SEISMIC MICROZONATION OF THE CITY OF CATANIA FOR THE ETNA EARTHQUAKE (M=6.2) OF FEBRUARY 20, 1818 ABSTRACT

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1 SEISMIC MICROZONATION OF THE CITY OF CATANIA FOR THE ETNA EARTHQUAKE (M=6.2) OF FEBRUARY 20, 1818 S. Grasso 1, M. Maugeri 2 and L. Spina 3 ABSTRACT The geotechnical zonation of the subsoil of the city of Catania suggests a high vulnerability of the physical environment to be added to site amplification of the ground motion phenomena. These elements concur on the definition of the Seismic Geotechnical Hazard of the city of Catania, that should be correctly evaluated, through geo-settled seismic microzoning maps. Based on the seismic history of Catania, the Etna earthquake of February 20, 1818 (M=6.2), has been considered as level II scenario event (base operative earthquake). Despite of its lower magnitude, a medium size earthquake, such as the 1818 Etna event, has to be accounted for the seismic hazard assessment of Catania, since it may cause heavy damage to the most urbanized area. According to historical data, the epicentre was located along the south-eastern flanks of the Etna Volcano, close to the municipal area of Catania. This earthquake is considered a tectonic earthquake and is associated to the northern segment of the Ibleo-Maltese fault system. This fault system is the major seismogenic structure of Eastern Sicily, and it is considered the responsible of the major historical earthquakes which struck this area and that devastated the city. The near-fault strong ground motion, evaluated at the conventional bedrock, as suggested by the Eurocode EC8: Design of Structures for Earthquake Resistance (CEN, 2002), has been computed through a hybrid stochastic-deterministic method (EXWIM) (Laurenzano et al., 2004). This method simulates rupture propagation along finite fault and solves the 3-D fullwave propagation in inelastic media with a vertically heterogeneous structure. In order to evaluate an exhaustive scenario, different slip distributions and hypocenters have been considered. The structural model assumed is representative of the Eastern Sicily area; the ground motion was computed for a regular grid of receivers sampling the urbanized area of Catania. The results consist of threecomponent waveforms in terms of displacement, velocity and acceleration. After evaluating the seismograms at the bedrock by the 3-D model, the ground response analysis at the surface, in terms of time history and response spectra, has been obtained by a 1-D non-linear model. In particular the study has regarded the evaluation of site effects in correspondence of the database of about 1200 boreholes and water-wells available in the data-bank of the Research Project: 1 Ph. D. in Geotechnical Engineering, University of Catania, Faculty of Engineering, Viale Andrea Doria n. 6, Catania (Italy). 2 Full Professor in Geotechnical Engineering, University of Catania, Faculty of Engineering, Viale Andrea Doria n. 6, Catania (Italy). 3 Ph. D. student in Geotechnical Engineering, University of Catania, Faculty of Engineering, Viale Andrea Doria n. 6, Catania (Italy).

2 Detailed Scenarios and Actions for Seismic Prevention of Damage in the Urban Area of Catania (Maugeri, 2000). According to the response spectra obtained through the application of the non-linear model, the city of Catania has been divided into some zones with different peak ground acceleration at the surface, to which corresponds a different value of the Seismic Geotechnical Hazard. A ground shaking map for the central area of the city of Catania was generated via GIS for the level II base operative scenario earthquake. The map represents an important tool for the seismic improvement of the buildings, indispensable for the mitigation of the seismic risk. Introduction According to the frequency and the importance of the seismic effects suffered in past times, Eastern Sicily must be considered one of the most high seismic risk areas in Italy. Today, on such a densely populated territory, a huge patrimony of historical and industrial buildings is placed. The most destructive earthquakes which affected Eastern Sicily are six events of the intensity equal or bigger than IX degree of MCS scale in the epicenter area, occurred in1169, 1542, 1693, 1757, 1818 and These can be considered among the most catastrophic earthquakes ever occurred on the Italian territory, and they give evidence of the high seismicity of the Iblean area. The city of Catania in the South-Eastern Sicily has been affected by some destroying earthquakes of about magnitude 7.0+ in past times. The area to the south of Volcano Etna, on the east of the Ibleo-Maltese escarpment, to the south of the graben of the sicilian channel and on the east of the overlapping front of Gela, known as Iblean Area, is placed on area of contact between the African and the Euro-asiatic plates, and it is therefore a seismogenic area. It is reasonable to assume in Catania a maximum expected earthquake as a repetition of the 1169 or 1693 event, with intensity X-XI MCS and estimated magnitude between 7.0 and 7.4 (Azzaro and Barbano, 2000). The 1693 earthquake may be selected as a first level scenario event (maximum credible earthquake). As second and third level scenario events may be chosen respectively the 1818 earthquake (M S = 6.2) and the 1990 earthquake (M L = 5.4). The return periods estimated for the Catania site range from 250 to 500 years for an event like 1693, and from 40 to 90 years for an event such as 1818 (Azzaro et al., 1999). The Etna Earthquake (M=6.2) of February 20, 1818 The Etna earthquake that took place on February 20, 1818 was a medium size earthquake, but its effects were noticed over a vast area. In fact, this quake was sensed in almost every part of Sicily and in the south of Calabria (Imposa and Lombardo, 1985). The quake occurred at 18:20 (G.M.T.) and destroyed many villages on the south-east side of Mt Etna (Imposa and Lombardo, 1985). The villages of Aci Consolazione and Aci Platani were completely destroyed while S. Antonio, Acicatena, S.G.Galermo, Mascalucia and Nicolosi were badly damaged. The macroseimic field of this earthquake is represented in Fig. 1. The seismic shock damaged also the city of Catania as it is reported by the historic sources of the time. Also the area going from Piedimonte Etneo to Maletto was severely damaged. A total amount of 72 people died because of the great number of houses that collapsed. The highest mortality rate was to be found in Zafferana and it was due to the fact that many people were inside the church when the quake

