former salina at a river mouth and its foundations are partly made by wood pillars. The 7-m thick sedimentary cover, as deduced from borehole data, is composed by made ground, sands and silts washed by the ancient stream. It is underlain by about m of Eocene Flysch sandstones and marls (Figure ) overlying Mesozoic limestones. The station CARC is instrumented with a Guralp CMGT accelerometer and a 6-bits Lennartz MARS/OD digitizer, with STA/LTA trigger. The station DST is located at the basement of a one-story building, about km NE from CARC, directly on the Flysch unit. It has the same instrumentation as CARC along with a -bits Quanterra Q digitizer, connected to a Streckeisen STS- velocimeter and to a Kinemetrics Episensor accelerometer. The station TRI is situated km N from CARC inside a natural cave within Mesozoic limestones, at about m depth and it is considered our reference station for the analyzed regional events. It is equipped with a Streckeisen STS- velocimeter and a -bits Quanterra Q digitizer. The accelerations at TRI are derived from the broadband recordings. Five regional events have been recorded by CARC since its installation: a Ml. earthquake near Ampezzo (NE Italy), a Ml. event located in the Appennines near Bologna, a Mb.9 event in Bosnia-Herzegovina a Mb. event (the waveforms recorded by the three stations are shown in Figure ) in the Bovec-Kobarid area (NW Slovenia) and a Mb. event located near Rijeka (Croatia) (Table ). The station TRI was down due to technical problems at the time when the first two events occurred. These events did not trigger the accelerometer of DST (Guralp CMGT accelerometer and Lennartz MARS/ OD digitizer), but the related records could be retrieved from the accelerometric channels of the Q digitizer at DST, which is continuously acquiring data. All three stations have recorded the last three events reported in Table. In Table the recorded PGA are reported. All data have been corrected for the instrumental transfer function.. Local Response of the Station DST We first focus our analysis on the DST station in order to retrieve the related local response with respect to the TRI station. In fact, although the instrumentation is fixed on rock, the Eocene Flysch unit is highly fractured. On the other hand, the lack of events and the consequent need of analyzing the largest number possible, suggests to take DST into account as a reference site for the two events not recorded by TRI. We analyze the local response of DST in terms of spectral ratios. First we use the standard spectral ratio technique on three regional events (Table ) recorded by both DST

6 Downloaded At: 9: 9 May 7 Site Effects Analyses in the Old City Center of Trieste (NE Italy) 7 cm/s FIGURE North components of the signals recorded by CARC, DST, and TRI related to the event in Bovec-Kobarid (/7/, M l.9). The record of TRI is differentiated from velocities. s and TRI. In this way we retrieve the transfer function of DST related to the reference site TRI. This result is compared to the receiver function of DST, obtained applying the H/V technique on five regional events (Table ). Finally, we record seismic ambient noise at this station and we compute H/V spectral ratios of these signals. At this stage, the local response analyses are performed using both average Response and Fourier spectra of earthquakes and the two results are then compared. We use Response spectra to obtain the local site characterization, since they provide information on the building response to earthquake ground motion. The SSR technique is nowadays widely used to infer the site local response from earthquake ground motion records. Firstly introduced by Borcherdt [97], it consists in dividing the spectrum observed at the investigated site by that observed at a nearby reference station, possibly on competent bedrock. The underlying assumption is that having the two sites similar source and path effects, the resulting spectral ratio represents an estimate of the site response. Both Fourier and % damping Response spectra of these signals are computed and the related ratios are shown in Figure. The ratios are calculated for frequencies up to Hz. The upper cut in frequency is due to the instrumental response of the STS- broadband sensor. The Fourier spectra are averaged and smoothed. In doing so, we compute the mean amplitude for a. Hz frequency moving window, with. Hz overlap. Both Fourier and Response spectra show variable amplifications for the different analyzed events, and they are in very good agreement. In particular, the DST east component exhibits a clear amplification at Hz for two events and at a frequency slightly greater than Hz for

