GEOTECHNICAL MODEL FOR SANTIAGO DE CUBA CITY Z.C. Rivera 1 , J. García 2 , D. Slejko 3 , A. Medina 4 1 National Centre of Seismological Researches, Santiago de Cuba, Cuba 2 GEM Foundation, Pavia, Italy 3 Ist. Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy 4 National Centre of Mineral Resources, Santiago de Cuba, Cuba Introduction. Santiago de Cuba city is located in the south-eastern part of the island, close to the coast and to the major seismogenic fault of the region, the Oriente transform fault system. The largest area of the city is framed in the basin of Santiago de Cuba, the terrain is semi- mountainous and the main geographical feature is a closed basin named Bay. The predominant Quaternary sediments in the superficial layers are represented by sand, gravel, clay, sandstone, marl, and limestone. The bedrock is formed by volcanic and volcanogenic sedimentary rocks of Paleogene age. The city is also crossed by numerous active faults, which can represent a major threat in the case of local amplification depending of the specific soil type. For this reason, several geological, tectonic, geotechnical, and geophysical studies have been carried out in the city during the last years (Perez et al. , 1994; Gonzalez et al. , 1997; Medina et al. , 1999; Fernández et al. , 2000; Zapata, 2000; Rivera, 2000; Méndez et al. , 2001; Alvarez et al. , 2004; García, 2007; Arango et al. , 2009; Rivera et al. , 2011), aimed at defining a correct assessment of seismic hazard, seismic microzonation and site response, considering deep geological sections, soil type maps, and analyzing the geotechnical properties of the terrains. Fig. 1 – Geological map of Santiago de Cuba city, at the scale 1:25,000 (modified from Medina et al., 1999), the red lines are the superficial projection of the Quaternary faults and the solid dots are the geotechnical boreholes (green) and geological wells (blue) used in the study. 330 GNGTS 2013 SESSIONE 2.2
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GEotECHNiCal modEl for SaNtiaGo dE CuBa CityZ.C. rivera1, J. García2, d. Slejko3, a. medina4
1 National Centre of Seismological Researches, Santiago de Cuba, Cuba2 GEM Foundation, Pavia, Italy 3 Ist. Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy4 National Centre of Mineral Resources, Santiago de Cuba, Cuba
Introduction. Santiago de Cuba city is located in the south-eastern part of the island, close to the coast and to the major seismogenic fault of the region, the Oriente transform fault system.
The largest area of the city is framed in the basin of Santiago de Cuba, the terrain is semi-mountainous and the main geographical feature is a closed basin named Bay. The predominant Quaternary sediments in the superficial layers are represented by sand, gravel, clay, sandstone, marl, and limestone. The bedrock is formed by volcanic and volcanogenic sedimentary rocks of Paleogene age.
The city is also crossed by numerous active faults, which can represent a major threat in the case of local amplification depending of the specific soil type.
For this reason, several geological, tectonic, geotechnical, and geophysical studies have been carried out in the city during the last years (Perez et al., 1994; Gonzalez et al., 1997; Medina et al., 1999; Fernández et al., 2000; Zapata, 2000; Rivera, 2000; Méndez et al., 2001; Alvarez et al., 2004; García, 2007; Arango et al., 2009; Rivera et al., 2011), aimed at defining a correct assessment of seismic hazard, seismic microzonation and site response, considering deep geological sections, soil type maps, and analyzing the geotechnical properties of the terrains.
Fig. 1 – Geological map of Santiago de Cuba city, at the scale 1:25,000 (modified from Medina et al., 1999), the red lines are the superficial projection of the Quaternary faults and the solid dots are the geotechnical boreholes (green) and geological wells (blue) used in the study.
The aim of this work is the definition of a geotechnical model for the Santiago de Cuba basin for the future 2D modelling of the expected ground motion determined by considering a scenario earthquake and the amplification caused by the local soil response.
Basic data used. The correct characterization of a site for seismic hazard purposes requests a good knowledge of several topics: geology, tectonics, geotechnics, geophysics, topography, and bathymetry.
a. Geology. Geological data are the basic information for the site characterization. We have used the geological-tectonic map of Santiago de Cuba city at the scale 1:25,000 (Medina et al., 1999), integrated with additional information from the stratigraphic lexicon (Carrillo et al., 2009) and further available literature related to this topic. The cited geological map of Santiago de Cuba city (modified from Medina et al., 1999) is reported in Fig. 1: the red lines are the superficial projection of the Quaternary faults and the solid dots are the geotechnical boreholes and geological wells available and used in this study. The Santiago de Cuba basin is characterized by low complexity in lithological variability, some geological formations are widely disseminated in the city broader area. Tab. 1 shows the geological formations and a brief description of these.
Tab. 1 - Lithostratigraphic information of the geology formations in the study area.
