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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

GÓNGORA, E., WALLENSTEIN, N., IBÁÑEZ, J., CARMONA, E., POSADAS, A. (2008) – Utilização da análise das componentes principais (ACP) no estudo da geometria da fonte na crise sísmica do Faial, entre 1998 e 2002 in: Sismo 1998 – Açores (2008). Uma década depois. SOUSA OLIVEIRA, C., COSTA, A. e NUNES, J. C. (eds.). Cap. 7, p.111-117. ISBN 978-989-20-1235-3.


On July 9th, 1998, a seismic crisis started near the Faial Island, in a typical premonitory-mainshock-aftershocks sequence. The main earthquake (ML = 5.9) was followed by a few thousands of aftershocks, located in a relatively narrow area, allowing a good seismic source characterisation.  This type of seismic crisis have been registered several times in the Azores region, in the past, also characterised by a pronounced decrease in the number of events and respective released energy in the following days, with some non periodic peaks.


Data compiled by the Azores Seismological Surveillance System (SIVISA, 1998-2004) was used in this study, from which a sub-database of 2,833 seismic events, from July 1998 to September 2002, was selected. These events were relocated, using two different (1D) velocity models: a regional (MAC) and a local (FAI), showing no significant differences.


The main objective of the present work was to characterize the seismic source geometry through the application of the multivariate statistical technique designated Principal Components Analysis (PCA). With this approach a large amount of data (with multiple variables) can be reduced, while keeping their representativeness of the contained information, as well as their characteristics (Jolliffe, 1986).


The application of this method begins with the selection  of hypocentre clouds , searching for their best-fitting ellipsoids and then selecting the most flattened ones (Posadas, 1991), which were named by Michelini and Bolt (1986) as Local Rupture Ellipsoid (LRE). The planes defined by half-axis of those ellipsoids allowed us to define main seismogenic faults of this crisis.


The application of the Spatial Principal Components Analysis (SPCA), lead us to admit that in the first three months of the crisis, the main fracture series trends to WNW-ESE (N80-60W), dipping 90º to 60º NE. In the following period the more significant direction was NE-SW (N30-50E), dipping SE (80º to 50º). All the obtained directions (both primary and secondary) were controlled by stress fields common to several tectonic accidents affecting the Azores.