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Evaluation of the local seismic response in the area of Catania (Italy)

G. Lombardo1, R. Rigano1, S. Gresta1, H. Langer2, C. Monaco1 & G. De Guidi1

1Dipartimento di Scienze Geologiche, Università di Catania 2Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania

Abstract

The evaluation of seismic site response in the urban area of Catania was tackled by selecting test areas having peculiar lithological and structural features, potentially favourable to large local amplifications of ground motion. The two selected areas are located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response was evaluated using a spectral ratio technique taking the horizontal-to-vertical component ratio of ambient noise. Inference from microtremor measurements are compared with results from synthetic accelerograms and response spectra computed at all drillings available for this area. The microtremor H/V spectral ratios evaluated at soft sites located within the downtown profile tend to be smaller than that usually reported in the literature for such soils. A tendency for amplifications to peaks near 2 Hz is observed only in some sites located on recent alluvial deposits. Evidence for amplifications of site effects (frequency range 4-8 Hz) were observed in the sampling sites located on the fault, with a rapid decrease of spectral amplitude just a few tenths of a meter away from the discontinuity. Numerical simulations evidenced the importance of geolithological features at depth levels even greater than 20-30 m. Besides this, the results strongly confirm the importance of the subsurface geological conditions in the estimate of seismic hazard on urban scales. Keywords: local seismic response, microtremors, H/V spectral ratio, Catania downtown, fault zone.

© 2004 WIT Press, www.witpress.com, ISBN 1-85312-736-1Risk Analysis IV, C. A. Brebbia (Editor)

1 Foreword

The city of Catania is located in an intensively active tectonic area, which implies a particularly high seismic hazard. The high level of activity corresponds to the development of large earthquakes with intensities up to X-XI EMS’98 and magnitudes ranging between 6 and 7.1 [1, 2]. Examples of major south-eastern Sicily earthquakes are the events of 4 February 1169 (I0=X), 10 December 1542 (I0=IX-X) and 9/11 January 1693 (I0=IX and X-XI, respectively), this last causing many thousands of victims and shattering an area of about 15,000 km2 [3, 4]. Recently, the 13 December 1990 earthquake caused severe damages, despite its moderate magnitude (MS=5.5). In particular this earthquake cleared up that seismic hazard is not only a matter of large but rather rare events, but represents an actual and frequently occurring problem. It is therefore of primary interest to evaluate the seismic response of the urbanized area from moderate to strong seismic input. In particular, the effects on sites having topographical, structural and/or lithological conditions favourable to large local amplifications of ground motion have to be examined. The distribution of seismic energy depends indeed on the lithology, as well as on the complexity of morphologic and structural features of the investigated areas. Experiments performed in California [5] and, recently in Italy, during the 1998 Umbria-Marche earthquakes [6] showed that both active and inactive faults behave like wave-guide when struck by a wave front. Many authors have recently demonstrated that the H/V spectral ratios from microtremor measurements are consistent in shape with H/V spectral ratios from earthquake recordings, and with earthquake spectral ratios using a reference bedrock station [7]. This method is quite sensitive to the features of the layering of sediments itself which strongly affect the frequency band and the shape of the dominant peaks in the H/V spectra [8]. The use of ambient noise has therefore become very attractive in recent times because it is both fast and cheap. It is particularly suitable in urban areas where the nature of the outcropping geological units is masked by city growth and anthropic intervention on the surface geology. The aim of the present study is to investigate peculiar site responses in two test sites located in the Catania area. The first zone was selected in the historical downtown of the city. It crosses the most representative lithotypes outcropping in the study area and reliable information about the lithostratigraphic units and their geotechnical parameters are available from well data. The second one, located immediately to the north of the town, is crossed by a dip-slip fault and an eruptive fracture. It was chosen in order to investigate focusing effects in the gouge zone of a fault, where stationary waves are generated by the impedance contrast between the “cataclastic” area and the surroundings rocks [9].

2 Geological settings

The surface geology of Catania is mostly characterized by massive and scoriaceous lavas that derive from several flows that in both pre-historical and


104 Risk Analysis IV

historical times covered the area. The volcanic rocks infill deep entrenched valleys that formed the original morphology of the sedimentary substratum that, at present, outcrops only in small areas. This is made up of a marly clays succession, up to 600 m thick, containing levels of pre-Etnean submarine and subaerial basalts. The overlying terraced deposits, are characterised by several metres thick sands, conglomerates and silty clays. Horizons of detritus several meters thick outcrop in different zones of the town. It can be due both to slope detritus as well as archaeological materials and ruins of buildings struck by the last destructive earthquakes of 1169 and 1693.