3 took place. A lot of superficial and subterranean movements were noticed in the areas shocked by the quake. Deep fractures, several metres long were discovered mainly in Acicatena, S.G. La Punta, Viagrande and Pozzillo. These events as a whole show that the quake could reach the peak of IX on MCS (Imposa and Lombardo, 1985) or I max = 9-10 (Boschi et al., 1995). The isoseismal map explains that the earthquake was perceived almost in every part of Sicily from Siracusa to Noto and Palermo. This seism was sensed in Malta and the south of Calabria as well. Shocks following the ones of February, 20 were recorded up to March, 2 and mainly those occurred on February, 21 and 28 were felt in Acireale. By adopting macro-seismic methods it was assessed that the focal depth was about 5 km (Imposa and Lombardo, 1985). The 1818 event caused damage and ruin in many localities of the Etna eastern flanks. The broad distribution of the ground shaking is typical of a crustal regional earthquake (Barbano and Rigano, 2001; Azzaro and Barbano, 2000) rather than a shallow depth volcanic Etnean event. The maximum damage is localized eastward of the epicentre. However, the damage distribution seems incomplete towards West and South, probably because of the scarcity of human settlements. The hypothesis of activation of a deeper, blind fault of the Malta escarpment beneath the eastern side of the volcano during the 1818 earthquake is supported by the evidence of secondary faulting of the Ibleo-Maltese system, NNW-SSE oriented (Fig. 2), along the eastern flank of Mt. Etna (Laurenzano et al., 2004) as well as the occurrence of a tsunami along the Ionian coast (Azzaro and Barbano, 2000). Figure 1. Shocked localities from the earthquake of February 20, 1818 (from Monachesi and Stucchi, 2000).

4 Figure 2. Secondary faulting of the Ibleo-Maltese system, NNW-SSE oriented (from Laurenzano et al., 2004). Zoning For Earthquake Ground Motion Zoning for ground motion is therefore an essential part of the information necessary to evaluate the overall nature of geotechnical hazards (ISSMGE-TC4, 1999). Earthquake ground motions are affected by several factors such as source, path and site effects. Assessment of ground motions depends on the following: regional seismicity, attenuation of ground motion and local site effects on ground motion. Three different ways of describing microzonation can be used as it has been specified by the Manual for Zonation on Seismic Geotechnical Hazards (ISSMGE-TC4, 1999) on how to zone the seismic geotechnical hazards: general zonation (Grade-1), detailed zonation (Grade-2) and rigorous zonation (Grade-3). Local site effects on ground motions are considered to be the most significant factor in the zoning of ground motions (ISSMGE-TC4, 1999). The reliability of the zoning developed using Grade-1 approaches may be improved by incorporating additional data and information. Additional investigations are likely to be required for improved assessments. Grade-2 methods require considerably more soil information than Grade-1, and this requires new subsurface investigations for evaluating the geotechnical properties at specific sites. These investigations include geotechnical investigations such as penetration tests, geophysical testing and soil sampling from boreholes for laboratory tests (ISSMGE-TC4, 1999). Local site effects can be defined as the modification of predicted rock outcrop reference motions to give actual motions at the local site in question. The accuracy of zoning based on local site investigation data can be further enhanced using computer modelling of ground response. Computer codes which are now available include one-dimensional linear and

