PRELIMINARY GEOLOGICAL AND GEOTECHNICAL MODEL OF …
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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, [email protected]
Almeida, Isabel, CeGUL/GeoFCUL, Lisbon, Portugal, [email protected]
Santos, Jaime, ICIST/DECivil/IST, Lisbon, Portugal, [email protected]
Malheiro, Ana, LREC, Ponta Delgada, Portugal, [email protected]
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 1st 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).
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.
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
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.
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 VS along the NW-SE direction (Figure
5).
Figure 5 – a. Digital terrain model of the São Sebastião volcanic crater; b. Horizontal section of
VS (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.
a
b
M J
E
M
J
E
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
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 NSPT is lower than 20 but
bellow that depth the NSPT 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.
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