GNGTS 2011 SESSIONE 2.2

MODELLING OF AMBIENT NOISE HVSR IN A COMPLEX GEOLOGICAL AREA: CASE STUDY OF THE XEMXIJA BAY AREA, S. Pace1, F. Panzera 2, S. D’Amico 1, P. Galea 1, G. Lombardo 2 1 Department of Physics, , , Malta 2 Dipartimento Scienze Geologiche, Università di Catania, Italy The Maltese islands are exposed to a low-to-moderate seismic hazard. Seismic activity around the islands is generally of low magnitude; however more infrequent, large events in Sicily and as far as the Hellenic arc have affected the country in the past and caused considerable damage (Galea, 2007). The general public perception is one of unjustified complacency, and no comprehensive assessment of seismic risk has so far been carried out. Much of the building stock is of load-bea- ring unreinforced masonry, and is vulnerable to even moderate ground shaking. This study will address the component of seismic risk arising from local site amplification, which is the result of the particular local sedimentary geology. Ambient noise measurements will be used to infer shallow shear wave velocity structure, for which no systematic data exists on the islands. The Xemxija area (Fig. 1) on the NE coast of Malta was chosen as an initial investigation site owing to its important role as a touristic urban area, and due to the site’s complex geology and topo- graphic structure. The area studied spans approximately 2.7 km by 2.1 km and includes the hilltop area, valley and coastal region. Being an urban area and a summer resort, a sizeable proportion of Xemxija is built up, mainly consisting of holiday flats. The Pwales Valley, within Xemxija, an area highly exploited for agricultural functions, is part of an extensive block faulting system that affects the Northern part of the Maltese islands (Illies, 1981; Reuther and Eisbacher, 1985). The Pwales valley itself is a graben, defined by two faults within the horst-and-graben system, characterised by

Fig. 1 - Map of locations of about 100 measurements point and selected HVSR. The inset on the top-left shows also the location on the island of the study area.

299 GNGTS 2011 SESSIONE 2.2 a sequence of ENE striking normal faults (MEPA, 2004; Geological Map of the Maltese Islands 1993). The lithological sequence in Xemxija, like the rest of the island, is composed of marine sedi- mentary rocks. The rock succession is relatively simple, being composed of five main layers. Star- ting with the oldest layer, these are the Lower Coralline Limestone (LCL), the Globigerina Lime- stone (GL), the Blue Clay (BC), the Greensand Formation and the Upper Coralline Limestone (UCL). Each layer has a distinct composition resulting in diverse properties, such as resistance to erosion, dependent on rock hardness. The Upper Coralline, Globigerina and Lower Coralline strata are essentially considered as being hard rock, whereas the Blue Clay layer and the Greensand layer are softer (Pedley, 1978; 2002). The main outcrops over the Xemxija area are Upper Coralline on the hill sides and at the ridge tops and Blue Clay at dispersed points along the hill side, and a sub- stantial thickness of soil overlying the Upper Coralline at several locations, such as in the Pwales Valley. All layers lie horizontally but they are displaced at intervals by various faults and fractures. The exposed Upper Coralline layer itself is visibly extensively fractured. A dense microtremor measurement survey was carried out at Xemxija, using a Tromino® tro- mograph (Micromed SpA), with about 100 microtremor recordings performed at random positions, as well as along predetermined profiles. The time series obtained were processed using GEOPSY and Grilla® in order to compute the HVSR (horizontal to vertical spectral ratio) curves (Nakamu- ra, 1989). Fig. 1, demonstrates a few examples of the results. HVSR curves obtained from measu- rements performed in the valley give a large spread of resonant fundamental frequencies ranging between 2 Hz to over 11 Hz. This spread, as well as the equally variable peak amplitudes, is due to the variation in thickness of the soil that overlays the Upper Coralline stratum. This implication is strengthened by the fact, that on the neighbouring hilltop, where no or negligible soil is present, the frequency range is a smaller and more stable, spanning 1.19 Hz - 1.56 Hz.

Fig. 2 - Examples, at two different points, of modelling of observed HVSR. The best models used for the modelling are also reported.

300 GNGTS 2011 SESSIONE 2.2

Fig. 3 - HVSR profile running thorough the valley. HSVR at each point and along the profile are also reported. The inferred geology is also given.

Whereas the overlying soil layer in the valley is clearly responsible for the amplification, the expected response is not so obvious on the hill sides and top, where the hard Upper Coralline is out- cropping, at first sight implying that there should be no significant spectral ratio peaks. In fact, HVSR curves obtained from these areas all display a clear resonance peak of amplitude greater than 2 in most cases. These resultant fundamental peaks can be generally associated with the Blue Clay layer underlying the UCL. The fact that the UCL is significantly fractured, and visibly so especial- ly at cliff and ridge edges, could also contribute to the amplification effects. The presence of the Blue Clay layer gives rise to a velocity inversion. This feature causes the HVSR values to drop below 1 over a wide frequency range (Castellaro and Mulargia, 2009) as is evident in the case of points 14 and 15 (Fig. 3), as opposed to points 6 and 13, where the clay is at the surface and does not produce a velocity inversion. To perform preliminary modelling at several points we used the ModelHVSR program (Herak, 2008). When modelling the resonance curve, a trade-off exists between the shear wave velocity and the thickness of the low velocity layer, therefore some form of initial constraint must be applied. In this case, use was made of a variety of engineering texts and past geotechnical studies undertaken over the island, as well as borehole logs from Xemxija to obtain approximate values of layer thick- nesses and rock densities. Seismic velocity values were also utilised from a separate preliminary study undertaken in the same area utilising the ReMi®, MASW and Refraction methods (Panzera et al., 2011). Fig. 2 gives two examples of modelling results, using the above constraints. The models are for points XE14 along the hill side and XE4 in the valley. At point XE4, the predomi-

