PRELIMINARY GEOLOGICAL AND GEOTECHNICAL MODEL OF THE SÃO SEBASTIÃO VOLCANIC CRATER TERCEIRA ISLAND

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1 PRELIMINARY GEOLOGICAL AND GEOTECHNICAL MODEL OF THE SÃO SEBASTIÃO VOLCANIC CRATER TERCEIRA ISLAND MODELO GEOLÓGICO E GEOTÉCNICO PRELIMINAR DA CRATERA VULCÂNICA DE SÃO SEBASTIÃO ILHA TERCEIRA Lopes, Isabel, ICIST/DECivil/IST, Lisbon, Portugal, ilopes@civil.ist.utl.pt Almeida, Isabel, CeGUL/GeoFCUL, Lisbon, Portugal, moitinho@fc.ul.pt Santos, Jaime, ICIST/DECivil/IST, Lisbon, Portugal, jaime@civil.ist.utl.pt Malheiro, Ana, LREC, Ponta Delgada, Portugal, Ana.MM.Malheiro@azores.gov.pt ABSTRACT The São Sebastião village showed higher intensity values than the surrounding areas and an unexpected damage distribution within the village during the 1980 January 1 st earthquake (M=7.2). The village is located within a volcanic crater and the site effects felt are mainly due to the soft soils filling of the crater, to topographic effects caused by the geometry of the crater and to the trapping effect of the seismic waves within the crater, but these aspects are not enough to explain the damage differences felt within the village. From the several tests performed in the crater, the last seismic tests made in the village, namely Surface Wave Tests, P Seismic Refraction and S Seismic Reflection along with new geological surface inspection introduced new information that was recently confirmed by a borehole campaign. This data finally allow to define a preliminary geological and geotechnical model of the São Sebastião volcanic crater. RESUMO Durante o sismo de 1 de Janeiro de 1980 (M=7,2) a vila de São Sebastião apresentou valores de intensidade superiores aos das áreas circundantes e uma distribuição de danos inesperada dentro da área da vila. São Sebastião desenvolve-se sobre uma cratera vulcânica e os efeitos de sítio verificados devem-se ao enchimento de solos moles, a efeitos topográficos causados pela forma da cratera e ao aprisionamento das ondas no seu interior. No entanto, estas causas não são suficientes para explicar as diferenças na distribuição dos danos sofridos. Dos vários ensaios realizados na cratera, os últimos ensaios sísmicos efectuados recorrendo ao Método das Ondas Superficiais, a Refracção Sísmica de ondas P e a Reflexão Sísmica de ondas S, juntamente com um novo reconhecimento geológico de superfície, introduziram informação nova que foi recentemente confirmada por uma campanha de sondagens. Estes dados permitiram finalmente construir um modelo geológico e geotécnico preliminar da cratera vulcânica de São Sebastião. 1. INTRODUCTION The study area, São Sebastião, is located in the SE end of Terceira Island in the central group of the Azores Archipelago (Figure 1). Terceira Island has felt in the historical times the effects of strong seismic events as the 1614 and 1841 earthquakes (Madeira & Brum da Silveira 2003). The last large earthquake, named the Terceira earthquake, occurred on January 1st 1980 with magnitude M=7.2 (Hirn et al. 1980) and showed that the seismic response of the São Sebastião volcanic crater is characterized by an amplification of ground movements inside the crater with respect to the surrounding area (Figure 2).

2 Figure 1 - Location of São Sebastião: a. Central Group of the Azores Archipelago; b. Terceira Island. Figure 2 - Isoseismic map of Terceira Island during the January 1st 1980 earthquake, showing the intensity anomalies (MM) in São Sebastião village (in: Montesinos et al. 2003) The local geology and geotechnical characteristics of the soil deposit play a strong role in the seismic ground shaking at the site. The site effects felt are mainly due to the soft soils filling of the crater, to topographic effects caused by the structure and to the trapping of seismic waves within the crater. Inside this volcanic crater, with an average diameter of 1100 m and a depth of about 50 m, is located the village of São Sebastião that has shown an anomalous seismic behaviour in a more local expression during the last earthquakes. In the 1980 earthquake some areas of the village have suffered more damage than others (Figure 3). During the last years some efforts have been made to understand the different site-effects occurred in the village. Some previous geological and geophysical studies were performed, (Malheiro 1998, Nunes 2000, Senos et al. 2000, Montesinos et al. 2003) but the information is merely qualitative, so it was not possible to have a rigorous and well-defined model of the crater. The results obtained by Lopes (2005) identified some remarkable differences in the soil velocity profile within the crater. Santos et al. (2007) described the main aspects obtained from these studies.

