Integration of Geological and Geophysical Data for Re
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ANNALS OF GEOPHYSICS, 61, 2, SE227, 2018; doi: 10.441/ag-7710 INTEGRATION OF GEOLOGICAL AND GEOPHYSICAL DATA FOR RE/ E“VALUATION OF LOCAL SEISMIC HAZARD AND GEOLOGICAL STRUCTURE: THE CASE STUDY OF ROMETTA, SICILY (ITALY) „ 1 2 1 1 1, Paolo Pino , Sebastiano D’Amico , Barbara Orecchio , Debora Presti , Silvia Scolaro *, Antonino Torre 1, Cristina Totaro 1, Daniela Farrugia 2, Giancarlo Neri 1 (1) Department of Mathematics, Computer Sciences, Physics, and Earth Sciences, University of Messina, Messina, Italy (2) Department of Geosciences, University of Malta, Malta Article history Receveid November 26, 2016; accepted December 22, 2017. Subject classification: Ambient noise; H/V; Seismic array; Site effects. ABSTRACT The village of Rometta, northern-eastern Sicily (Southern Italy), experienced severe damage during the most energetic earthquakes that occurred in eastern Sicily and southern Calabria in the last centuries. Geological maps indicate that Rometta primarily lies on a stiff plate of Upper Pliocene - Lower Pleistocene calcarenites and only to minor extent on Middle Pleistocene overlying clays. Rometta represents an interesting case study for site response investigation because of the apparent mismatch between the currently available geological knowledge and the level of damage caused by historical seismic events. The local seismic response has been investigated through a grid of 64 single-station measurements of ambient seismic noise by the Horizontal to Vertical Noise Spectral Ratio technique (H/V). Also, phase velocity dispersion curves from seismic array through the Extended Spatial Auto-Correlation method were analyzed. The H/V curves ob - tained show a ubiquitous frequency peak between 0.5 Hz and 0.9 Hz due to the deep interface between the metamorphic substrate and sedimentary sequence, and a secondary peak in the 2.5-15 Hz interval in most of the measurement points that may be related to a larger extent of the clay outcrops with respect to what is already known from geological investigations. Joint inversion of dispersion and H/V curves was also performed in order to obtain the velocity profile. Results acquired through the combined use of geophysical methods furnished useful information for seismic hazard evaluation where surface geology is not clearly visible because of urbanization and veg - etation cover, thus suggesting that a wide extent of clays may reasonably be the cause of past earthquake damage distribution. 1. INTRODUCTION Mw=7.1; 28 December 1908, I=XI MCS Mw=7.1, Locati et al., [2016]) that occurred in eastern Sicily and The village of Rometta is located in NE Sicily (Figure southern Calabria [Figure 1; Baratta, 1910; Locati 1), within a segment of the Neogene Apennine-Maghre - et al., 2016]. Also, macroseismic data (https://emid - bide orogenic belt, which developed in the Central ius.mi.ingv.it/CPTI15-DBMI15/) indicates that major Mediterranean Sea as a result of collision between Africa earthquake effects in Rometta were frequently greater and Europe and northwesterly subduction and roll-back compared to nearby locations thus leading us to in - of the Ionian slab [Ghisetti and Vezzani, 1982; Malin - vestigate the presence of heterogeneous geological set - verno and Ryan, 1986]. North-eastern Sicily presently tings potentially able to produce significantly separates the Tyrrhenian basin (to the NW) and the Io - differentiated seismic responses. nian basin (to the SE), both representing back-arc areas A quick estimate of the surface geology effects on [Gueguen et al., 1997; Faccenna et al., 2001]. seismic motion is provided by the Horizontal to Verti - Rometta experienced severe damage during the cal noise Spectral Ratio (HVSR) technique [Lermo and strongest historical earthquakes (for example 11 Jan - Chavez-Garcia, 1993; Konno and Ohmachi, 1998; Bard, uary 1693, I=XI MCS Mw=7.4; 5 February 1783, I=XI 1999; Bonnefoy-Claudet, 2006a; D’Amico et al., 2008, 1 PINO ET AL. In the present study, single-station horizontal to ver - tical spectral ratio technique (HVSR) and multi-station (seismic array) configurations have been used to deter - mine dynamical properties of the subsoil of Rometta from noise measurements. We performed 64 measure - ments of ambient noise by using the 3-component Tro - mino velocimeter (www.tromino.eu.), and we estimated shear wave velocity ( Vs) profiles by analyzing phase ve - locity dispersion curves from seismic arrays through the Extended Spatial Auto Correlation (ESAC) method [Ohori et al., 2002; Okada, 2003; Parolai et al., 2006; Albarello et al., 2011]. Results obtained from measurement analyses, com - pared with the local geology, furnishes new informa - tion useful for seismic risk mitigation. 