7 Downloaded At: 9: 9 May 7 TABLE Regional events recorded by the stations operating in Trieste (CARC, DST, TRI) Date M (Agency) Epicentral Area Distance, km Azimuth Data source // 9::.. M l (CSEM) Friuli 7 CARC-DST /9/ ::.. M l (CSEM) Appennino CARC-DST // :9:9.9.9 M b (CSEM) Bosnia CARC-TRI-DST /7/ ::.. M b (CSEM) Bovec 7 CARC-TRI-DST /9/ :9:6.7. M b (CSEM) Rijeka 76 CARC-TRI-DST

8 Site Effects Analyses in the Old City Center of Trieste (NE Italy) 9 Downloaded At: 9: 9 May 7 TABLE PGA(cm/s ) of the five regional events recorded by the three stations operating in Trieste Events Operating stations PGA(N) cm/s PGA(E) cm/s PGA(Z) cm/s 9 CARC.*.*.7* DST.9* 6.*.* TRI down down down 9 CARC.9. 9.* DST.6*.*.* TRI down down down 99 CARC 7.*..* DST.*.*.* TRI 7.*.* 9.6* 7 CARC.9.7. DST..7.9 TRI CARC DST.*.*.6* TRI.7*.*.9* the third event. In general, the response of DST related to TRI is quite flat for the north component, with an amplification factor around. On the average, the vertical component is, as expected, not amplified. The H/V approach applied to records of earthquakes, or receiver function technique, is a very useful tool for estimating local site characteristics, especially when no reference station is available. The method [e.g., Lermo and Chavez-Garcia, 99; Field and Jacob, 99; Theodulis et al., 996] consists of recording several earthquakes and computing the spectral ratio of the horizontal and vertical components on the S-wave portion of the records. The H/V approach is based on the assumption that the local geology does not amplify the vertical component significantly, at least for frequencies of interest in civil engineering (below Hz) and it is mostly used when no reference site can be found on non-weathered bedrock. Thus, the vertical component spectrum acts as a reference level with respect to the horizontal components ones. The receiver function method is thus useful to identify the fundamental frequency of a site but evaluates the amplification factor only at low frequencies [e.g., Lermo and Chavez-Garcia, 99; Bonilla et al., 997]. Average H/V response and Fourier spectra related to the station DST and the events of Table are shown in Figure. In general, both Fourier and Response spectra exhibit a good agreement showing a quite flat response of the horizontal components of DST related to the vertical one, and the amplification factor is around, as obtained using SSRs. On the other hand, the clear amplification around Hz displayed by the SSRs for the East component is not identifiable with the receiver function technique. Microtremors measurements are also a reliable tool to find the resonant frequency of a site, but not to give the amplification of the seismic ground motion [Lermo and Chavez- Garcia, 99; Lachet and Bard, 99]. In some cases, HVNSR does not evidence higher modes of resonance of the soft sediment [e.g., Lermo and Chavez-Garcia, 99] and in these cases the fundamental resonant frequency might even not be the frequency at which there is the maximum amplification. HVNSR at DST are shown in Figure 6 and they are

9 Downloaded At: 9: 9 May 7 DST station SSR TRI station East components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. DST station SSR TRI station North components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. DST station SSR TRI station Vertical components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. spectra ratio Fourier spectra ratio Fourier Fourier spectra ratio DST station SSR TRI station East components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. 6 DST station SSR TRI station North components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. 6 DST station SSR TRI station Vertical components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. Repsonse spectra ratio FIGURE Fourier (above) and Response (below) spectra ratio SSR of the regional events recorded at DST and TRI (Table ). East/Vertical components on the left, North/Vertical components in center, vertical components on the right.

10 Site Effects Analyses in the Old City Center of Trieste (NE Italy) DST stati on HVSR East/ Verti cal Friuli // Ml. App. bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. DST station HVSR North/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. Downloaded At: 9: 9 May 7 Fourier spectra raio 6 DST statio n HVSR East/ Verti cal Friuli // Ml. App. bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. Fourier spectra raio 6 FrequencyHz DST station HVSR North/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. 6 6 FrequencyHz FIGURE Fourier (above) and Response (below) spectra ratio HVSR of the regional events recorded at DST (Table ). East/Vertical components on the left, North/Vertical components on the right. flat and almost equal to, as reported by Lachet et al. [996], Lermo and Chavez-Garcia, [99], LeBrun et al. []. The spectra are derived using the free-software J-Sesame (SESAME Project, ) on certain time windows. The amplification of both NS and EW components is around. In studying the site response of DST, we have also to consider possible amplifications due to the building. Microtremors measurements have been carried out at the station and on the uppermost floor of the building at the same time, in order to possibly detect the resonance frequencies of the building. This test has been performed using two STS- broadband velocimeters and Quanterra Q digitizers during a time period of h. The ratio of the spectra related to the uppermost floor recordings and the to the basement are flat in the frequency range of interest.

