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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

TEVES COSTA, P., PACHECO, J.M., ESCUER, M., COUTINHO, R., ZANON, V., PIMENTEL, A., KUEPPERS, U. (2008) – Utilização de vibrações ambientais na caracterização dinâmica das camadas superficiais na cidade da Horta. In: Sismo 1998 - Açores. Uma década depois, Oliveira, C.S., Costa, A., Nunes, J.C. (Eds.), Ed. do Governo dos Açores e da Sociedade de Promoção e Reabilitação da Habitação e Infra-Estruturas, S.A., p. 137 - 149.​


Horta Platform is a geomorphologic region of Faial Island, located on its  SE sector. It is characterized by a smooth topography and gentle slopes. Horta town, with an area of about 8,5 km2, is located at the east side of the platform and its geologic setting is marked by two distensive fault systems, with general orientations of WNW-ESE and NE-SW. Those systems are associated with fissural basaltic volcanism that produced several cinder and tuff cones.


The stratigraphic sequence at Horta region encompasses, from the older to the younger deposits, (1) a group of very altered lavas and some basaltic tuff and scoria layers related with the neighbour central volcanoes  of Ribeirinha and Caldeira; (2) a sequence of basaltic lava flows and a few basaltic tuffs and scoria layers related with the development of Horta Platform and (3) a set of pumitic pyroclastic deposits, produced by the Caldera Central Volcano, with an overall thickness often larger than 2 m although, due to localized erosion, in several places it was removed and the underlying lava is now cropping out at the surface.


Overall, the lithology at Horta region is quite homogeneous and can be described as a sequence of basaltic lavas covered by pumitic pyroclastic deposits ranging in thickness from 0 to over 2 m.


Nakamura’s methodology developed for the analysis of ambient vibration has been used, by several authors, for the dynamic characterization of the surface layers in sedimentary or alluvial regions. We tried to apply this methodology in the town of Horta. However, due to the particular geologic cover of this town, composed by pyroclastic deposits with irregular thickness (between 0 and 10m), the interpretation of the H/V curve is very hard. We tried to associate the frequency peak of the H/V curves with the thickness of the surface layer. We used SHAKE program to calculate the transfer function of soil columns with several surface layer thicknesses. We tried to fit the H/V curve for points with the same surface layer thickness with the transfer function, adjusting the S wave velocities (there are no measures of this parameter). The results show a reasonable agreement, between the surface layer thickness and the H/V peak frequency, assuming S wave velocities between 150 and 175 m/s. However, it should be noticed that most of the physical parameters were estimated based on geology and lithology information.