Proceedings - Johannes Gutenberg-Universität Mainz

2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011)
EARTHQUAKE
ARCHAEOLOGY
INQUA PALEOSEISMOLOGY
AND ACTIVE TECTONICS
PLIO – PLEISTOCENE TECTONIC ACTIVITY IN THE SOUTHWEST OF PORTUGAL
Figueiredo, Paula M. (1, 2), Cabral, João (1,2), Rockwell, Thomas K. (3)
(1) IDL, Instituto Dom Luiz, pmfigueiredo@fc.ul.pt
(2) Geology Department, Science Faculty, Lisbon University
(3) Geological Department, San Diego State University
Abstract: Southwest Portugal, located close to the Eurasia-Núbia plate boundary, is characterized by moderate
seismicity, although strong events may occur, as in 1755 (Mw8), 1969, (Mw 7.9), and more recently in 2007 (Mw
6.3) and 2009 (Mw 6.1), all located in the offshore. No historical earthquakes with onshore rupture are known for
this region. Inland, recent neotectonic and paleoseismological studies corroborate seismogenic activity during the
Pleistocene, at the S.Teotónio–Aljezur–Sinceira Fault System, suggesting that these structures are active and are
potential sources for moderate seismicity. At the coastline, several features such as old beach sediments, paleo
abrasion platforms and paleo cliffs were recognized. A sequence of poorly preserved surfaces with thin deposits
may correspond to Pleistocene marine terraces, suggesting a higher uplift rate than expected for this region.
Key words: Pleistocene, Uplift, active tectonics, Portugal
INTRODUCTION
Southwestern Portugal is located close to the
Eurasia-Nubia plate boundary, near the Azores-
Gibraltar fracture zone. East of the Gloria transform
fault, this boundary becomes complex and diffuse,
where deformation is distributed across a few
hundred kilometres wide zone, related to the NW-SE
convergence of Iberia and Nubia at a rate of ca. 4-5
mm/yr. In this area is located the inferred
seismogenic source zone of the 1755 earthquake
and tsunami (estimated Mw 8), and also of the Mw
7.9 1969 event. (Zittelini et al., 2009).
Fig. 1: Main Neotectonic structures located in southwern
Portugal. Onshore structures: APF, Alentejo-Placencia
Fault; STASFS, São Teotónio-Aljezur-Sinceira Fault
System. Offshore Structures: PSF, Pereira de Sousa Fault;
MPF, Marquês de Pombal Fault; HSF, Horseshoe Fault;
GBF, Guadalquivir Bank Faults.
and other faults trending close to E-W, such as the
Guadalquivir Bank and SWIM faults (Figure 1). To
understand and study the recent tectonic activity in
this sector of Iberia, it is necessary to study the
individual inland structures but also inland
deformation evidences that may be related to some
of the referred offshore active structures. This
abstract depicts both perspectives.
DISCUSSION
The São Teotónio –Aljezur- Sinceira Fault System
The São Teotónio–Aljezur–Sinceira fault system
(STASFS) corresponds to the nearest inland brittle
structure that may correlate to the ongoing plate
boundary deformation in the offshore. It extends
NNE-SSW for 50 km, parallel and close to the
southwest Portuguese coast (Figure 1), and
comprises left-lateral strike-slip faults, deforming a
large regional abrasion platform ca. 10 km wide,
considered of probable late Miocene age, and which
was reoccupied during the Pliocene and the
Pleistocene. Four small Cenozoic tectonic basins,
filled with Miocene to Pleistocene sediments, occur
along the STASFS.
Post-Pliocene vertical displacements of up to 100 m
may have occurred related to tectonic activity in the
STASFS, but generally these only reach a few tens
of meters. Dias (2001) estimated a slip-rate of ca.
0.03-0.06 mm/yr, based on the vertical offset of
morphological features. However, as the main slip on
these structures is strike-slip, those values are a
minimum estimative. Dias (2001), taking into account
This seismogenic source zone is characterized by
several structures trending NNE-SSW to NE-SW, as
the Marquês de Pombal and the Horseshoe faults,
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