11 F. Fitzko et al. DST station HVNSR North/Vertical East/Vertical Downloaded At: 9: 9 May 7 F o ur i e r s pe c tra rat i o 6 FIGURE 6 Fourier spectra ratio of microtremors recorded at DST.. Estimation of Site Effects at CARC In analyzing the local response of CARC, we first derive the transfer function of CARC related to both the reference station DST (two events) and TRI (three events) (Table ). Then, we compare this result to the receiver function of CARC obtained using five regional events. Fourier and Response SSRs related to DST station are shown in Figure 7. Both show a clear amplification at Hz on the east and the north components. This frequency corresponds to the resonance frequency of the site if we use a simple -D model (.) vs f = (.) H (where v s is the S-wave velocity and H the sediment thickness; Bard and Bouchon, 9). Higher frequencies are also amplified but to a lesser extent. The vertical component shows a clear peak at Hz. Applying the D model to the resonant frequency found and to the known depth of the sedimentary layer gives us a Vs of m/s, compatible with the geotechnical characteristics of the soil beneath the station. If we look at SSR related to TRI (Figure ) for the three events reported in Table, we again see the very strong amplification (up to ) around Hz, already evidenced by the spectral ratio of the events recorded at CARC related to the ones acquired at DST (Figure 7). Other peaks appear at higher frequencies, possibly related to higher modes or building effects. The vertical component of CARC shows a relevant amplification around Hz, and for the Rijeka event around Hz. We have compared some records from a station installed at the surface just above TRI station in order to detect possible effects of the cave itself on the signals [Amoruso et al,

12 Downloaded At: 9: 9 May 7 CARC station SSR DST station East components Friuli // Ml. App. Bolognese /9/ Ml. CARC station SSR DST station North components Friuli // Ml. App. Bolognese /9/ Ml. CARCstation SSR DST station Vertical components Friuli // Ml. App. Bolognese /9/ Ml. Fourier spectra ratio Fourier spectra ratio Fourier spectra ratio CARC station SSR DST station East components Friuli // Ml. App. Bolognese /9/ Ml. CARC station SSR DST station North components Friuli // Ml. App. Bolognese /9/ Ml. CARCstation SSR DST station Vertical components Friuli // Ml. App. Bolognese /9/ Ml FIGURE 7 Fourier (above) and Response (below) spectra ratio SSR of the regional events recorded at CARC and DST (Table ). East components on the left, North components in center and vertical components on the right.

13 Downloaded At: 9: 9 May 7 CARC station SSR TRI station East components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARCstation SSR TRIstation North components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARCstation SSR TRI station Vertical components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. Fourier spectra ratio Fourier spectra ratio Fourier spectra ratio CARC station SSR TRI station East components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARC stationssr TRI station North components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARC station SSR TRI station Vertical components Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb FIGURE Fourier (above) and Response (below) spectra ratio SSR of the regional events recorded at CARC and TRI (Table ). East components on the left, North components in center and vertical components on the right.

