GRAVIMETRIC AND MICROSEISMIC CHARACTERIZATION OF THE GEMONA (NE ITALY) ALLUVIAL FAN FOR SITE EFFECTS ESTIMATION Furlanetto Eleonora, University of Trieste - Dip. Scienze della Terra, Trieste, Italy Costa Giovanni, University of Trieste - Dip. Scienze della Terra, Trieste, Italy Palmieri Francesco, OGS, Trieste, Italy Delise Andrea, University of Trieste - Dip. Scienze della Terra, Trieste, Italy Suhadolc Peter, University of Trieste - Dip. Scienze della Terra, Trieste, Italy
CS5 :: Poster :: Thursday - Friday :: Level 2 :: P533B
The urban area of Gemona (NE Italy) is mainly built on alluvial fan sediments that contributed to the destruction of the city during the Friuli earthquake, May-September 1976. Three accelerometric stations of the Friuli Venezia Giulia Accelerometric Network, run by the Department of Earth Sciences, University of Trieste, in collaboration with the Civil Defence of FVG, are set in Gemona for site effects estimation purposes. Using weak motion recordings of these stations, we are able to derive the H/V spectral ratio and also to apply the reference site technique. The result of these elaborations shows different resonant frequencies in the two sites (one on the fan, one on the sedimentary basin), when excited by the same event, and also different resonance frequencies at the same site when excited by different events. This can be explained with 2D or 3D site effects modelling, that requires, however, the characterization of the local subsoil structures, in particular the sediment-bedrock contact. We use gravimetric data to characterize a model with a homogeneous sedimentary layer of variable thickness along five selected profiles. The models are elaborated using the residual Bouguer anomaly and, as a constraint, three boreholes that reach the bedrock, geological outcrops and intersection points on the profiles. We derive the mean thickness along all the profiles and the bedrock undulations. We validate the modelling using a second approach based on seismic noise data acquired along the same profiles. The data are analysed to calculate the fundamental frequency of soft soil using Nakamura?s techniques, that is compared to resonance frequencies calculated from weak motion events. The lack of agreement confirms the 2D or 3D character of the alluvial fan. However, the computed thickness of sediments along all the profiles match the ones determined from gravimetric inversion results.
GRAVIMETRIC AND MICROSEISMIC CHARACTERIZATION OF THE GEMONA (NE ITALY) ALLUVIAL FAN FOR SITE EFFECTS ESTIMATION E. Furlanetto (1), G. Costa (1), F. Palmieri (2), A. Delise (1), P. Suhadolc (1)
(1) Dipartimento di Scienze della Terra, DST, University of Trieste, v. Weiss 1, 34127 Trieste, Italy (2) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, OGS, Borgo Grotta Gigante, 34100 Trieste, Italy
The urban area of Gemona (NE Italy) is mainly built on alluvial fan sediments that contributed to the destruction of the city during the Friuli event sequence, May-September 1976. Three accelerometric stations of the Friuli Venezia Giulia Accelerometric Network (RAF ), run by the Department of Earth Sciences, University of Trieste, in collaboration with the Civil Defence of FVG, are set in Gemona for site effects estimation purposes. Using weak motion recordings of these stations, the H/V spectral ratios are computed and the reference site technique applied. The result of these elaborations shows different resonant frequencies at the two sites (one on the fan, one on the sedimentary basin), when excited by the same event, and also different resonance frequencies at the same site when excited by different events. This can be explained with 2D or 3D site effects modelling, that requires, however, the characterization of the local subsurface structures, in particular the sediment-bedrock contact. Gravimetric data are used to characterize a model with a homogeneous sedimentary layer of variable thickness along five selected profiles. The models are elaborated using the residual Bouguer anomaly and, as a constraint, three boreholes that reach the bedrock, geological outcrops and intersection points on the profiles. The sediment thickness along all the profiles and the bedrock undulations are retrieved. A second approach based on seismic noise data acquired along the same profiles is also applied. The data are analysed to calculate the fundamental frequency of soft soil using Nakamura's technique. The results are compared to the resonance frequencies calculated using weak motion events recorded by the RAF stations. The lack of agreement confirms the 2D or 3D character of the alluvial fan. However, the computed thickness of sediments along the profile analyzed match the ones determined from gravimetric inversion results. The Gemona’s RAF stations