Figure 1: Geological map of Catania downtown.

In particular, the historical downtown area chosen for the simulation, including the oldest buildings in the city surrounded by the 16th century fortifications, is characterized by a complex sequence formed by soft sediments, constituted by terraced deposits (T6 and T7, see Monaco et al. [10]), interbedded between the marly-clayey bedrock and upper massive and scoriaceous volcanic


Risk Analysis IV 105

layers (fig. 1). These latter have been attributed to the 252 A.D. lava flow, drilled in the northern border of fortifications, and 1669 lava flow which surrounds the western and southern border of the fortifications. The terraced deposits are mostly composed of dark epiclastic sands and microconglomerates, up to 30 m thick, containing levels of quartzouse sands and pebbles of quartzarenites [10]. As regards the marly-clayey bedrock, two interlayered levels of massive basalts, up to 30 m thick, have been drilled in the area of the Ancient Theatre by several wells. Finally, in the ancient part of the city, the uppermost stratigraphic horizons consist of up to 10 metres thick levels of earthquake ruins.

Figure 2: Geological settings of the northernmost area of Catania.

In the second test area, located in the northernmost part of Catania town, lava flows outcrop extensively. The flows originated in historical time (253 A.D. and 1381) and covered altered lavas of pre-historical age.

3 Methodology

In the present study, site response was evaluated using spectral ratio technique taking the horizontal-to-vertical component ratio of ambient noise [11]. Inferences from microtremor measurements were compared with results from synthetic accelerograms and response spectra computed at the drillings available for the area. A validation of these methodologies using data from earthquake recordings was not attempted since the number of recorded data is, at present time, not enough to represent significantly the different site conditions. About 200 time histories of microtremors were recorded in 40 measurements sites that are roughly aligned in three profiles; the first, trending about N-S is located in the historical part of the town (fig. 1) and the remaining two are perpendicular to the fault located in the northern part of the city (fig. 2). Measurement sites had an average spacing of a few hundred metres in the downtown profile, while in the two remaining sections, spacing ranged from ten metres up to one hundred metres. Time histories of ambient noise were recorded using a three-component 1-Hz Mark L4C 3-D seismometer connected to a 12-bit



analog-to-digital converter and a notebook PC. Sampling frequency was 100 Hz, two antialiasing filters cut higher frequencies with a 10 db/oct slope. At each site five time series were recorded, each with a length of 120 s. The time series were base-line corrected in order to remove spurious offsets and low-frequency trends. After the application of a 10% cosine-tapered window, a Fast Fourier Transform algorithm was applied to obtain spectra in a frequency band 0.5 - 15 Hz. The lower limit of 0.5 Hz was assessed through the low-frequency cut off where the microtremor standard deviation becomes equal to the natural noise fluctuation of the 12-bit digitizer. Amplitude spectra of the 3 components were corrected for their instrumental response. For each time history the amplitude spectrum of the vector composition of horizontal components was divided by the vertical component spectrum, providing the mean H/V spectral ratio at each measurement site. A running smoothing function of 0.1 Hz was also used, both on the spectra of each single component and on the final ratio, to reduce spectral fluctuations. The standard deviation evaluated for H/V spectra obtained at the bedrock (clayey sites) is of the order of ± 0.5 units, around mean amplitude values not exceeding 2 units. Based on these fluctuations only spectral peaks higher than 3 units were taken into account as statistically significant. In the present study, the results obtained with the aforesaid methodology were corroborated through a synthetic approach. The simulation of strong ground motion in the urban area of Catania took advantage from the large amount of shallow borehole data that were available [10] whereas information about the deeper geology of the area was obtained from Agip [12] and Lentini [13]. Finally, the knowledge on the seismotectonic setting in South-eastern Sicily has been boosted by the analysis of the 13 December 1990 earthquake and its aftershocks. These events form the first digital data set of the area disclosing the possibility of investigating spectral source parameters, attenuation and seismic scaling laws of the south-eastern Sicily seismic zone [14, 15, 16, 17]. The approach used here contains various simplifications, such as the use of a 1D structural model or the limitation to SH-waves. These simplifications, which can be justified for the given overall conditions of the studied area, render the calculus simple and fast and permit accounting for the high frequency content of the signal. The synthetic accelerograms were simulated according to the procedure proposed by Boore [18], modified by Langer [19]. Its essence is the modelling of the source time function in terms of acceleration as a band-limited random sequence of random pulses, and accounting for the wave propagation effects by using Haskell layer matrices. Global source parameters are accounted for by applying a band-pass filter to the Gaussian white noise, i.e. C M0 S(f, f0) P(f, fmax) where C is a constant for geometrical spreading and radiation pattern, M0 the seismic moment of the event, f0 the corner frequency, S(f,f0) = f2/(1+(f/f0)2), P(f,fmax) = (1+(f/fmax)2q)-1/2, q the parameter of the steepness of the high frequency decay (here q=4). The corner frequency f0 can be related to the size of the source (its radius r0) after Brune [20] by: f0 = 0.372 c/r0, where c is the shear-wave velocity. Finally the seismic stress drop is given by: t = 7M0/(16r0