14 Site Effects Analyses in the Old City Center of Trieste (NE Italy) Downloaded At: 9: 9 May 7 997]. We have found no significant deamplification due to the cave in the frequency range of interest, and in particular a ratio of around the frequency of Hz. As previously pointed out, the basic assumption in using H/V spectra ratio for site effects estimation is that the vertical component is free from local amplifications. This fact is not always true, especially at sites located near sub-vertical discontinuities of lithologic or tectonic origin which affect the vertical motion in the same proportion as the horizontal one, however it is mainly observed at higher frequencies. HVSR at CARC are shown in Figure 9. There is a good agreement with SSRs at low frequencies, as already pointed out by many authors [e.g., Lermo and Chavez-Garcia, 99; Field and Jacob, 99]. The frequency of Hz is amplified, as already revealed by SSR analyses, but the amplification factor is smaller. In this article we have used the full signals for our analysis. However, we have performed HVSR and SSR analyses also on subsequent portions of the signal, but have found no significant differences with respect to the full-signal results. Microtremor measurements have been carried out at the station site (basement) and on the third floor of the building to investigate possible effects of the building on the spectral frequency and amplification content. Microtremors have been recorded at the station CARC on the basement and on the uppermost floor of the building at the same time. In the test, two STS- broadband velocimeters and Quanterra Q digitizers have been recording continuously during a time period of h. CARC station HVSR East/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARC station HVSR North/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mbl. Fourier spectra ratio Fourier spectra ratio 6 6 CARC station HVSR East/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mb. CARC station HVSR North/Vertical Friuli // Ml. App. Bolognese /9/ Ml. Balkan Pen. // Mb.9 Bovec /7/ Mb. Rijeka /9/ Mbl. 6 6 FrequencyHz FIGURE 9 Fourier (above) and Response (below) spectra ratio HVSR of the regional events recorded at CARC (Table ). East/Vertical components on the left, North/Vertical components on the right.

15 6 F. Fitzko et al. CARC station HVNSR North/Vertical East/Vertical CARC station microtremors: upper floor/basement NS EW Z Downloaded At: 9: 9 May 7 F o ur ir e s pe ctra rat i o 6 Fou rir e s pe ctra rat i o 6 FIGURE Fourier spectra ratios of microtremors recorded at CARC. On the left, HVNSR at the station. On the right, each component recorded on the uppermost floor is divided by the correspondent one recorded on the basement. Fourier spectra of selected time windows have been computed using the free-software J-Sesame and the results are shown in Figure. The HVNSR of CARC evidences the clear Hz peak, and the amplification factor is comparable to HVSR performed using weak motion data. The Fourier spectra ratios of uppermost floor vs. basement recordings [Figure ] show large amplifications at frequencies around Hz, and a very small peak at Hz, suggesting that the amplification at Hz is due to the site, rather than to the building. If we use the model (.) H T = 9. (.) D proposed by the Uniform Building Code [9] for the resonance frequencies of the building in which CARC is instrumented, we obtain frequencies around Hz and in the frequency range of 7 Hz as well. This model, far away from being a structural analysis, is, however, a simple model in good agreement with microtremor measurements (Figure ).. Discussion and Conclusions Trieste, though not affected by strong earthquakes in its history, is located close to a set of active faults. Most of its downtown is built on thick soft sediments, which could reasonably amplify the earthquake ground motion in case of an event, but no site effects analyses have been carried out in the city so far. Therefore, we believe that a detailed study of the local response of Trieste downtown should be of some concern in terms of seismic hazard. This is hence a first attempt to derive useful information on local ground motion amplification in the area using accelerometric data. A high urban-noise level, along with the moderate seismicity of the area, result in few earthquake weak motion recordings having been obtained at CARC. In particular, beside the reference station TRI located inside a cave on limestone bedrock, another station (DST) on Flysch bedrock has been used for computing SSR, in order to analyze the largest