1976 GMT M GESC station located on alluvial fan sediments. 6May 20:00:08 6.4 Operating form 1993 until now.
9May 00:53:42 5.3
11September 16:31:08 5.1
11September 16:34:53 5.6 The two stations on basin sediments: 15September 03:15:17 5.8 GETM station operating from 1993 to 2000; 15September 09:51:16 6.1 GEDE station operating form 2000 until now. Biggest aftershocks of the Friuli 1976 earthquake occurred in the area of Gemona. GEPF station located on bedrock in a tunnel. Events recorded by the RAF from 1993 to 2005 in the Friuli Venezia Reference Station operating from 1993 until now. Giulia area. White rectangle indicates the Gemona area.
The Gemona area complexity H/V spectral ratio and reference site technique applied to signals recorded from Gemona accelerometric stations are shown in the figure below on the right. The results for 6 events with different azimuths are reported. Comparing the amplifications (SSR techniques) obtained at the two different sites, for the same event, they are quite different. It is clearly visible that the resonant frequencies are spread over a wide range, even for different events recorded at the same site. This might be exlained with 2D or 3D site effects. Therefore, an investigation of the characteristics of the local subsurface in the area of the stations is needed.
Carnia, 14/02/2002, GESC station HVSR East/Vertical GESC station HVSR North/Vertical GESC station SSR GEPF East component GESC station SSR GEPF North component Md 4.9 (OGS) 12 12 30 30 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Carnia 14/02/02 Md 4.9 Carnia 14/02/02 Md 4.9 Carnia 14/02/02 Md 4.9 Carnia 14/02/02 Md 4.9 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Pontebba, 12/12/2005, Pontebba 12/12/05 Ml 3.3 Bordano, 02/01/2005, Venzone, 08/02/2005, 8 Pontebba 12/12/05 Ml 3.3 8 Pontebba 12/12/05 Ml 3.3 20 Pontebba 12/12/05 Ml 3.3 20 AVERAGE AVERAGE AVERAGE AVERAGE Md 2.5 (OGS) Ml 2.5 (DST) Ml 3.3 (DST)
4 4 10 10
Fourier Spectral Ratio Fourier Spectral Ratio Fourier Spectral Ratio Fourier Spectral Ratio FAN
0 0 0 0 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 Frequency (Hz) Frequency (Hz) Frequency (Hz) Frequency (Hz)
Bovec, 31/08/2005, Ml 3.1 (DST) GEDE station HVSR East/Vertical GEDE station HVSR North/Vertical GEDE station SSR GEPF station East component GEDE station SSR GEPF station North component 12 12 30 30 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Bovec 12/04/98 Ms 5.7 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Bordano 02/01/05 Md 2.5 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Venzone 08/02/05 Ml 2.3 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Bovec 31/08/05 Ml 3.1 Pontebba 12/12/05 Ml 3.3 Pontebba 12/12/05 Ml 3.1 Pontebba 12/12/05 Ml 3.3 Pontebba 12/12/05 Ml 3.3 AVERAGE AVERAGE AVERAGE AVERAGE Bovec-Krn, 12/04/1998, 8 8 20 20 Ms 5.7 (OGS)
4 4 10 10 Fourier Spectral Ratio Fourier Spectral Ratio Fourier Spectral Ratio Fourier Spectral Ratio
BASIN
0 0 0 0 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 Frequency (Hz) Frequency (Hz) Frequency (Hz) Frequency (Hz)