3).



Strong ground motion is seriously affected by wave propagation effects caused by changes due to absorption, reflection and refraction at the boundaries of the geological structures. In particular the subsurface geological structure is of principal importance in this context. The use of Haskell matrices in the sense of a 1D model is certainly a simplification, which however can be justified for the given conditions in the urban area of Catania following the analysis carried out by Bard and Bouchon [21]. As a further simplification we restricted ourselves to SH-waves since these are the most important ones for our purposes and it can be shown that SV-waves behave similarly as SH-waves for the steep ray incidence occurring in our models.

Table 1: Geotechical parameters of subsurface layers in Catania downtown (a) and in the northern part of Catania (b).

In the test area located in the northern part of the town, besides the previously described methodology, a simple 1-D modelling [22] was used to validate and interpret the results of H/V spectra ratio technique. In this method, well known as “Successive use of two layers”, the physical model adopted is a resonant cavity close to the lower extremity, which generally coincides with the contact between the basement and the overlaying formations. The strongest earthquake scenario for Catania is the shock that occurred on 11 January 1693. Its epicentre was situated probably offshore, in a distance of about 20-40 km from the Catania downtown area. The seismic moment was estimated to about 2.5x1019 Nm, corresponding to a surface wave magnitude of over 7. Assuming a global stress drop of 100 bars we derive a source area of ca 300 km2. Several seismic logs are available for the study area so we were able to derive standard values for all the formations making up the geology within both the test areas (table 1).



4 Results and discussion

Preliminarily to the description of results obtained it must be specified that since noise measurements were made in densely urbanized areas, it was necessary to check the effect of nearby buildings. Tests performed showed out that the natural period of buildings does not significantly contaminate the peak response at sites [23, 24]. The main H/V spectral ratios obtained within the urban area are shown in fig. 3. Spectra obtained from measurements on the clayey basement show a flat shape in the frequency band 2-10 Hz [23]. A similar behaviour is observed for all the sites located on massive lavas (45a, 37m, 51m). Spectral peak amplifications at a frequency of 2 Hz are observed in some sites located on recent alluvial deposits (50a, 51a). Moderate amplifications are observed at sites located on detritus (4m, 49m, 50m).

Figure 3: Geolithological cross-section of Catania downtown with modelled peak accelerations and examples of H/V microtremor spectra.

At soft sites like sands and gravelly sands (23m, 35m, 36m, 46m), amplification of the microtremor H/V spectral ratios tend to be smaller than that usually reported in the literature for such soils. Similar behaviour was observed in other surveys performed in the Catania area [25, 26]. A moderate impedance contrast of sand and gravel deposits with respect to the basement clays may be hypothesized as a possible explanation for the diffuse low-amplitude H/V spectral ratios at soft sites [23]. Figure 3 also shows the geological cross - section performed in the Catania downtown area, which is plotted together with the values of simulated peak



accelerations and 5% pseudo-acceleration at 1 and 5 Hz, respectively. From our simulations we conclude that the local geolithological features do not significantly affect the estimated PSA at 1 Hz. On the contrary, the evaluation of peak spectral acceleration in case of relatively high-frequency oscillators e.g. small buildings (5 Hz), depends on the shallow lithological features of the site, but is sensitive to the contribution of deeper reflectors as well. The lithostratigraphic situation at a distance of about 1800 m from the origin of the profile is a typical example. In this case, seismic waves propagating in the low impedance clayey basement are trapped between two higher impedance reflectors given by two massive lava bodies. The synthetic modelling of the strong ground motion parameters reveals a strong dependence on the subsurface structure, in particular concerning the peak accelerations and response accelerations at 5 Hz. Strong amplifications have to be expected in particular at outcrops of detritic material. Besides this, numerical simulations evidenced the importance of geolithological features at depth levels even greater than 20-30 m. In particular, reduced amplitudes of peak ground accelerations and response accelerations are predicted for sites, where we hypothesized the presence of high-impedance lava lenses enclosed in weaker material, such as clays or sands.