5 non-linear analyses and two - and three-dimensional analyses; these are necessary to achieve Grade-3 zoning. Ground response analyses are usually performed on a site specific basis and can form the basis of very reliable zoning maps. The methods of one-dimensional ground response analysis are useful for level or gently sloping sites underlined by horizontal bedrock. Such conditions are not uncommon and one-dimensional analyses are widely used in geotechnical earthquake practice. The most widely used 1 D analytical method for site response is to make use of the multiple reflection model for the propagation of S-waves in a one-dimensional column (e.g. Haskell, 1953). The soil column is modelled as a series of horizontal layers. These layers are subjected to base motions that are considered representative of those likely to occur in the region of interest. In the mapping of an area, amplification factors are determined for each element of the mesh. The mesh used in the present work is 40 x 40 m in size. The data from empirical correlations or from direct measurements of shear-wave velocity are used to build up the subsurface ground model for each mesh. Specific parameters required for this analysis are shearwave velocity, density, damping factor and thickness of layers. Non-linearity of site response is one of the major issues in evaluating site effects as soils exhibit strong strain-dependency of modulus and damping characteristics. Evidence of non-linear behaviour has been detected in observed earthquake ground motion records. This aspect is considered in most 1-D codes, like the SHAKE code (Schnabel et al., 1972; Idriss, 1990) and the GEODIN code (Frenna and Maugeri, 1995) used in the present work, that are both computer codes based on equivalent linear analysis. Also some 2-D codes, like the QUAD4 code (Idriss et al. 1973) and the SPEM code (Priolo 1999), take into account soil non-linearity. A 3-D code has been used for the evaluation of the ground motion at the bedrock in this work. Deterministic Hazard Scenario Earthquake A deterministic level II scenario event, i.e. the operative damaging earthquake (I MCS = VII-VIII) has been evaluated in the present work, referring to the 1818 event (Azzaro and Barbano 2000). Although probabilistic hazard tools may be helpful in selecting the scenario earthquakes, a deterministic shaking description was preferred in this work to a probabilistic one, also because it can better account for the complex geology of the area and the associated local site amplification. The experience of other countries in damage and loss estimation studies (see Shinozuka et al., 1997) suggests the use of a deterministic approach as a valid support to the decision makers, because it improves the ability to predict the amount and location of damage. Furthermore, such an approach is apt to provide rapid evaluations and monitoring of information immediately following a strong earthquake, in the emergency management phase. The deterministic level II hazard scenario earthquake has been evaluated by the 3-D hybrid stochastic-deterministic method EXWIM (Laurenzano et al., 2004). This method has been used to compute strong motion seismograms in the near-field of extended earthquake source. It solves the 3-D full-wave propagation in anelastic media with a vertically heterogeneous structure. The computational procedure consisted in: 1) discretizing the area into a grid of elementary point sources; 2) computing the wavefield generated by each point source with unitary seismic moment; 3) computing seismograms for each point source according to the given distribution of slip; and 4) summing up each contribution synchronized in time to simulate the

6 propagation of the rupture. Seismograms have been computed up to a maximum frequency of 20 Hz. The deterministic-stochastic transition has been set at Hz. The maximum fault size has been set at 26.6 km x 16.6 km, and it has been discretized into 4895 elementary sources, with inter-spacing of 300 m. The ground motion has been computed at a regular grid of receivers covering the Catania municipal area. This area is sampled with 470 receivers (Fig. 3a), with inter-spacing of 320 m (Laurenzano et al., 2004). A pure normal fault mechanism is assumed for the Etna earthquake. Three different fault sizes, corresponding to large, medium and small size, have been fixed respectively. The average value is of 16 km x 10 km, while the others correspond to an increase/decrease of 2/3 of the average length. Fig. 3b shows an example of large size source with indicated the seismic moment distribution. a) b) Figure 3. a) Location of the 470 receivers; b) size source (Laurenzano et al., 2004). Geology and Geotechnical Sub-Surface Conditions One of the most important input information for the ground response analysis is a subsurface model that represents the variation in thickness of the upper 30 m soil layers. This model should at least contain information on the depth of the bedrock (Vs = 800 m/s), or on the depth where the shear wave velocities approach a value that is comparable to rock. The backbone of the urban area is represented by marine terraces carved on a sedimentary substratum. This is made up of a Lower-Middle Pleistocene succession of marly clays, up to 600 m thick, upward evolving to some tens of meters of coastal sands and fluvial-deltaic conglomerates, referred to the Middle Pleistocene. The sedimentary substratum is dissected by deeply entrenched valleys filled with thick lava flows, which represent most of the rocks cropping out in the city. The geotechnical model for the city of Catania has been detected by the stratigraphic log of borings, characterised by variable degrees of accuracy; some are accompanied by in situ and/or laboratory tests. In total, the database assembled includes about 1200 boring locations, with a density distribution of the investigation points highly varying from site to site. Processing of this information has resulted in the via GIS geo-settled map of geotechnical units. Because of their relevance on the estimation of local ground shaking and site effects, data from in-hole geophysical surveys (Down- and Cross-Hole measurements) have been examined with special attention, particularly for S wave velocity measurements. Down-Hole data were available from investigations at several different test sites, in the urban area and in the industrial zone. They

