Il Convegno è aperto a tutti coloro che operanonel campo della Geofisica della Terra Solida

Il Convegno è organizzato nei seguenti temi:

7° Convegno Nazionale

* sessioni co-organizzate da:

Segreteria del Convegno

Fino al 9 novembre 2007:

c/o Istituto Nazionale di Oceanografiae di Geofisica Sperimentale - OGS

Borgo Grotta Gigante 42c34010 SGONICO (TS)tel. 040 2140339fax 040 327307

Angela Marchettoamarchetto@ogs.trieste.itgngts@ogs.trieste.it

Durante il Convegno:

c/o Consiglio Nazionaledelle Ricerche

Aula ConvegniPiazzale Aldo Moro 700197 ROMAtel. 06 49933555 – 06 49933188

• Le note potranno essere presentate oralmente o come poster.

• I tempi per la presentazione delle note sono 15 min +5 min di discussione.

• I poster saranno esposti per tutta la durata del Convegno.

• Il presentatore del poster dovrà essere disponibile presso il proprio posterdurante le ore previste dal programma.

• Per poter presentare le note, gli Autori dovranno registrarsi e quindidepositare un “Riassunto Esteso” di 1-4 pagine dal 21 settembre all’8ottobre 2007 sul sito GNGTS: www2.ogs.trieste.it/gngts/ secondo i formatie le modalità indicati nello stesso. D

esi

gner:

Nin

oBon –

OGS,

Trie

ste

Sta

mpa:

Ste

lla, Tr

iest

e

GRUPPO

DI GEOFISICA

DELLA TERRA SOLIDA

NAZIONALE

EUROPEAN

ASSOCIATION OF

GEOSCIENTISTS &

ENGINEERS

S E Z I O N E I T A L I A N A

Tema 1: Geodinamica: osservazioni e modelliinterpretativi

Sessione 1.1. Sismicità e sismogenesiConvenor: E. Eva (INGV, Genova) e P. Gasperini (Univ. Bologna)

Sessione 1.2. Geodinamica e tettonofisicaConvenor: R. Caputo (Univ. Ferrara) e E. Mantovani (Univ. Siena)

Sessione 1.3. Aree vulcaniche e geotermicheConvenor: E. Del Pezzo (INGV, Napoli) e A. Siniscalchi (Univ. Bari)

Tema 2: Sismologia per la pianificazione territoriale e la progettazioneSessione 2.1. Sismotettonica

Convenor: P. Galli (Dip. Prot. Civ., Roma) e S. Solarino (INGV, Genova)

Sessione 2.2. Caratterizzazione sismica del territorioConvenor: D. Albarello (Univ. Siena) e G. Naso (Dip. Prot. Civ., Roma)

Sessione 2.3. Scenari di scuotimento e di dannoConvenor: A. Masi (Univ. Basilicata) e M. Mucciarelli (Univ. Basilicata)

Tema 3: Geofisica Applicata*Sessione 3.1. Applicazioni della Geofisica all’Ingegneria, ai Beni Culturali

ed all’AmbienteConvenor: G. Ranieri (Univ. Cagliari) e L. Zanzi (Politecnico Milano)

Sessione 3.2. Sismica di esplorazione per indagini ad alta e bassa profonditàConvenor: G. Böhm (OGS, Trieste) e M. Pipan (Univ. Trieste)

Sessione 3.3. Esplorazione tramite metodi di potenzialeConvenor: C. Del Negro (INGV, Catania) e M. Fedi (Univ. Napoli)

Roma13-15 novembre 2007

Consiglio Nazionaledelle Ricerche

Piazzale Aldo Moro 7

Convegno

Nazionale

26º

Istruzioni per gli Autori:

Features of seismic microtremor signals within a building S. Lepore (1), V. Di Fiore (2), A. Rapolla (1,3) (1) Department of Earth Sciences, University Federico II, Complesso Universitario di Monte S. Angelo, Naples. (2) Consiglio Nazionale delle Ricerche (CNR), Istituto per l’Ambiente Marino e Costiero, Naples. (3) Consorzio Inter-Universitario per la prevenzione dai Grandi RIschi (C.U.G.RI.), Naples. Microtremors are omnipresent low amplitude oscillations that take place on the Earth surface. They are caused by a variety of different, spatially distributed, mostly unrelated and often continuous sources of human and natural origin. Because of source variety, microtremors are a more or less stationary stochastic process without a defined phase spectrum (Bormann, 2002). The microtremor amplitude spectrum, instead, is dependent on the soil properties and on the natural period of generating sources. The purpose of this work is to describe an experience on how a building changes the characteristics of microtremor signals, moving from the free earth outside the building to its basement and to the upper floors. In order to perform this study, a comparison was first made between the acceleration signal amplitude at the free earth and inside the building, in time domain. Then, an amplitude spectral analysis of the free earth acceleration signal was carried out to verify the stability of the spectral pattern. Finally, to examine the changes, the outside signal spectra were compared with the signal spectra of the simultaneous data recorded inside the building at all floors. Data acquisition was carried out continuously for several days at the Complesso Universitario di Monte S. Angelo of the University Federico II of Naples, outside and inside the building, where our geophysics laboratory is located, which is made of reinforced concrete and has four floors. A reference external seismic station was put at the free earth into a pyroclastic clay layer, thirty centimeters beneath the surface. This station was equipped with a three-component velocimeter that shows a flat band between 1 and 80 Hz in the velocity vs frequency response curve. A sixteen bits ADC (analog digital converter) data acquisition card was connected to the velocimeter. The card was linked to a computer in order to record and process the data. Inside the building a internal seismic station was placed at all floors, equipped with the same acquisition and recording instruments of the external one. This expedient enabled us to obtain the best comparison between signals inside and outside the building. A time derivative was applied to get an acceleration signal, together with a digital notch filter to remove 50 Hz power frequency. Five-minute records of the microtremors acquired at the external station are reported in Fig.1. In part (A), signal 1 corresponds to a record at 4:03 in the night on May 7th 2007, signal 2 at 10:03 in the morning, signal 3 at 16:03 in the afternoon, and signal 4 at 22:03 in the evening. In part (B), numbers have the same meaning, but the acquisition day is May 26th 2007. May 7th was a working day, while May 26th was a weekend day with a reduced human activity: the different level of microtremors is reflected by the higher amplitude of the signal in (A) with respect to (B). Then, the microtremor signals detected at the internal station were analyzed, and the relative acceleration amplitude was compared with the one recorded at the external station. Gradual amplitude attenuation has been noticed, moving from the free earth to the basement and to the upper floors of the building. In Fig.2 the five-minute record of microtremors acquired at the internal station on the working day August 1st 2007 at the 16:03 is shown, to be compared with the five-minute recording on May 7th 2007 at 16:03 detected by the external station. In the figure, signal V corresponds to the vertical component, signal H1 to the horizontal N-S component, and signal H2 to the horizontal E-O component. One-hour acquisition windows were employed for the spectral analysis of the signals recorded outside and inside the building, for remedying to local signal variations (Kagami et al., 1982). Since the frequency range of interest fell between 1 Hz and 10 Hz, the signal was filtered with a Butterworth lowpass filter of the tenth order with a 13 Hz corner frequency to remove higher frequencies from the spectrum. Moreover, the resultant signal was smoothed with a triangular moving average algorithm, to better visualize the pattern of the spectral acceleration amplitude as a frequency function, either at the free earth, either inside the building. Since in the low-frequency band horizontal components vary much more than the vertical one at soft sites (Cara, 2003), the modulus of the two horizontal components H and the vertical component V were studied separately.

