Grevemeyer, Ingo, ed. (2015) RV POSEIDON Fahrtbericht / Cruise Report POS430, POS440, POS460 & POS467 - Seismic hazards to the southwest of Portugal; POS430 - Le-Seyne-sur-Mer - Portimao (7.4. - 14.4.2012), POS440 - Lisbon - Faro (12.10. - 19.10.2012), POS 460 - Funchal - Portimao (5.10. - 14.10.2013), POS467 - Funchal - Portimao (21.3. - 27.3.2014). . GEOMAR Report, N. Ser. 024 . GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany, 43 pp. DOI 10.3289/GEOMAR_REP_NS_24_2015.

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Abstract

The plate boundary at the eastern terminus of the Azores-Gibraltar transform fault between Africa and
Iberia is poorly defined (Fig. 1.1). The deformation in the area is forced by the slow NW-SE convergence
of 4 mm/yr between the oceanic domains of Iberia/Eurasia and Africa and is accommodated over a 200-
300 km broad tectonically-active deformation zone. The region, however, is also characterized by large
earthquakes and tsunamis, such as the 1969 Mw=7.9 Horseshoe Abyssal Plain earthquake and the
November 1, 1755 Great Lisbon earthquake with an estimated magnitude of Mw~8.5. The exact location
of the source of the 1755 Lisbon earthquake is still unknown. Recent work may suggest that the event
occurred in the vicinity of the Horseshoe fault, an oblique thrust fault. In addition, the Gorringe Bank, a
~180 km-long and ~70 km-wide ridge with a relieve of ~5000 m, has been considered being a potential
source of the Lisbon earthquake. Deep Sea Drilling (DSDP) and rock samples indicated that the bank is
mainly composed of serpentinized peridotites with gabbroic intrusions, perhaps being created by
overthrusting of the Horseshoe Abyssal Plain onto the Tagus Abyssal Plain in NW direction. Further, the
Horseshoe Abyssal Plain is marked by the presence of compressive structures with a roughly NE-SW
orientation and E-W trending, segmented, crustal-scale, strike slip faults that extend from the Gorringe
Bank to the Gibraltar Arc in the eastern Gulf of Cadiz, which were called “South West Iberian Margin” or
SWIM faults. The fault system may mark a developing Eurasia-Africa plate boundary.

Two local seismic networks were operated in the area to investigate natural seismicity and seismic
harards. First, a network of 14 ocean-bottom seismometers (OBS) was operated between April and
October 2012 in the vicinity of the Horseshoe fault between 10°W to 11°W, and 35°50’N to 36°10’N.
OBS were deployed during RV Poseidon cruise POS430 and recovered during cruise POS440. From
October 2013 to March 2014 a second network of 15 OBS monitored seismicity at the Gorringe Bank.
OBS were deployed during RV Poseidon cruise POS460 and recovered during cruise POS467. Both
networks benefitted from seismic stations operated in Portugal and provided in the order of 50 to 90 locale
earthquakes occurring within or in the vicinity of each network. Most earthquakes in the Horseshoe
Abyssal Plain occurred at a depth of 40-50 km, either in oceanic mantle or unroofed continental mantle.
The large source depth of events observed in the Horseshoe Abyssal Plain supports the idea that large
catastrophic earthquakes, like the Great Lisbon earthquake of 1755, may indeed occur in the area. At the
Gorringe Bank seismicity was generally shallower, occurring at <30 km depth.

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