Abstract
Abstract. The segment of the Africa–Eurasia plate boundary between the Gloria Fault and the Strait of Gibraltar has been the setting of significant tsunamigenic earthquakes. However, their precise location and rupture mechanism remain poorly understood. The investigation of each event contributes to a better understanding of the structure of this diffuse plate boundary and ultimately leads to a better evaluation of the seismic and tsunami hazard. The 31 March 1761 event is one of the few known transatlantic tsunamis. Macroseismic data and tsunami travel times were used in previous studies to assess its source area. However, no one discussed the geological source of this event. In this study, we present a reappraisal of tsunami data to show that the observations data set is compatible with a geological source close to Coral Patch and Ampere seamounts. We constrain the rupture mechanism with plate kinematics and the tectonic setting of the area. This study favours the hypothesis that the 1761 event occurred in the southwest of the likely location of the 1 November 1755 earthquake in a slow deforming compressive regime driven by the dextral transpressive collision between Africa and Eurasia.
Highlights
The coast along the southwest Iberian margin is prone to earthquakes and tsunamis
A complex system of faults accommodates the stress driven by the present-day tectonic regime that is constrained by NW–SE plate convergence between Africa and Eurasia at ∼ 4 mm year−1 (Argus et al, 1989; DeMets et al, 1994) and by the westward migration of the Cadiz Subduction slab ∼ 2 mm year−1 (Gutscher et al, 2012; Duarte et al, 2013)
We present the results of hypotheses A-MS and B
Summary
The coast along the southwest Iberian margin is prone to earthquakes and tsunamis. The earthquake and tsunami catalogues for the Iberian Peninsula and Morocco report three tsunamigenic earthquakes in the 18th century: 1722, 1755 and 1761 (Mezcua and Solares, 1983; Oliveira, 1986; Baptista and Miranda, 2009). The source location obtained by Baptista et al (2006) places the 1761 event southwest of the Southwest Iberian Margin (SWIM) in the outer part of the Gulf of Cadiz (Fig. 1). The SWIM is dominated by large NE–SW-trending structures limiting the Horseshoe Abyssal Plain (HAP) (Fig. 1). Large WNW–ESE-trending dextral strike-slip faults (the northern WNW–ESE-trending lineament, LN, and the southern WNW–ESE-trending lineament, LS) further characterise the SWIM cutting through the Gulf of Cadiz to the HAP (Zitellini et al, 2009; Terrinha et al, 2009; Rosas et al, 2009) (Fig. 1). We start with a reappraisal of previous research, we analyse the tectonic setting of the area and propose a source compatible with plate kinematics. We use Cadiz and Lisbon observations in 1755 and 1761 to compare the sizes of the events
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