Fig.1 - Amplitude-time records of the microtremors acquired at the external station

Fig.2 - Amplitude-time record of the microtremors acquired at the internal station Some of the spectra of the microtremors recorded at the external station are reported in Fig.3: a stability of the signal pattern for all frequencies has been noticed. Then, it was analyzed how the microtremor signal acceleration amplitude spectrum at the free earth is modified inside the building under examination. An evaluation of the microtremor spectrum at various floors was necessary, because a spectral change is foreseeable in correspondence of each level. Hence, the internal station was moved among all floors along the same vertical line. One spectrum for each floor is reported in Fig.4. With respect to the V spectra reported in Fig.3, the V spectra in Fig.4 results ever more attenuated for frequencies higher than 6 Hz. For the modulus of the two horizontal components,

A1

A2

A3

A4

B1

B2

B3

B4

V

H1 H2

H1 H2

instead, amplification has been noticed with respect to the amplitude values of the H spectra in Fig.3, that becomes increasingly higher passing from the basement to the top floor for frequencies higher than 4 Hz.

Fig.3 - The amplitude-frequency spectra of the vertical component (V) and of the modulus of the two horizontal components (H), associated to the microtremors recorded at the external station in different days and at the same hour (23:00-23:59).

Fig.4 - The amplitude-frequency spectra of the vertical component (V) and of the modulus of the two horizontal components (H), associated to the microtremors recorded at the internal station in different four days at the same hour (23:00-23:59). Curve 0 refers to the basement floor, 1 to the first floor, 2 to the second floor, and 3 to the third floor.

(V)

(V)

(H)

(H)

0

1 2

3

3

0-1-2

This is consistent with the results of Satake (1996) obtained by building vibration tests and it is possibly due to the fact that the building movement under ground motion evolves predominantly along the horizontal components. The study of the spectra inside the building along the modulus of the two horizontal components H and the vertical component V is very helpful also for the knowledge of the building characteristic frequency. It is reported in literature (Gamal, 2003), in fact, that source effects are removed through the calculation of H/V spectral ratio, allowing a more proper analysis of the natural vibration modes and of the corresponding amplification relative to each floor. As a consequence, the spectral ratio was estimated for each floor in different hours on various days: the value of the peak frequency for each floor of our building falls between 4.5 Hz and 5 Hz, as shown in Fig.5, where it is marked by an arrow. The obtained results seem to be very helpful in contributing to the evaluation of seismic induced vibrations on a building, even if the amplitude of microtremors is very small.

Fig.5 - The H/V spectral ratio obtained from the spectrum of microtremors recorded at internal station from 23:00 to 23:59. Curve 0 refers to the basement floor, 1 to the first floor, 2 to the second floor, and 3 to the third floor. The arrows mark the peak frequency for each floor of our building. Bormann P.; 2002: New manual of seismological observatory practice. IASPEI Cara F., Di Giulio G., and Rovelli A.; 2003: A study on Seismic Noise at Colfiorito (Central Italy) and on implications for the use of H/V spectral ratios. Geophys. Res. Lett., 30, 1–4 Gamal M.A., and Ghoneim E.; 2003: Using microtremors to determine the natural frequencies of vibration for structures in Cairo city. IASPEI SHR E–Newsletter, 4, 215–241 Kagami H., Duke C.M., Liang C.G., and Yutaka O.; 1982: Observation of one to five second microtremors and their application to earthquake engineering. Part II. Evaluation of site effect upon seismic wave amplification due to extremely deep soil deposits. Bull. Seism. Soc. Am., 72, 987–998 Satake N., and Yokota H.; 1996: Evaluation of vibration properties of high-rise steel buildings using data of vibration tests and earthquake observations. Journal of Wind Engineering, 59, 265–282

3

2

1

0

04/06/07

08/05/07

08/07/07

02/08/07