3 Figure 3 - Distribution of damage in São Sebastião village for the 1980 January 1st earthquake, showing the location of the strong motion stations and of the church in the village main square (in: Montesinos et al. 2003) In 2006 the investigation proceeded with the acquisition of seismic data, namely additional surface wave tests, P seismic refraction and S seismic reflection along with new geological surface inspection. In 2007 a new borehole campaign was performed and the resulting information confirmed the hypothesis on the geological structure proposed before by Santos et al. (2007). This paper presents the preliminary geological and geotechnical model of the São Sebastião volcanic crater, using all available data at the present moment. 2. GEOLOGICAL SETTING Terceira Island grew from E to W, starting during early to middle Pleistocene, and is composed by four major strato-volcanoes: the oldest structure is the Serra do Cume-5 Picos volcano, after in central position in the island it is possible to find the Guilherme Moniz and Pico Alto volcanoes and in the western area the Santa Bárbara volcano (Lloyd & Collis 1981, Montesinos et al. 2003, Madeira 2005). There is a basaltic fissure zone that crosses the island and that along with the Pico Alto and Santa Barbara volcanoes are thought to be still active (Madeira 2005). The volcanic products extruded are very heterogeneous and include ignimbrites, pyroclastic and pumice deposits, basaltic, andesitic and trachytic lava flows, scoria, tuffs, among others (Lloyd & Collis 1981, Montesinos et al. 2003, Madeira 2005). São Sebastião crater has an average diameter of 1100 m and a depth of about 50 m. The detailed volcanological map of São Sebastião and surrounding area was made by Nunes (2000). A simplified version of the map is presented in Figure 4 and shows that the geological formations of the area have mainly basaltic composition. Nunes (2000) showed that the northern side of the

4 crater rim cuts basaltic lava flows while in the southern part is identified a lahar deposit. The eastern side of the crater shows a scoria cone (Monte das Cruzes) and inside (SE side) there is an outcrop of basaltic lavas of the same nature as the ones that appear on the NW-W side. The depression is filled mostly by slope deposits of different nature and composition and by fluvial deposits. Figure 4 - Simplified volcanological map of the São Sebastião area (adapted from Nunes, 2000). While there is generally agreement on the type of the geological materials found in the São Sebastião area, the nature of the depression is still under discussion. Montesinos et al. (2003) propose a pit crater structure for the area while Madeira (2005) agrees with the idea proposed by Lloyd & Collis (1981) of a phreatomagmatic nature. Lloyd & Collis (1981) had proposed this genetic nature for São Sebastião because such a large crater could only be formed by a very explosive event as the phreatomagmatic explosions. Madeira (personal communication, 2006) justifies that the lahar deposit is a phreatomagmatic flow deposit, resultant from the formation of the crater and couldn t be associated to other source because it doesn t appear anywhere else in the surrounding area. The phreatomagmatic volcanoes, usually designated by maar, are low standing volcanoes with very wide bowl shaped craters that can range from a few hundreds of meters to about 3 km. They result from the contact of basaltic magma with water (the water level, an aquifer or the sea water) that produces a blast of fine-grained particles and a steam explosion. As they usually form holes in the surface, afterwards they get filled with water, forming lakes (Cas & Wright 1987, Francis 1993, Fisher et al. 1997). In the opinion of Madeira (personal communication, 2004) the soft shallow sediments of the actual São Sebastião crater are derived from the former existence of a lake, presently filled by sediments. The understanding of the geological nature and evolution of the crater is highly dependent on the volcano-stratigraphic relation between the different geological materials and is very important for the comprehension of the site effects occurred in the village.