Figure 4: Geolithological profiles in the northern area of Catania and H/V microtremors spectral ratios.

A similar tight connection between local site response and significant impedance contrast of materials interested by wave propagation can be postulated for seismic waves propagating through the gouge zone of a fault. Such interesting feature may be inferred from the spectral ratios H/V along the cross-



sections (fig. 2) of the tectonic structure located in the northernmost part of Catania area. The obtained H/V spectral ratios show a tendency towards the amplification of site effects in the frequency range 4.0-8.0 Hz, with dominant spectral peaks at about 6.0 Hz in the sampling sites located on the fault (fig. 4). A rapid decrease of spectral amplitude is observed just a few tenths of meters away from the discontinuity. A similar tendency towards spectral peak amplification at frequency values of about 5 Hz is observed for H/V spectra obtained in some sites located on the eruptive fracture zone (site 48 in A-A’). In some cases, amplification peaks related to peculiar local conditions, such as altered lavas and detritus, are observed (e.g. sites 11, 15, 31, 52).

Figure 5: Comparison between 1-D modelling of site response and experimental data in the northern area of Catania: average peak

line), theoretic transfer function (light line), Nakamura H/V spectral ratio (dashed line). The input data and the results from modelling using Dobry method are shown in the bottom panel.

The inspection of the spectral ratios features, sets into evidence that peaks at about 1.8 Hz are quite common in this area (i.e. sites 15, 30, 32, 33, 46, 47, 52). In order to validate the experimentally observed spectral peaks and to interpret them in terms of the resonance effects due to the local stratigraphy, a 1-D modelling using the Dobry et al. [22] and the Haskell-Thomson methods was performed (fig. 5). The theoretic transfer functions for SH vertically incident waves were therefore computed, using data available either from literature or from field measurement. In particular, the S-wave velocity for the first thirty meters of depth was drawn from seismic tomographic data available, whereas, for greater depth values of shear wave velocities were taken from the shallower part of crustal models known in literature for the Etnean area [27, 28, 29].



ground acceleration computed using the scenario earthquake (dark

Dominant spectral peaks observed in both the theoretic transfer function and the average peak ground acceleration share the frequency range of 1.8-2.0 Hz. Such values are very similar to what experimentally observed in the H/V spectra, although theoretical spectral peaks underestimate experimental spectral amplitude. The results obtained through the Dobry et al. [22] method show that the surface bounding downwards the resonant layer is at depth of about 60 meters in the ambit of lavas (table 2).

5 Concluding remarks

The present study faced the problem of site response evaluation in the urban area of Catania by investigating on possible amplification effects that can be observed through detailed studies on some peculiar lithostratigraphic sequences and structural features. The results, on the two selected test sites, strongly confirm the importance of the subsurface geological conditions in the estimate of seismic hazard at urban scale and allow us to draw the following considerations: - The H/V microtremor spectral ratios do not show pronounced spectral peak amplifications in most of the sampling sites investigated along the downtown profile. Significant amplifications are only observed in sites located on the recent alluvial deposits, while some moderate amplifications appear in a few sites located on the detritus and pyroclastics. The absence of amplifications in most of the sampling sites can be explained in terms of the poor wave velocity contrast between the lithotypes forming the sedimentary sequence outcropping in the study area. - Numerical modelling highlights the importance of taking into account, besides the features of shallow lithotypes, the existence of deeper structures as well. - Significant amplification effects are observed through numerical simulations especially when the 5 Hz – acceleration responses are computed. Such response represents the expected loading of one or two story buildings that are indeed the majority of the existing edifices in the downtown area. - The existence of significant amplification effects due to the focusing of seismic energy in the gouge zone of a fault was clearly observed. Although such effects sensibly decrease few tens of meters away from the fault area, their occurrence has to be taken into account in evaluating the site response in town expansion planning. In our opinion, the most outstanding finding of present research is that it further demonstrates the importance of detailed studies in the evaluation of seismic response linked both to the peculiar lithostratigraphic sequences and the amplification effects due to structural discontinuities. The detection of tectonic structures when they are masked by anthropic actions and city grows as well as the monitoring of their amplification effects is particularly important.



Acknowledgements

This work was carried out in the framework of the researches supported by INGV-GNDT project: “Detailed scenarios and actions for seismic prevention of damage in the urban area of Catania”.

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