IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Oral Presentation 1734
Geological recognition of tsunami and storm deposits in two coastal areas
of Portugal
Mr. Pedro J M Costa
Earthquake Engineering and Seismology Division, IC IST Technical University of Lisbon
Suzanne A. G. Leroy
Abrupt marine invasions such as tsunamis and storms are particularly devastating for coastal areas.
They may also leave a permanent record in sedimentary deposits. In historical times the most
destructive tsunami that affected Europe was the Lisbon AD 1755. The effects and consequences of the
AD 1755 tsunami are presented. Sedimentological criteria to identify abrupt marine invasions in the
stratigraphic column are discussed. This work aims to contribute to a better understanding of the
signature left by abrupt marine invasions in coastal stratigraphy by investigating the nature of the
sedimentary record associated with tsunamis in a region of their known impact. A wide range of proxies
was used to detected tsunami and storm deposits in two coastal areas of Portugal. The techniques used
include stratigraphic description, grain size analysis, digital and x-ray photography, magnetic
susceptibility, macrofossils analysis, geochemical analysis and 210Pb and Optically Stimulated
Luminescence dating. The investigated areas (e.g. Lagoa de bidos (Central Portugal) and Martinhal
(South Portugal)) were affected by the AD 1755 tsunami. The locations have similar geomorphological
features and are both susceptible to major abrupt marine invasions. Results show that an abrupt event
deposited unique sedimentary units in both locations. A similar age for the event was established. A
considerable number of tsunami sedimentary characteristics were detected in both units. Moreover,
other abrupt marine invasions were detected in the lithostratigraphy of the study areas. However, a key
outcome of this research is the demonstration of the difficulty of distinguishing between sedimentary
deposits laid down by tsunamis, and those deposits resulting from storm action; consequently the
geological record of tsunamis almost certainly underestimates their frequency.
Keywords: tsunami, storms, portugal