16 Site Effects Analyses in the Old City Center of Trieste (NE Italy) 7 Downloaded At: 9: 9 May 7 number possible of events recorded by CARC and to characterize the local response of the Flysch unit present in most part of the city. The SSR approach applied to this latter station has evidenced a quite flat response on the horizontal components and an amplification factor of about, whereas the vertical component is, as predicted, not amplified. The receiver function technique is consistent with these results and the HVNSR approach at DST shows a flat response on both components and an amplification factor, as expected, slightly lower. The site effects studies performed in the old city center of Trieste, using different techniques, have all evidenced relevant amplification for frequencies around Hz, consistent with the resonance frequency of the soft sedimentary cover, obtained using a simple -D model. This result is also supported by continuous microtremors measurements performed on the basement and on the uppermost floor of Palazzo Carciotti, which show amplifications around Hz and not around Hz. These results are particularly important if we consider that most of the edifices built in this part of the city have similar dimensions as Palazzo Carciotti, suggesting that the ground motion amplifications found in this study could be reasonably unsafe for a large area. Acknowledgments This study was supported by the Interreg III-a Italia-Austria contract Reti sismologiche senza frontiere nelle Alpi sud-orientali and by the Interreg III-b Alpine Space Sismovalp Seismic hazard and alpine valley response analyses. Thanks are due to the anonymous reviewers for constructive comments that helped us to improve the paper. References Aki, K. [99] Local site effects on weak and strong ground. motion, Tectonophysics,, 9. Amoruso, A., Crescentini, L., De Luca, G., Scarpa, R., Abril, M., and Cirella, A. [997] Underground earth strain and seismic radiation measurements with a laser interferometer and a dense small-aperture seismic array, Annali di Geofisica,, 99. Aoudia, A. [99] Active Faulting and Seismological Studies for Earthquake Hazard Assessment, Ph.D. Thesis, University of Trieste pp. BenedettiI, L. [999] Sismotectonique de l Italie et des Regions adjacentes Ph.D. Thesis, Univerisy Paris 7, pgs. Bard, P.-Y. and Bouchon, M. [9] The two-dimensional resonance of sediment-filled valleys, Bull. Seism. Soc. Amer, 7, 9. Bonilla, F. L., Steidl, J. H., Lindley, G. T., Tumarkin, A. G., and Archuleta, R. [997] Site amplification in the San Fernando Valley, California: variability of site-effect estimation using the S-cods and H/V methods, Bull. Seism. Soc. Amer, 7, 7 7. Borcherdt, R. D. and Gibbs, J. F. [97] Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 96 earthquake, Bull. Seism. Soc. Am., 66, 67. Duval, A.-M., Bard, P. Y., Lebrun, B., Lacave-Lachet, C., Riepl., J., and Hatzfeld, D. [] H/V technique for site response analysis. Synthesis of data from various surveys, Boll. Geofis. Teor. Appl., ( ), 67. Fäh, D. and Suhadolc, P. [99] Application of numerical wave-propagation techniques to study local soil effects: the case of Benevento (Italy), Pure Appl. Geophys., (), 6. Field, E. H. and Jacob, K. [99] A comparison and test of various site-response estimation techniques, including three that are not reference-site dependent, Bull. Seism. Soc. Amer.,, 7. Lachet, C. and Bard, P. Y. [99] Numerical and theoretical investigations of the possibilities and limitations of the Nakamura s technique, J. Phys. Earth,,

17 F. Fitzko et al. Downloaded At: 9: 9 May 7 Lachet, C., Hatzfeld, D., Bard, P. Y., Theodulidis, N., Papaioannu, C., and Savvaidis, A. [996] Site effects and microzonation in the city of Thessaloniki (Greece), Bull. Seism. Soc. Amer, 6, Lebrun, B., Hatzfeld, D., and Bard, P. Y. [] Site effect study in urban area: experimental results in Grenoble (France), Pure appl. Geophys.,, 7. Lermo, J. and Chavez-Garcia, F. J. [99] Site effects evaluation using spectral ratios with only one station, Bull. Seism. Soc. Am.,, 7 9. Lermo, J. and Chavez-Garcia, F. J. [99] Are microtremors useful in site response evaluation?, Bull. Seism. Soc. Amer.,, 6. Nakamura, Y. [99] A method for dynamic characteristic estimations of subsurface using microtremor on the ground surface Quart. Rep. Railway Tech. Res. Inst. (RTRI),,. Nogoshi, M. and Igarashi, T. [97] On the propagation characteristics of microtremor, J. Seism. Soc. Jpn.,, 6. Site EffectS assessment using AMbient Excitations SESAME [] European Commission, nr. EVG-CT- 6. Theodulis, N. Bard, P. Y., Archuleta, R., and Bouchon, M. [996] Horizontal-to-vertical spectral ratio and geological conditions: the case of Garner Valley Downhole Array in southern California, Bull. Seism. Soc. Amer., 6, 6 9. Uniform Building Code [9] International Conference of Building Officials, Whittier, California.