Description des sites ´ etudi´ es

Cinq des sites NERIES sont ´etudi´es en d´etails dans ce travail de th`ese : Bolu, Cerreto, Colfiorito, Knidi et Tolmezzo. Leur description est r´edig´ee en anglais car elle faisait partie du livrable NERIES - JRA4 D5 (NERIES-JRA4, 2009a), associ´e `a l’´etude sur les sites 2D/3D (section 4.3).

Bolu

Bolu is located in a large alluvial plain, and the site is known from borehole investi- gation to lay on very stiff to hard sandy clays and clayey sands, at least down to 30 m depth. No information is available at greater depth, and the only measurements allowing in depth investigations are the ambient vibration array measurements performed during the NERIES project. Their dispersion curve inversion results in a 1D profile where no strong velocity contrast separates bedrock and sediments (Figure 2.3a). In the following, we refer to the H/V results obtained on this ambient vibration acquisition, which consists in 4 concentric arrays of 5, 15, 45 and 100 m radius.

48 2. Description et reconnaissance des sites Cerreto

Cerreto site is in a narrow river valley, whose western and eastern sides are filled with travertine and alluvia respectively (Figure 2.4 a et b). Both array and profile acquisitions are located on the eastern side of the valley. The profile (35 m long) was acquired at the edge of the valley, lying both over alluvial deposits on its western part, and over mountain side debris on its eastern part (Figure 2.4b). Moreover, the three smallest arrays (of 5, 15 and 20 m radius) are spread on alluvial deposits, whereas the largest one (about 80 m radius) lays both on alluvial deposits and on travertine. Shear wave velocity in the upper meters is comprised between 160 and 200 m/s.

Colfiorito

The Colfiorito basin is a 3-km wide intramountain basin in the Appenninic arc. Its sedi- mentary fill consists of alluvial deposits (lateral debris fans mixed with lacustrine sandy- clayey deposits) overlaying limestone and marls of the Umbria-Marche Meso-Cenozoic sequence. The bedrock topography was determined by Di Giulio et al. (2003) with bore- holes and extensive seismic refraction and geoelectrical surveys (Figure 2.4c). Two loca- tions were investigated within the NERIES project. The first one is close to the strong motion station (Colfiorito SM, Figure 2.4c). In section 4.3, we use data from one array acquisition with 2 concentric arrays of 15 and 100 m radius (2^nd location for Colfiorito in Table 2.2), and one 115 m long active seismic profile (3^rdprofile for Colfiorito in Table 2.2).

At this location, the shallow geological structure consists of horizontally layered Eocene marsh with shear wave velocity increasing from 70 to 150 m/s in the first 10 m (Figure 2.4f). The bedrock interface dips slightly from 50 to 60 m depth eastwards (Figure 2.4c).

Fig. 2.2 – Processing of surface waves at the Volvi site (right page).

a) and b) Dispersion curve comparison :a) Rayleigh and b) Love dispersion curves measured from surface waves or computed from Vs profiles estimated with different methods. [NERIES FK : average of the FK and HRFK dispersion curves, NERIES SPAC : dispersion curve inverted from auto-correlation curves, NERIES MASW P1 - P3 : dispersion curves measured by MASW on the 3 profiles, REFRACTION : theoretical dispersion curves of the model derived from re- fraction analysis, SESAME FK et SPAC : dispersion curves measured in previous investigations durong the SESAME project].

c) to f) Active seismics : c) Vs profiles resulting from the inversion of the MASW dispersion curves. d) and e) Comparison of measured (black) and inverted (colors scaled with misfit) Ray- leigh and Love MASW dispersion curves respectively. f) Misfit color scale of graphs c) to e).

g) to m) Passive seismics : g) Vs profiles resulting from the inversion of the passive seismics dispersion and auto-correlation curves. h) and i) Comparison of measured (black) and inverted (colors scaled with misfit) passive seismics Rayleigh and Love dispersion curves respectively. j) and k) Comparison of measured (black) and inverted (colors scaled with misfit) Rayleigh auto- correlation curves for the 3 largest and 3 smallest rings respectively. l) Misfit color scale common to all other graphs. m) Comparison of measured H/V ratios (green) and theoretical Rayleigh ellipticity curves of the inverted profiles (black).

2.1. Les sites NERIES 49

50 2. Description et reconnaissance des sites The second location (Colfiorito BE, Figure 2.4c) is at the NW edge of the basin. It was investigated with a 69 m long seismic profile (1^st profile for Colfiorito in Table 2.2). The refraction analysis shows that Vs in the first 10 m increases from 120 to 300 m/s, and that the bedrock below the profile (with Vs ≈ 1000 m/s) dips from 15 to 25 m depth (Figure 2.4e).

Knidi

Knidi site is located on a large slope in a hilly area. The geology of the upper part of the subsurface is essentially composed of conglomerates and sandstone (geological map). The refraction analysis of the active seismic data indicates a shallow dipping interface (Figure 2.3b) separating a soft layer (Vs≈100 m/s) from a much stiffer one (Vs≈500 m/s). This dip is weak in absolute value (1 m vertically against the 35 m length of the profile), but is relatively significant compared with the depth of the interface itself, varying from 1 to 2 m.

Tolmezzo

Finally, the strong motion station in Tolmezzo is located on rock, on top of a ridge and above a hydroelectric dam on the western side of the hill (Figure 2.4g and h). The shear wave velocity estimated from borehole measurement increases from 800 to 1200 m/s in the first 20 m and remains around this velocity at least down to 60 m depth. The array acquisition (2^nd location for Tolmezzo in Table 2.2, with 5 and 15 m radius arrays) is close to the strong motion station.

Fig. 2.3 – Geological and topographical information on the sites used for the study on 2D/3D sites (section 4).

a) BOLU : Vs profiles with misfit < 1 resulting from surface wave inversion of ambient vibration measurements.

b) KNIDI : Seismic structure deduced from SH refraction analysis.

2.1. Les sites NERIES 51

Fig.2.4–Geologicalandtopographicalinformation(suite)onthesitesusedforthestudyon2D/3Dsites(section4). a)andb)CERRETO:a)PhotographtakenfromtheSouth(fromA.Rinaldi),withpositionofthelargestarray(triangles),thesmall arrays(rhombus)andthe35mlongseismicprofile(doubleblackarrow).Blackdashedline:positionofthecross-sectionpresentedinb).b) Geologicalcross-sectionofthevalleywithpositionofthelargestarrayandtheseismicprofile(doubleblackarrows).1)alluvia,2)travertine, 3)moutainsidedebris,4)carbonatebedrock.ModifiedfromS.Martino(personalcommunication) c)tof)COLFIORITO:c)Locationofthetwoacquisitionsclosetothestrongmotionstation(SM),andthebasinedge(BE)plottedon thedepthofbedrockmapfromDiGiulioetal.(2003).d)Correspondingcross-sectionbelowthebasin.e)andf)seismicstructuresinduced fromrefractionanalysiswithamultidippinglayeralgorithmatBEandSMrespectively. g)andh)TOLMEZZO:g)aerianvueofthesite.Redcircle:positionofthestrongmotionstation.h)West-Eastgeologicalcross-section.

52 2. Description et reconnaissance des sites