Holocene palaeotsunami catalogue of SW Iberia
Introduction
Seismic activity in the Gulf of Cadiz appears to be related to Azores–Gibraltar transform fault zone (Gloria Fault). The tectonic pattern of this area operates as a strike-slip movement in the western part (Azores) and as a north–south compression to the east (Gibraltar) (Udías et al., 1976, Buforn et al., 1988). Commonly, the 1755 Lisbon earthquake epicentre has been located 200 km southwest of San Vicente Cape (Fukao, 1973, Martínez Solares et al., 1979). Recently, however, drill core and seismic profile data suggest an incipient subduction area west of Portugal where the tsunami origin may possibly have been located (Baptista et al., 1998, Zitellini et al., 1999, Zitellini et al., 2001). Data from off-shore southern Portugal revealed a number of active faults that may represent an earthquake and tsunami hazard to the coast of SW Iberia and NW Africa (Gràcia et al., 2003a, Gràcia et al., 2003b, Gràcia et al., 2010, Terrinha et al., 2003, Zitellini et al., 2004).
Off-shore studies are related to multidisciplinary marine geological and geophysical campaigns focused on the study of the active tectonics of SW Iberia. Recent papers have tried to highlight the potential of the turbidite record as a marine palaeoseismic indicator, as well as the correlation with the on-shore tsunamites record (Garcia-Orellana et al., 2006, Vizcaino et al., 2006, Gràcia et al., 2010).
The studies focused on on-shore record of these events have also attempted to distinguish features produced by tsunami and other extreme wave events (EWE), such as severe storm surges (Lario et al., 2010). Most surveys of EWEs in the area have concentrated on: Guadalquivir (Lario et al., 1995, Lario et al., 2001a, Lario et al., 2001b, Lario, 1996, Ruiz et al., 2004, Ruiz et al., 2005, Cáceres et al., 2006), Tinto-Odiel (Lario, 1996, Ruiz et al., 2007, Morales et al., 2008), and Guadalete (Lario et al., 1995, Lario, 1996, Dabrio et al., 1999, Luque et al., 2001, Luque et al., 2002) estuaries, the littoral lowland on the south-eastern coast of the Gulf of Cadiz (Luque, 2002, Whelan and Kelletat, 2003, Whelan and Kelletat, 2005, Alonso et al., 2004, Luque et al., 2004), and the Algarve (Andrade, 1992, Dawson et al., 1996) (Fig. 1). These studies described the sedimentary record of high energy events in the Gulf of Cadiz and concluded that most of them were tsunamigenic in origin. Following this conclusion, recurrence intervals were deduced (Morales et al., 2008, Ruiz et al., 2008a). The reported sediments are usually interbedded in estuarine or spit barrier sedimentary units that act as effective sedimentary sinks, likely to preserve geomorphological features that witness the palaeoenvironmental changes. Lario et al. (2010) summarised these studies and concluded that destructive events in this area created by EWE have a periodicity of 1200–1500 y, but not all were generated by tsunamigenic events.
Section snippets
Off-shore record of earthquake-triggered turbidites
Although some work suggested the presence of earthquake-triggered turbidites in this area (Thomson and Weaver, 1994, Lebreiro et al., 1997) it was only recently that the first paper about turbidite seismology in the SW Iberian margin appeared, studying the sediments related to 20th century earthquakes (Garcia-Orellana et al., 2006). Vizcaino et al. (2006) studied four gravity cores and found some events recorded as debris flows and turbidites that allowed estimation of a recurrence interval of
Record of EWE on-shore
Geomorphological and sedimentological features generated by extreme wave events (EWE) are common along the coasts of the Gulf of Cadiz, and have been assigned to either tsunami or storm surges (Lario et al., 2010). During recent years, numerous studies have tried to characterise the sedimentological features of tsunami and storm events, and concluded that the deposits generated by both types of events exhibit similar textural, structural and sedimentary properties (Morton et al., 2007, Morton
Discussion: on-shore evidence of palaeotsunami and their correlation with the off-shore record
The record of earthquake-triggered turbidites off-shore in this area (Gràcia et al., 2010) and the extreme wave events (EWE) that reached the SW coast of Iberia (Lario et al., 2010) allow correlation of these events, assignment of a tsunamigenic origin to some of the off-shore deposits, and identification of those regional events affecting the entire study area, generated by seismic sources in the Gulf of Cadiz area (Fig. 2).
The oldest EWE is recorded in Valdelagrana spit barrier system (Lario,
Conclusions: on-shore catalogue of palaeotsunami and recurrence intervals
Comparison of the on-shore geological record with the recently published off-shore records of palaeotsunami in the SW coastal area of the Iberian Peninsula (Gulf of Cadiz) makes it possible to identify five main EWE’s generated by tsunami affecting the area for the last 7000 y, prior to the well-known 1755 Lisbon tsunami-earthquake. These five events have calibrated ages of ca.7000–6800 cal BP, ca.5500–5000 cal BP, ca. 3900–3600 cal BP and ca.2200–2000 cal BP (Fig. 2), consistent with the
Acknowledgements
The research was funded by Projects EU NEAREST, CGL-2008-4000/BTE, CGL-2008-3998/BTE, GRACCIE (CSD-2007-00067) and ACI2008-0726. It is a contribution to IGCP Projects 588 and 567 and of the INQUA Focus Area on Palaeoseismicity and Active Tectonics.
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2021, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :Beside these long-term processes the Portuguese coast has been exposed to short-term high energy extreme wave events that have affected the environs and humans along its shores. Special attention is given in this case to the earthquake and subsequent tsunami that devastated coastal regions of Portugal (and especially its capital Lisbon), Spain and Morocco on November 1st 1755, and has been subject to numerous studies (see tsunami catalogues for reviews: Baptista and Miranda, 2009; Kaabouben et al., 2009; Lario et al., 2011; Maramai et al., 2014). Furthermore, storms and energetic wave conditions are relatively frequent (Costa et al., 2001).