7 include measurements in different lava flows, tuffs, sandy and marly clays and alluvial finegrained deposits. It must be noted that the shear waves velocity Vs was evaluated on the basis of both empirical correlations with in situ or laboratory tests and by direct measurements. The map of shear waves velocity, in the upper 30 m of soil, for the whole area of Catania, is reported in the Fig. 4. Figure 4. Map of shear waves velocity, in the upper 30 m of soil. The boring data have been summarised according to a simple lithological description, through the choice of few fundamental classes. In a second detailed phase of the geotechnical characterisation of the various units identified, characteristic (average) values of some representative geotechnical parameters have been evaluated. The following soil parameters were considered: physical characteristics (unit weight, moisture content, grain size, Atterberg s limits); shear strength (in drained and undrained conditions); shear wave propagation velocity Vs. Ten test sites have been chosen for a very accurate geotechnical characterisation. In these test sites, in addition to the boreholes, SPT and routine laboratory tests, the following tests have been carried out: Cross-Hole (CH) and/or Down-Hole (DH) tests, Resonant Column Tests (RCT) and Cyclic Loading Torsional Shear Tests (CLTST) (Cavallaro et al., 2005). By way of example some boring logs with the indication of the Vs profiles are reported in Fig. 5a. Microzonation of the Ground Motion for the City of Catania The three-components seismograms, computed at each of the 470 receivers, for a total number of 1188, 360, and 48 seismograms for the large, medium, and small fault sizes, respectively (Laurenzano et al., 2004) have been used for the microzonation of the city of Catania. Fig. 5b shows an example of the seismograms and acceleration response spectra computed at the surface receivers. These seismograms have been de-convoluted at a depth of 30

8 m, which is the depth to be considered, as suggested by the Eurocode (CEN 2002) for the evaluation of local site amplification effects. The de-convolution suggests an average reduction factor of 1.75 for peak ground acceleration, slightly less than the factor of 2 generally used in the Engineering practice. The time history of synthetic accelerograms reduced by a factor of 1.75 has been used as input accelerograms for local site effects evaluation with 1-D code, in correspondence of the database of 1200 boreholes and water-wells available in the data-bank of the work. The 1-D site response using very detailed soil stratigraphy in the upper 30 m has been compared with 3-D synthetic accelerograms at the surface evaluated using a necessarily very simplified soil stratigraphy. a) b) Figure 5. a) Example of boring logs with the indication of the Vs profiles; b) example of the seismograms and acceleration response spectra computed at receivers (Laurenzano et al., 2004). A ground-shaking map for the city of Catania was generated via GIS for the level II scenario earthquake. The shaking description is given in terms of peak ground acceleration. For this method of hazard estimation only the zero period spectral acceleration (or PGA, peak ground acceleration) has been used. Ground shaking scenario has been constructed in terms of PGA, to satisfy the demand by the method used to obtain a Grade-3 map of the seismic geotechnical hazard for the city of Catania, according to the Manual (ISSMGE-TC4, 1999). In the southern part of the city of Catania, where 3-D synthetic accelerograms were not evaluated, an appropriate attenuation relation has been used. The 1-D ground motion parameters were evaluated at about 120,000 points in the query database system, with a pixel resolution of 40x40 m, at all points of the study area, so with much more detail in comparison with the 3-D ground motion parameters evaluated only for the northern part of the city, using a mesh of 500 x 500m. In Fig. 6 is presented the ground shaking map in terms of predicted PGA values for the city of Catania, generated via GIS for the level II operative earthquake. Conclusions On the basis of the proposed method it has been possible to obtain a detailed evaluation of the spatial variability in seismic responses, which can be used as an improved basis for seismic microzonation mapping.

