RESUMO: Na segunda quinzena de janeiro de 2018 realizou-se um estudo
da macrossismicidade associada ao sismo de Arraiolos de dia 15 de janeiro.
No presente trabalho descreve-se a metodologia usada na recolha de
informação, o tratamento dos dados e a sua representação na forma de mapa
de intensidades. A distribuição geográfica das intensidades mostra um
alongamento da isossista de intensidade IV na direção N-S e no sentido sul.
Este alongamento poderá estar relacionado com uma variação lateral da
atenuação, ou com um efeito de sítio. Outra hipótese, menos provável, é
que este padrão se deva ao efeito de diretividade originado por uma rutura
unilateral com propagação de norte para sul, ao longo de um plano do mecanismo focal. Esta possibilidade, conjugada com evidências para a existência
de estruturas ativas WNW-ESE descritas em trabalhos anteriores, levanta
a hipótese do designado “cluster sísmico de Arraiolos” estar associado a
uma interseção de falhas ativas.ABSTRACT: We performed a study about the macrossismicity associated to the
Arraiolos (Portugal) earthquake, that occurred on January 15th, (M = 4.9), in
order to construct a map of seismic intensities. The fieldwork took place in
the second half of January, using the questionnaire available in the website of
the Portuguese Institute of the Sea and the Atmosphere (IPMA). We provided
the same questionnaire on the internet, and collected responses from many
parts of the country. The results obtained allowed to construct a map of intensities showing a clear elongation of the intensities in a N-S direction. This
orientation can be explained by the differences in the behaviour of seismic
waves depending on the nature of the rocks or by a directivity effect associated
with the rupture along a fault with this direction. On the one hand, an
earthquake of magnitude 4.9 will hardly have the capacity to produce a
directivity effect, however on the other hand the analysis of the Portuguese
Geological Map seems to discard the first hypothesis. The IPMA provides on
its website, a strike slip focal mechanism for this earthquake (E-W dextral and
N-S sinistral). The conjugation of this focal mechanism with the distribution
of intensities, seems to reinforce the hypothesis of rupture along an N-S fault
plane. Similar focal mechanisms are known, associated with other earthquakes
that occurred in this region, but previous works propose that they are related, not
toN-Sruptures, buttoWNW-ESE structures present in the region, with geological and geomorphological evidence of recent tectonic activity. In this type of
studies, there is always a factor ofsubjectivity inherent to the sensitivity of the
people who were questioned, so these data are not sufficient to clearly affirm
that the earthquake occurred associated with a N-S-trending fault. The combination of these data with the information published in previous works allows
to consider the possibility that the "Arraiolos seismic cluster" corresponds to
an intersection of active WNW-ESE and N-S strike slip faults. Assuming that
there are alternating ruptures in both systems, temporary block occurs in the
system that has been cut, leading to a local stress concentration. Though, a
movement that in depth can be ductile, aseismic throughout most of the area
of the faults, in the zone of intersection of the two systems the crust will have
to respond necessarily as brittle, generating earthquakes and explaining the
existence of this seismic cluster.info:eu-repo/semantics/publishedVersio