Age Geological formation Geological description Depth (m)Q4 var = Varadero Marine beach deposits. 5Q4 río = Río Macío Deposits of alluvial valleys. 10-20
Q1sup js = Jaimanitas Biodetrital limestones, very fossiliferous
with shell and corals. 10
N2su crt = Camaroncito Calcarenite with gravel. 4
N2sup-Q1
inf rm = Río Maya Coralline limestone, dolomite, clays and intercalation of polimictic conglomerates. 30-80
N2inf stg = Santiago Calcareous argillite, silty sands with
b. Tectonics. The identification of the active geological faults in the study region was based on those located onshore and proposed as active in the last 20 M.a. (of Miocene to Holocene) by Medina et al. (1999). These faults are not capable to generate a medium/large earthquake, but play an important role in the local amplification characteristics. Moreover, it is very important to have information on the deep geometry of the faults (e.g., dip, locking depth).
c. Geotechnics. In order to obtain a valid description of the variation of the soil properties with depth, we have used the geological and geotechnical information coming from several geotechnical surveys: 550 soil profiles from geotechnical boreholes performed by a local engineering and geological research institution (ENIA-Santiago) belonging
to the Cuban Ministry of Construction, and 11,120 geological borings from the Geominera East company (EGMO), collected in a dataset by Mendez et al. (2001). All this information was implemented into a GIS database.
d. Geophysics. The information from regional geophysical surveys (aeromagnetic, magnetic, gravimetric, seismic and geo-electric) was taken from Medina et al. (1999). In addition some local geophysical surveys were developed: electrical resistivity (tomography) and seismic refraction, in order to validate the physical-mechanical properties of the soil profiles and the characteristics of the rock basement in terms of VS.
e. Topography. All sites have been characterized by their elevation on the basis of the Terrain Elevation Model of the study region.
f. Bathymetry. As the study region comprises an offshore sector, it was necessary to use the bathymetric data (Diaz et al., 1998) to determine the sea depth and in this way to make a better interpretation of the geotechnical profiles that cross the bay area.
The geotechnical model for the future modelling of site effects. The final goal of this work is the estimation of the expected ground motion amplification due to the local soil response in the Santiago de Cuba city broader area. To reach this aim, a procedure was designed that accounts for all available information.
The bedrock, in terms of layer with a VS greater than 800 m/s, represents the starting layer from which the modelling of the local amplification is computed and it is made of hard or very hard rock, according to physical-mechanical properties. The geometry and the characteristics of the bedrock have been derived from the data of the geotechnical borings (lithology of the different strata and their physical-mechanical properties), the description of the geological formations (Nagy et al., 1983; Carrillo et al., 2009), the stratigraphic behaviour, and the geophysical information based on the surveys done.
Sixty-one profiles have been constructed to identify the different layers of the sedimentary cover. More precisely, the study area has been divided from north to south and from east to west by a suite of equidistant profiles, with the distance between them fixed at 500 m. We have used a custom VBA code (Create Geologic Cross Sections--eXacto Section v. 2.0, ArcMap 9.3) developed by the Illinois Geological Survey to build-up the geologic cross-sections. The topographic surface (using DEM values), the geologic units and their contact
Fig. 2 – 3D representation of the geotechnical model, the superficial geology is represented as in Fig. 1.
points (on the surface and along depth), the location of existing wells/boreholes (in a radius of 250 m), and the geologic material constituting the different strata are represented on the resulting cross-sections. In addition, the faults present in the region have been reported by a procedure developed ad hoc for the present study. This procedure takes into account the exact location of the fault on the surface and its strike and dip. In such a way it is possible to project in depth the fault and then to analyze how it can modify the behaviour of the different strata. Fig. 2 shows a 3D representation of the geotechnical model: the superficial geology is characterized in a manner similarly to Fig. 1 and the profiles illustrate the geologic interpretation in depth. Furthermore, data about the bathymetry of the bay have been added to define the depth of the sea.
Each profile is geo-referenced and the contact between the different layers in depth was modified by hand when it was required. The soil properties, such as type of material, density, and VS, were assigned to each layer: these parameters are the input data in local response modelling.
After having defined all profiles, the geotechnical model of the study area has been developed. An E-W oriented geological cross-section is represented in Fig. 3, the acronyms for geology are the same as in Tab.1. It is the framework from which it is possible to identify the sectors to be modelled for deriving the expected soil amplification.
Both 1D and 2D modelling techniques are expected to be applied. In sectors where the geotechnical model shows a homogeneous soil behaviour in depth (almost flat parallel layers), an equivalent linear 1D analysis is planned, according to the computer code PSHAKE (Sanò and Pugliese, 1991). Conversely, in areas of complex stratification with strong lateral variations, a linear 2D analysis is planned with the BESOIL code (Sanò, 1996). This code applies a fast numerical calculation of the seismic wave propagation in space, based on the boundary element technique.
Conclusions. For seismic response modelling it is worth to have a good geotechnical model of the investigated area. Good geological, geotechnical, topographical, geophysical, tectonic and seismic data contribute in a fundamental manner to the construction of a geotechnical model.
A geotechnical model for the Santiago de Cuba broader area has been defined on the basis of the analysis of all the available geological and geophysical information. Moreover, 61 stratigraphic profiles, which cover the entire study area, have been constructed being the profiles calibrated on data coming from a huge number of boreholes. The obtained geotechnical model is intended to be used for 1D and 2D modelling of the local soil amplification.
Fig. 3 – E-W oriented geological cross-section, the acronyms for the geology are the same as in Tab. 1 and the red lines represent the deep geometry of the faults.
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