See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/234201767 Tsunamigenic potential of mass sliding in the Gorringe Bank (Gulf of Cadiz, SW Iberia) Article · April 2010 CITATION READS 1 425 8 authors, including: Filippo Zaniboni Claudio Lo Iacono University of Bologna Spanish National Research Council 87 PUBLICATIONS 1,150 CITATIONS 154 PUBLICATIONS 2,154 CITATIONS SEE PROFILE SEE PROFILE Gianluca Pagnoni Stefano Tinti University of Bologna University of Bologna 99 PUBLICATIONS 1,171 CITATIONS 318 PUBLICATIONS 5,602 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Algero-Balearic basin View project TSUJAL View project All content following this page was uploaded by Eulàlia Gràcia on 10 March 2016. The user has requested enhancement of the downloaded file. G 33446 2nd pages Large, deepwater slope failures: Implications for landslide-generated tsunamis Claudio Lo Iacono1*, Eulàlia Gràcia1, Filippo Zaniboni2, Gianluca Pagnoni2, Stefano Tinti2, Rafael Bartolomé1, Douglas G. Masson3, Russell B. Wynn3, Nuno Lourenço4, Manuel Pinto de Abreu4, Juan José Dañobeitia1, and Nevio Zitellini5 1Unidad de Tecnología Marina, CSIC, 08003 Barcelona, Spain 2Universitá di Bologna, Departimento di Fisica, 40127 Bologna, Italy 3National Oceanography Centre, Southampton SO14 3ZH, UK 4Estrutura de Missão para a Extensão da Plataforma Continental, P-2770047 Paco De Arcos, Portugal 5Istituto di Scienze Marine, CNR, 40129 Bologna, Italy ABSTRACT Deepwater landslides are often underestimated as potential tsunami triggers. The North Gorringe avalanche (NGA) is a large (~80 km3 and 35 km runout) newly discovered and deepwater (2900 m to 5100 m depth) mass failure located at the northern fl ank of Gor- ringe Bank on the southwest Iberian margin. Steep slopes and perva- sive fracturing are suggested as the main preconditioning factors for the NGA, while an earthquake is the most likely trigger mechanism. Near-fi eld tsunami simulations show that a mass failure similar to the NGA could generate a wave >15 m high that would hit the south Por- tuguese coasts in ~30 min. This suggests that deepwater landslides require more attention in geo-hazard assessment models of southern Europe, as well as, at a global scale, in seismically active margins. INTRODUCTION Almost all tsunamigenic landslides studied to date have been gener- ated in subaerial coastal settings or shallow marine areas, typically at the shelf edge or upper continental slope (Bondevik et al., 2005; Chaytor et al., 2007; Billi et al., 2008; ten Brink et al., 2009; Lo Iacono et al., 2011). The possibility that a submarine landslide generated at depths >3000 m can represent a tsunami risk for neighboring coastal areas has generally Figure 1. Bathymetric map of Gorringe Bank showing the location of been neglected, mainly due to lack of data required to assess this potential the North Gorringe avalanche (NGA). White dots represent earthquake ≤ hazard and because only large-volume landslides will generate dangerous epicenters of 4 < Mw 6 recorded since A.D. 1915 (Instituto Geográfi co tsunami waves at these water depths. Nacional, 2011). Red dots represent earthquakes of Mw > 6.0 (numbers refer to year and M ) (Instituto Geográfi co Nacional, 2011). Yellow dots A comprehensive multibeam map of the Gulf of Cádiz (Zitellini et w locate sediment cores. Main active faults are also located (Zitellini et al., 2009) has revealed a large, deepwater landslide located on the north- al., 2009). GF—Gorringe fault; MPF—Marquês de Pombal fault; SVF— ern fl ank of Gorringe Bank, referred to hereafter as the North Gorringe São Vicente fault; HF—Horseshoe fault. The white rectangle indicates avalanche (NGA). Here we describe the morphology of the NGA, discuss the area of the Figure 2. Inset: Plate-tectonic setting of the southwest the most likely triggering mechanism, and evaluate its maximum near- Iberian margin (box) along the Eurasian and African plate boundary. AGFZ—Azores Gibraltar fracture zone. fi eld tsunamigenic potential using numerical simulations. We show that the NGA, although deep and far from shore, may have generated a high- amplitude near-fi eld tsunami that would have hit the Portuguese coast, demonstrating that deepwater landslides need to be included in geo-haz- Zitellini et al., 2004) corresponding to one of the largest gravity anomalies ard assessments related to tsunamis. in the world (Souriau, 1984). Drilling and submersible investigations have revealed a heterogeneous composition, dominated by serpentinized peri- GEOLOGICAL SETTING dotites and gabbros, with less abundant basalts and a thin cover of recent The southwest Iberian margin hosts the present-day convergent sediments from a few meters to tens of meters thick (Auzende et al., 1978; boundary between the European and African plates (4.5–5.6 mm/yr), an Girardeau et al., 1998). Deep fractures and near-vertical faults cut through area characterized