9 (a) (b) Fig. 6. Ground shaking map in terms of predicted PGA for the Catania city, for level II operative scenario earthquake of February 20, 1818 (M=6.2): a) central area; b) the whole area. This method has a clear advantage above the traditional way of microzonation because it incorporates any a-priori geological and geotechnical knowledge into the model and can yield microzonation. The carried out procedure was to evaluate the design ground acceleration at the conventional bedrock (30 m according to Eurocode) by the 3-D EXWIM code and after to evaluate the response at the surface by the 1-D non-linear GEODIN code. The use of non-linear code is needed when the given area is shacked by destructive earthquakes, as in the case of Catania, which was affected, in the past, by very strong earthquakes. A ground-shaking map for the city of Catania was generated via GIS for the level II operative scenario earthquake. However, the proposed procedure requires a very large number of boreholes and accurate dynamic laboratory investigations for detecting soil non-linearity. References Azzaro, R., Barbano, M.S., Moroni, A., Mucciarelli, M.and Stucchi, M. (1999). The seismic history of Catania. Journal of Seismology, 3, 3, Azzaro, R. and Barbano, M.S. (2000). Seismogenetic features of SE Sicily and scenario earthquakes for Catania. The Catania Project: earthquake damage scenarios for a high risk area in the Mediterranean, part I: seismotectonic framework and earthquake scenarios. CNR-Gruppo Nazionale per la Difesa dai Terremoti, Roma, pp Barbano, M.S., and Rigano R. (2001). Earthquake sources and seismic hazard in Southeastern Sicily, Ann. Geofis., 44, pp Boschi, E., Guidoboni, E. and Mariotti,D. (1995). Seismic effects of the strongest historical earthquakes in the Syracuse area, Annali di Geofisica 38, 2 (May), pp

10 Cavallaro A., Grasso S. and Maugeri M. (2005). Site Characterisation and Site Response for a Cohesive Soil in the City of Catania. Proc. of the Geotechnical Earthquake Engineering Satellite Conference. Osaka, Japan, 10 September 2005, CEN (2002). Design of Structures for Earthquake Resistance. Part 1: General Rules, seismic actions and rules for buildings. Draft No. 5, Revised Final Project Team Draft (Prestage 49). Brussels: European Committee for Standardization (CEN), May 2002, 197 pp. Frenna, S.M. and Maugeri, M. (1995). GEODIN: a Computer Code for Seismic Soil Response. Proceeding of the. 9 th Italian Conference of Computational Mechanics, Catania, Italy, June, (in Italian), pp Haskell, N.A. (1953). The dispersion of surface waves on Multi-layered Media, Bull. Seism. Soc. Am., Vol.43, pp Idriss, I.M., Lysmer, J., Hwang, R. and Seed, H.B. (1973). A computer program for evaluating the seismic response of soil structures by variable damping finite element procedures. UCB EERC Report n Idriss, I.M. (1990). Response of soft soil sites during earthquakes, Proc. H. Bolton Seed Memorial Symposium, Imposa, S. and Lombardo, G. (1985). The Etna earthquake of February 20, In: Postpischl, D. (ed), Atlas of Isoseismal Maps of Italian Earthquakes, PFG-CNR, Quad. Ric. Scie. 2A, 114, Bologna, pp ISSMGE-TC4 (1999). Manual for Zonation on Seismic Geotechnical Hazards (Revised Version). The Technical Committee No. 4 for Earthquake Geotechnical Engineering of the ISSMGE, Japanese Geotechnical Society of SMGE. Laurenzano G., Priolo E., Klinc P. and Vuan A. (2004). Near fault earthquake scenarios for the February 20, 1818 M=6.2 Catanese event. Proc. of the Fourth International Conference on Computer Simulation in Risk Analysis and Hazard Mitigation: Risk Analysis 2004, Rhodes, September 2004, Maugeri, M Detailed Scenarios and actions for seismic prevention of damage in the urban area of Catania. Framework Program of the National Group for the Defence Against Earthquakes (National Group of Geophysics and Volcanology), financed by the Department of Civil Defence, June Monachesi, G. and Stucchi, M. (eds), (2000). DOM 4.1: an intensity database of damaging earthquakes in the Italian area, GNDT-CNR open file report, 2 vv., Milano. Web Site: Priolo, E. (1999). 2-D spectral element simulations of destructive ground shaking in Catania (Italy). J. of Seismology, 3(3), pp Schnabel, P.B., Lysmer, J. and Seed, H.B. (1972). SHAKE A computer program for earthquake response analysis of horizontal layered sites, Report No. EERC 72-12, University of California, Berkeley, 88pp.