Gravimetric characterization of the Gemona fan A second way to characterize the fan: seismic noise measurements Gravimetric data were collected over 5 profiles on the fan: 3 profiles are transversal (NS profiles) and 2 Seismic noise data were recorded along the RED coloured profile at 22 sites, using as noise sites the same coaxial (EW profiles) to the fan axis. Planning the gravimetric net (40 m step), two gravimetric sites were locations used before for gravimetric measurements. constrained to coincide with the locations of GEPF and GESC stations. 3 boreholes that reach the bedrock H/V spectral ratios were performed using J-SESAME program (Atakan et al, 2003), recording time windows (Stefanini, 1986)were used as costraint, as well as geological outcrops and intersection points on the pofiles. of 10min and processing windows of 20s. Below the results for the gravimetric data inversion. The processing was made using 2.6 g/cm3 as density for Toestimate the depth of the sediments a simple 1D the bedrock, and 1.8 g/cm3 as a density for the sedimentary layers (Broili et al, 1980). The assumed Model was used: f =b/4 H. sedimentary layer is a single, homogeneous layer with no intrusions. The shear velocity attributed to sediments varies from 0.4km/s to 0.6km/s.
400 RED Profile
350 GESC
SURFACE TOPOGRAPHY
300
250 Height (m)
200 CONCLUSIONS Gravimetry 150 ??? 0.4 km/s ????0.5 km/s ????0.6 km/s Average thickness 105 m 78 m !The mean thickness of the sediments along all 100
0 0.4 0.8 1.2 1.6 the profiles and the bedrock undulations are well offset (km) defined. 65 m !The thickness of sediments calculated with the 2 different approaches, using unrelated constraints, are in agreement. REFERENCES Atakan K., Bard P.Y., Moreno B., Roquette P., Tento A. (2003): “Report on the Multiplatform 103 m H/V Processing Software J-SESAME”, WP03-Technique: Data Processing, Deliverable !The lack of agreement between fundamental D09.03, SESAME Project, Department of Earth Sciences, University of Bergen Broili L., Carraro F., Cucchi F., Onofri R., Stefanini S., Ulcigrai F. (1980): “Comune di Gemona frequency of soft soil calculated using del Friuli: studio geologico per la ricostruzione”, 95 pp., Tip. Naz. Trieste Nakamura’s technique (H/V ratio) and Furlanetto E. (2004). Indagini geofisiche per caratterizzare al struttura del sottosuolo del conoide di Gemona del Friuli. Graduate Thesis, University of Trieste, 190 pp. resonance frequencies calculated from weak Site E ffect S assessment using AM bient E xcitations - SESAME (2001): European motion events confirms the 2D or 3D character of Commission, nr. EVG1-CT-2000-00026. Stefanini S. (1986): “Litostratigrafie e caratteristiche idrologiche di pozzi della pianura the alluvial fan. friulana, dell'anfiteatro morenico del Tagliamento e del campo di Osoppo e Gemona”, pp. 145-174, Trieste !This result encourages us to extend the noise measurements over all the fan to characterize its ACKNOWLEDGMENTS This study was financially supported by contracts INTERREG IIIB Alpine Space Project 106 m "Sismovalp: Seismic risk in alpine valleys", “Contratto tra Dipartimento di Scienze della Terra e Regione Autonoma FVG, Direzione Regionale della Protezione Civile per la gestione della Rete Accelerometrica del Friuli Venezia Giulia (Contratto 6/06, Delibera Giunta Regionale Details of the 5 selected profiles, anomaly and model, 3425, dd 29/12/05)”, Progetti di ricerca di particolare interesse per il Dipartimento della Protezione Civile - convenzione ProCiv-INGV 2004-06: Progetto esecutivo S4 "Stima dello and their location on the fan (Furlanetto, 2004). scuotimento in tempo reale o quasi reale per terremoti significativi in territorio nazionale".