overall by moderate seismic activity but where large the bank, defi ning individual blocks (Girardeau et al., 1998) and likely historical earthquakes and tsunamis have been generated, among them making this area prone to slope failure. Although Gorringe Bank has been the Great 1755 Lisbon Earthquake and Tsunami (Baptista et al., 1998) modeled as a tectonic source for tsunamis (Gjevik et al., 1997), its threat (Fig. 1). Gorringe Bank, located within the plate boundary zone ~170 km for landslide-generated tsunamis has not been considered previously. offshore the southwestern Portuguese coast, is a 200-km-long, 80-km- wide NE-SW–trending asymmetric seamount, more steeply sloping to the DATA AND METHODS north, rising almost 5000 m above the surrounding Tagus and Horseshoe Swath-bathymetric data from Gorringe Bank were acquired with a abyssal plains (Fig. 1). Gorringe Bank is a Middle Miocene northwest- 12 kHz Simrad EM120 multibeam echo sounder on the RV Dom Carlos vergent thrust block of crustal and mantle rocks (Girardeau et al., 1998; and processed to a 60 m grid (Figs. 1 and 2). High-resolution multichan- nel seismic profi les were acquired onboard the RV Urania using a 465 in3 *E-mails: [email protected]; [email protected]. air gun source and a 48-channel, 1200-m-long streamer (Zitellini et al., GEOLOGY, October 2012; v. 40; no. 10; p. 1–4; Data Repository item 2012262 | doi:10.1130/G33446.1 | Published online XX Month 2012. GEOLOGY© 2012 Geological | October Society 2012 of America.| www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 G 33446 2nd pages Figure 3. A: Multichannel seismic profi le across the North Gor- ringe avalanche (NGA). Red line delineates the main body of the avalanche. The thickness of the deposit (0.25 s TWTT) is ~225 m, assuming an average velocity of 1800 m/s. M-PQ—Miocene to Plio- Figure 2. Three-dimensional bathymetric image of the NGA. Dashed cene–Quaternary; Cr-Eo—Cretaceous to Eocene. B: Line drawing of lines indicate approximate limit of the avalanche recognized from the 3.5 kHz echo sounder record collected in the distal depositional seafl oor morphology. The main depositional area consists of a rock area of the NGA. H/T—hemipelagite/turbidite sequence; ST—sandy cluster with glided blocks. Yellow dots locate analyzed sediment turbidite interval. cores. Black lines depict the BS14 multichannel seismic profi le in Figure 3A and the subbottom profi le in Figure 3B. likely due to the erosive action of the mass failure. The 3.5 kHz profi le collected in the distal depositional area of the NGA shows an ~10-m-thick 2004). Subbottom profi les (3.5 kHz) were acquired to select the coring unit of transparent facies below an 8–9-m-thick uppermost unit of well- sites. Three sediment cores, MD03-2701, SW-15, and JC27-33, were col- stratifi ed facies (Fig. 3B). Sediment cores show the top of the transparent lected adjacent to the NGA deposit (Figs. 1 and 2) during cruises onboard facies to correspond to a coarse sandy turbidite lying below a 9-m-thick the RV Marion Dufresne (2003), RV Urania (2005), and RV James Cook hemipelagite/turbidite sequence. AMS radiocarbon dating above the (2008), respectively. Radiocarbon dating was performed on hemipelagic sandy turbidite yields an age of 26–30 cal. k.y. B.P. (late Pleistocene). Due sediment samples. Dates were calibrated using a ΔR value of 95 ± 15 to the thickness of the sandy interval, we infer that a single mechanism 14C yr (Gràcia et al., 2010) and the Marine04 curve (Hughen et al., 2004) may have simultaneously triggered the turbidity current and the NGA fail- included in the OxCal 4.0 software. The volume of the NGA used in the ure. The late Quaternary age of the NGA is also supported by the thin tsunami model was calculated by subtracting the present-day bathymetry sedimentary layer draping the rock cluster in the seismic profi le (Fig. 3A). of the evacuation area from a simulated prefailure bathymetry obtained by Steep slopes, pervasive fracturing, and lithological heterogeneity interpolating the depth values of the NGA main headscarp and the edges of Gorringe Bank may be preconditioning factors for the NGA failure. of the slide (see the GSA Data Repository1 for further details). Land- On the southwest Iberian margin, an earthquake is the most likely trig- slide modeling and tsunami generation and propagation were simulated ger mechanism for the NGA, since earthquakes are also known to trigger separately by means of two different codes: UBO-BLOCK2 and UBO- coeval widespread turbidity currents in this area (Gràcia et al., 2010). A TSUFE, respectively. These codes, originally described by Tinti et al. minimum moment magnitude (Mw) of 6.0 has been suggested to generate (1994, 1997), have been extensively applied to the study of historical and large-volume landslides with near-fi eld tsunamigenic impact (Tappin et recent tsunamis (Tinti et al., 2006; Ranguelov et al., 2008).