Neotectonics of the SW Iberia margin, Gulf of Cadiz and Alboran Sea: a reassessment including recent structural, seismic and geodetic data

Abstract

S U M M A R YWe use a thin-shell approximation for the lithosphere to model the neotectonics of the Gulf of Cadiz, SW Iberia margin and the westernmost Mediterranean, in the eastern segment of the Azores-Gibraltar plate boundary. In relation to previous neotectonic models in the region, we utilize a better constrained structural map offshore, and the recent GPS measurements over NW Africa and Iberia have been taken into account, together with the seismic strain rate and stress data, to evaluate alternative geodynamic settings proposed for the region. We show that by assuming a relatively simple, two-plate tectonic framework, where Nubia and Eurasia converge NW-SE to WNW-ESE at a rate of 4.5-6 mm yr −1 , the models correctly predict the amount of shortening and wrenching between northern Algeria-Morocco and southern Spain and between NW Morocco and SW Iberia, as estimated from both GPS data and geological constraints. The consistency between modelled and observed velocities in the vicinity of Gibraltar and NW Morocco indicates that forcing by slab sinking beneath Gibraltar is not required to reproduce current horizontal deformation in these areas. In the Gulf of Cadiz and SW Iberia, the modelling results support a diffuse Nubia-Eurasia Plate boundary, where the convergence is accommodated along NNE-SSW to NE-SW and ENE-WSW thrust faults and WNW-ESE right-lateral strike-slip faults, over an area >200 km wide, in good general agreement with the distribution of the seismic strain rate and associated faulting mechanisms. The modelling results are robust to regional uncertainties in the structure of the lithosphere and have important implications for the earthquake and tsunami hazard of Portugal, SW Spain and Morocco. We predict maximum, long-term average fault slip rates between 1-2 mm yr −1 , that is, less than 50 per cent the average plate relative movement, suggesting very long return periods for high-magnitude (M w > 8) earthquakes on individual structures.

Educating for Earthquake Science and Risk in a Tectonically Slowly Deforming Region

“…Puzzlingly, a clear relationship between clusters and lineations of earthquakes and geologically mapped faults does not exist, opening several interesting questions about the local seismotectonics (Custódio et al, 2016). Thin-sheet modeling suggests that maximum long-term fault slip rates are between 1 and 2 mm=yr, resulting in very long (> 1000 years) return periods for M w > 8 earthquakes (Cunha et al, 2012;Matias et al, 2013).…”

“…However, tectonic deformation occurs at low rates, and therefore the intervals between strongly felt earthquakes are long (Vilanova and Fonseca, 2007;Baptista and Miranda, 2009;Cunha et al, 2012). The need to educate for earthquake risk is recognized at a high level by educators, scientists, and civil protection authorities (Wallenstein and Rebelo, 2010; Autoridada Nacional de Protecçã Civil 2011), but educational efforts are hampered by a mix of cultural and sociological factors, namely the lack of a prevention culture and a generally low earthquake risk perception (Lima, 1993).…”

Ambient Noise Recorded by a Dense Broadband Seismic Deployment in Western Iberia

“…Although Portugal has been repeatedly affected by onshore and offshore moderate-to-large earthquakes (Fukao, 1973;Johnston, 1996;Stich et al, 2005;Vilanova and Fonseca, 2007;Fonseca and Vilanova, 2010;Teves-Costa and Batlló, 2011), active seismogenic structures remain poorly understood. The interaction between Nubia and Iberia is thought to create a broad area of deformation, which in turn results in a pattern of diffuse seismicity (Buforn et al, 1995;Borges et al, 2001;Cunha et al, 2012;Bezzeghoud et al, 2014). A proper understanding of how large earthquakes are generated in such slowly deforming environment is lacking.…”

Development of a methodological framework for the assessment of seismic induced tsunami hazard through uncertainty quantification: application to the Azores-Gibraltar Fracture Zone

“…Because the modelled area encompass different seismotectonic domains, the overall AGFZ area was split into two main seismic source zones in order to feed the meta-model database. This was done to represent an oceanic domain to the West where normal to transtensive earthquakes mainly occur within a thin oceanic crust and a 5 continental domain to the East where reverse to transpressive earthquakes mainly occur on a thickened oceanic to continental crust (Buforn et al, 1988;Molinari and Morelli, 2011;Cunha et al, 2012). As a first order analysis, we considered the oceanic domain (hereafter called oceanic shelf) west of the 10°W meridian and the continental domain (hereafter called continental shelf) east of this meridian, coinciding roughly with the base of the continental slope facing the Portuguese coastline.…”

“…Both structures were modelled taking into account available maps (Cunha et al, 2012;Duarte et al, 2013) and fault parameters (Stich et al, 2007;Grevemeyer et al, 2017) summarized in considering their location (Figure 3). Similarly, fault parameters are considered uniformly distributed and are randomly sampled in their range of variation (Table 5), for a total of nearly 10,000 tsunami scenarios.…”