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 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.

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.

Hirn, A.; Haessler, H.; Trong, P. H.; Wittlinger, G. & Victor, L. A. M. (1980). “Aftershock

sequence of the January 1st, 1980 earthquake and present day tectonics in the Azores”

Geophysical Res. Lett., 7, pp. 501-504.

IGEOE (2002). Military Map of Portugal, Series M889 (Azores), Page 25 -Ribeirinha

(Terceira), scale 1:25.000, Instituto Geográfico do Exército, Lisbon (in Portuguese).

Lloyd, E. F. & Collis, S. K. (1981).“Geothermal Prospection – Ilha Terceira, Açores”,

Geological Report, SRCI-GRA, Ponta Delgada, 96 p.

Lopes, I. M. F. (2005). “Small Strain Characterization of Soils. Application of the Surface

Wave Method”, PhD Thesis in Geotechnics, Lisbon University, 369 p. (in Portuguese).

Madeira, J. & Brum da Silveira, A. (2003). “Active tectonics and first paleoseismological

results in Faial, Pico and S. Jorge islands (Azores, Portugal)”, Annals of geophysics, Vol.

46, Nº 5, 733-761.

Madeira, J. (2005). “The volcanoes of Azores Islands: a world-class heritage. Examples from

Terceira, Pico and Faial Islands”, Field Trip Guide Book, IV International Symposium

ProGEO on the conservation of the Geological Heritage, 104 p.

Malheiro, A. M. (1998). “Borings in São Sebastião – Terceira Island” Technical Note Nº 97/98,

PPERCAS Project, Regional Laboratory of Civil Engineering, Ponta Delgada (in

Portuguese).

Montesinos, F. G.; Camacho, A. G.; Nunes, J. C.; Oliveira, C. S. & Vieira, R. (2003). “A 3-D

gravity model for a volcanic crater in Terceira Island (Azores)”, Geophysical Journal

International, nº154,1-14.

Nunes, J. C. (2000). “Geological study of the volcanic depression of São Sebastião (Terceira

Island- Azores) and surronding area”, Geological Map, PPERCAS project, Azores

University, Ponta Delgada (in Portuguese).

Santos, J.; Lopes, I.; Chitas, P.; Oliveira, C. S. (2007). “Site characterization of a volcanic crater

in Terceira island – Azores” Proc. 4th Int.Conference on Geotechnical Earthquake

Engineering., Thessaloniki, CD-ROM, paper no.1720, 14 p.

Senos, M. L.; Cruz, J. & Pena, J. A. (2000). “Seismic noise study of São Sebastião, in Terceira

Island and Flamengos, in Faial Island”, PPERCAS project, Meteorological Institute,

Lisboa (in Portuguese).