5 3. PRELIMINARY GEOLOGICAL AND GEOTECHNICAL MODEL Santos et al. (2007) based on the preliminary results of the seismic tests and on the surface geological inspection made in 2006 proposed a hypothesis that considers the presence of a lava tongue within the crater. Under this hypothesis, the outcrop of intermediate lava that appears in the SE limit of the crater (Figure 4) can be the end of the lava tongue that flowed into the crater and outcrops in that area due to its lower altitude relatively to the surroundings. This hypothesis is in agreement with the topography and is compatible with the surface wave tests results presented by Lopes (2005) that indicated an increase of V S along the NW-SE direction (Figure 5). a b M J E M J E Figure 5 a. Digital terrain model of the São Sebastião volcanic crater; b. Horizontal section of V S (m/s) for 2 m depth resultant from surface wave tests (Lopes, 2005); In both are figures are located the seismic strong motion stations: M- Misericórdia; J Junta; E- Escola. The location of the boreholes performed in 2007 (Figure 6) was decided based not only on this hypothesis but also trying to obtain the closest position to the location of the seismic strong motion stations at Misericórdia, Junta and Escola. The 2007 borehole campaign consisted on four boreholes with SPT tests. SPT samples and undisturbed samples were collected for laboratory tests which are currently ongoing (physical and mineralogical identification, triaxial and dynamic tests). The borehole results allowed to confirm the hypothesis proposed by Santos et al. (2007). In two of the boreholes, the ones placed near to the south part of the crater, was crossed a layer of basaltic lavas consistent with the presence of a lava tongue in the NW-SE direction. Due to the confirmation of this hypothesis the surface wave data from Lopes (2005), which were blind interpreted before, are being reprocessed to better represent the internal structure of the crater. In this preliminary stage of combining and processing all seismic, geological and geotechnical data (Figure 6), was already possible to start sketching some cross-sections (Figure 7) that allow a better understanding of the internal structure of São Sebastião volcanic crater. Some doubts still remain regarding the dip of the internal borders of the crater and the total thickness of the soil that fills the crater. Not knowing the direction of the crater borders doesn t allow to decide between the two proposed crater genesis which results, for further modelling purposes, in considering both situations.

6 Figure 6 Location of the main tests performed in São Sebastião volcanic crater (Basis map: IGEOE, 2002). The SE-NW direction locates the cross-section presented in Figure 7. Figure 7 Simplified interpreted cross-section (scale elevation factor 10 ) on the southern part of the São Sebastião volcanic crater (direction located on Figure 6) crossing two of the 2007 boreholes near the seismic strong motion stations of Escola (E) and Junta (J). This cross-section in Figure 7 was made using as basis the topography of the IGEOE (2002) map and the volcanological map by Nunes (2000) simplified in Figure 4 and after was included

7 extra information from the seismic tests and boreholes. The nomenclature used for the geological formations is the same presented by Nunes (2000). The lava tongue appears with a V shape, which can be an indication of a valley in the paleomorhology. This hypothesis is also confirmed by the presence of an organic soil deposit detected by the boreholes. The ongoing laboratory tests indicate that the Fluvial and slope deposits are mainly composed by sandy soils with less than 20% of fines. The layers showing a higher percentage of fines are essentially non-plastic materials. The SPT test results show that until more or less 15 m of depth the N SPT is lower than 20 but bellow that depth the N SPT starts increasing. 4. CONCLUSIONS A considerable amount of tests were already made in the São Sebastião volcanic crater. The last results from seismic and geological-geotechnical tests started to bring valuable information on the internal structure of the crater that helps justifying the damage distribution differences within the interior of the crater. The combined processing and interpretation of all data is still in a preliminary stage. The laboratory tests on the soft soil filling of the crater are currently ongoing so the geotechnical information about the soil is still very limited. The available data allowed to construct a preliminary geological and geotechnical model of the crater. After this preliminary analysis, considering both crater genesis, further site effects 2-D numerical modeling is needed in order to understand the damage distribution. This modeling work can add information on the structure of the crater borders. ACKNOWLEDGEMENTS This work was developed under the research activities of ICIST (Institute for Structural Engineering, Territory and Construction of Instituto Superior Técnico) and CeGUL (Geology Centre of Universidade de Lisboa) and was funded by FCT (Science and Technology Foundation) under the research project POCI/CTE-GEX/58579/2004 and the scholarship SFRH/BPD/26547/2006. The authors would like to acknowledge Prof. José Madeira (GeoFCUL) for the fruitful discussions about the geology of volcanic areas and for sharing his ideas about the possible origin of the São Sebastião crater. REFERENCES Cas, C. A. & Wright, J. V. (1987). Volcanic successions: Modern and ancient, Allen & Unwin, 528 p. Fisher, R. V.; Heiken, G. & Hulen, J.B. (1997). Volcanoes. Crucibles of Change, Princeton University Press, 317 p. Francis, P. (1993). Volcanoes: a planetary perspective, Oxford University Press, Oxford, 443 p., 1993.

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