301 GNGTS 2011 SESSIONE 2.2 nant 6.5Hz peak, with a maximum amplitude of almost 6, is well modelled by a 9.4m thick surfi- cial layer of soil, as expected, whilst along the sides and on top of the hill or ridge, where soil is sparse, the dominant peak may be modelled by a thick, buried clay layer underlying the hard Upper Coralline stratum, as implied by previous knowledge of the geology. Fig. 3 is a preliminary attempt at modelling the subsurface geology based on fitting a number of HVSR curves.taken along the pro- file marked in red in Fig. 1. It clearly shows the complexity and variability of the site’s geology. The soil thickness within the valley, increasing towards the centre of the basin, is clearly reflected in the variation of resonance frequency across the valley., while the depth of the buried clay layer appears to have an effect on the amplitude value. The relatively flat response curve obtained at point 5, pro- bably indicates that the faulting process has resulted in either a lack of clay below the point, or else that the clay is too deep to have an effect on the resonance curve. Modelling of HVSR data using ambient noise has been shown to be a useful tool in microzo- nantion studies. No VS30 values are presented here, but at first glance from preliminary models it is evident that this parameter could be misleading and inadequate in certain situations (Gallipoli and Mucciarelli, 2009). In particular, when no surface low-velocity layer is present and the buried clay is further than 30m deep, such as in the case of the measurement at point XE15, the VS30 value, in accordance with standard guidelines such as NEHRP, would predict negligible amplification. In such instances the site would be classified as a hard rock site, whereas in reality, the buried clay layer could produce considerable amplification that must be taken into consideration in seismic risk studies. References Castellaro S., and Mulargia F.; 2009: The effect of velocity inversions on H/V. Pure and Applied Geophysics, 166, 567-592. Gallipoli M.R., and Mucciarelli M.; 2009: Comparison of Site Classification from VS30, VS10, and HVSR in Italy. Bull. Seismol. Soc. Am., 99, 340–351. Galea P.; 2007: Seismic history of the Maltese islands and considerations on seismic risk. Annals of Geophysics, 50, N.6. Herak M.; 2008: ModelHVSR—A Matlab® tool to model horizontal-to-vertical spectral ratio of ambient noise. Computers & Geosciences, 34, 1514–1526. Illies J.H.; 1981: Graben formation: the Maltese Islands, a case history. Tectonophysics, 73, 151-168. Nakamura Y.; 1989: A method for dynamic characteristics estimation of subsurface using microtremors on the ground surface. Quarterly Reports of the Railway Technical Research Institute Tokyo, 30, 25-33. Oil Exploration Directorate, 1993; Geological Map of the Maltese Island. Sheet 1 – Malta - Scale 1:25,000. Office of the Prime Minister, Malta. Panzera F., Pace S., D’Amico S., Galea P., Lombardo G.; 2011: Preliminary Results on the Seismic Properties of Main Lithotypes Outcropping on Malta. (this volume) Pedley H.M., House M.R., Waugh B.; 1978: The geology of the Pelagian block: the Maltese Islands. In: A.E.M. Nairn, W.H. Kanes and F.G. Stehli (Eds.), “The Ocean Basin and Margins. Vol. 4B: The Western Mediterranean”. Plenum Press, London, pp. 417- 433. Pedley H.M., Clark M., Galea P.; 2002: Limestone Isles in a Crystal Sea: The Geology of the Maltese Islands. P.E.G. Ltd, Malta. ISBN:99909-0-318-2 Reuther C.D. and Eisbacher G.H.; 1985: Pantelleria Rift - crustal extension in a convergent interpolate setting, Geol. Rndsch, 74, 585-597.

AMBIENT NOISE MEASUREMENTS FOLLOWING THE CHRISTCHURCH, 2011 EARTHQUAKE: RELATIONSHIPS WITH PREVIOUS MICROZONATION STUDIES, LIQUEFACTION AND NON-LINEARITY M. Mucciarelli Dept. of Structures, Geotechnics and Engineering Geology, Basilicata University, Potenza, Italy During a quick field survey in Christchurch after the February 2011 earthquake it was possible to collect microtremor measurements as close as possible to accelerometric stations that recorded the event, in the most damaged areas, and finally trying to achieve a good spatial coverage. This allowed a comparison with the microzonation map produced by Toshinawa et al. (1994), and in par- ticular with their map of HVSR frequencies and peak values. There is a general agreement in the frequency map, with frequency decreasing from the western part of the city and moving toward the

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