2. GEOLOGICAL SETTING AND STRATIGRAPHIC FIGURE 1. The map shows the location of Rometta village, NE FRAMEWORK OF ROMETTA HILL Sicily, and the epicenter (stars) of historical events producing effects of least of degree VII (MKS scale; The main tectonic features of the study area, located https://emidius.mi.ingv.it/CPTI15-DBMI15/) in in the Peloritani Mts. (Figure 1), is represented by a NE- Rometta. The year of earthquakes occurrence and the relative magnitude are reported. The inset shows the SW and NNE-SSW oriented normal fault system, which northwestern trending convergence between African causes brittle deformation of the contractional archi - and European plates (black arrows) [Calais et al., tecture [Bonini et al., 2011] and controls the actual set - 2003; Nocquet and Calais, 2004; Nocquet, 2012] and ting of this sector of the Sicilian Tyrrhenian coast the Appennine-Maghrebides orogenic belt (grey line) [Neri et al., 2009]. [Guarnieri and Carbone, 2003; Di Stefano et al., 2007]. The extensional tectonic activity of the Peloritani Mts. relates to the opening of the Tyrrhenian Sea and more Castellaro and Mulargia, 2009; Vella et al. 2013; recently of the Messina Straits [Bonini et al., 2011]. In Paolucci et al., 2015; Scolaro et al., 2018]. This tech - this area paleozoic high-grade metamorphic rocks out - nique relies on the recording and analysis of the envi - crop extensively (Aspromonte Nappe, Aut.), locally cov - ronmental ground noise, both of natural and ered by heterogeneous sedimentary Middle Upper anthropogenic origin, and uses the Fourier amplitude Miocene - Middle Pleistocene successions [Lentini et al., spectral ratio between the horizontal to vertical com - 2000; APAT, 2008]. ponents of the recorded signals [Nogoshi and Igarashi, The landscape of the Peloritani Mts. reflects the tec - 1971; Nakamura, 1989; Bonnefoy-Claudet, 2006a and tonic setting of the area, in particular the intense recent 2006b]. The basic hypothesis of seismic ambient noise dynamics related to strong tectonic uplift [Montenat et theory is that a resonance peak in the H/V ratio may be al., 1991; Westaway, 1993; Tortorici et al., 1995]. Vertical interpreted both in terms of SH-wave resonance in su - movements during the late Quaternary controlled the perficial layers or in terms of ellipticity if Rayleigh sur - shaping of the landforms as indicated by isolated hills and face waves predominate [Nogoshi and Igarashi, 1971; NW-SE elongated hilly ranges with steep slopes deeply Lachet and Bard, 1994; Scherbaum et al., 2003]. The cut by V-shaped valleys, tectonically structured in a half- wavefield is a combination of both types of waves and graben downdropped toward the Tyrrhenian sea. then the H/V curve contains information about the Rometta hill is surrounded by cliffs and escarpments shear wave velocity profile in shallow sediments. Also, (Figure 2a). This geomorphological framework clearly it is commonly accepted that, although the single com - shows the sequence of lithological units outcropping ponents of ambient noise can show large spectral vari - (Figure 2, plots a and b), allowing relatively easy field ations as a function of natural and cultural observations for reconstruction of the lateral and verti - disturbances, the horizontal to vertical noise spectral cal distribution, geometries and thicknesses of the pre- ratio tends to remain invariant, therefore preserving the Middle Pleistocene. As inferred by geological maps fundamental frequency peak [Cara et al., 2003]. [Lentini et al., 2000; APAT, 2008, see plots 2c, 2d] 2 LOCAL SEISMIC HAZARD RE-EVALUATION IN NE SICILY FIGURE 2. (a) Picture of north-western sector of Rometta hill showing abrupt geomorphological framework made of cliffs and steep slopes. Dotted lines illustrate the vertical sharp transition between geological formations. (b) Schematic geological sec - tion of Rometta hill. Geological maps of the study area: (c) APAT [2008], (d) Lentini, 2000. Legend: prevailing parag - neiss of paleozoic metamorphic Complex - Aspromonte Unit (PMAa and UA); undifferentiated lithofacies of Middle-Upper Miocene San Pier Niceto Formation consisting of well consolidated thick sands and arenites weakly cemented (PCTb and Mar) passing upwards to conglomeratic (PCTc and Mac); stiff upper Messinian evaporitic limestones (GSa and Mc); con - solidated and massive Middle Pliocene sandy marls of Massa S. Lucia Formation (FSL); stiff calcarenites massive and cross-stratified of Upper Pliocene - Lower Pleistocene (ROE and PQ) with at the top marly clay Middle Pleistocene (ROEa and Qa) both attributed to Rometta Formation. The latter have been evidenced by red contour. Rometta village is mainly set on the Upper Pliocene- tapering to 10-20 m along the southern rim of Rometta Lower Pleistocene calcarenites (Rometta Formation Aut.) Hill, and are intermediate (40