The 1856 Djijelli (Algeria) earthquake and tsunami: source parameters and implications for tsunami hazard in the Balearic Islands

Abstract

Abstract. In 1856, one (or two) destructive earthquake(s) occurred off Djijelli (Algeria) and probably triggered a tsunami in the western Mediterranean Sea. Following recently published results of marine campaigns along the North-Algerian margin, a new source hypothesis for the earthquake has been proposed, and is constituted with a set of three "enéchelon" fault segments positioned in agreement with previous studies of this earthquake and with macroseismic data available. The geometrical parameters for this source, in agreement with a Mw = 7.2 earthquake, display an average 40 • NW dip, a 80 • strike and mean dimensions of 80 km (length) × 20 km (width). A coseismic slip of 1.5 m is consistent with an average convergence rate of about 5-6 mm/yr and a recurrence period of 300-400 years. They are then introduced in the tsunami modelling code to study the propagation across the Mediterranean Sea with a special attention towards the Balearic Islands. A focus on the two major towns, Palma (Majorca) and Mahon (Minorca) Harbours shows that these places are not the most exposed (maximum water heights less than 1 m) by tsunami waves coming from this part of the African margin. Specific amplifications revealed by modelling occur off the southern coast of Minorca and the southeastern coast of Majorca, mostly related to submarine bathymetric features, and are able to produce coastal wave heights larger than 1 to 2 m as offshore Alcalfar (Minorca). A deep submarine canyon southward Minorca leads to the amplification of waves up to two times on both sides of the canyon. However these modellings could not be compared to any historical observations, non-existent for these sites. This work is a contribution to the study of tsunami hazard in western Mediterranean based on modelling, and offers a first assessment of the tsunami exposure in the Balearic Islands.

Tsunami hazard assessment of Guadeloupe Island (F.W.I.) related to a megathrust rupture on the Lesser Antilles subduction interface

“…The role of strike angle has been previously discussed by Roger and Hébert (2008) concerning the impact of Algerian tsunamis in the western Mediterranean, or by Barkan et al (2009) and their modelings of the 1755 Lisbon tsunami in the North Atlantic Ocean. More generally, knowledge of the focal mechanisms of the seismic rupture, including rake angle and slip value determination, will help to propose the best scenario able to reproduce as well as possible tsunami generation by earthquakes as demonstrated, for example, by Geist (1998), Geist and Dmowska (1999) or Okal and Synolakis (2004).…”

“…Nonlinear terms are taken into account, and the resolution is carried out using a Crank-Nicolson finite difference method centered in time and using an upwind scheme in space. This method has been already successfully applied for studies in the Mediterranean Sea, the Pacific and Atlantic Oceans (Hébert et al, 2001;Roger and Hébert, 2008;Roger et al, 2010b). Wave propagation is calculated over 5 levels of nested grids (2 levels, grid 1 and 3 have been introduced for numerical stability) prepared using the same data set constructed for ComMIT preliminary tests (Fig.…”

Tsunami hazard at the Western Mediterranean Spanish coast from seismic sources

“…If we only consider the islands then the sources that pose the greatest threat are the S-3, S-5, S-6 and S-8, giving each one a total of 9 points of threat on the islands. The source with less danger from the North of Algeria would be the S-7, proposed as possibly responsible for the earthquake and tsunami of Djijelli in 1856 (Álvarez-Gómez et al, 2010b;Roger and Hébert, 2008).…”

Impact of a tsunami generated at the Lesser Antilles subduction zone on the Northern Atlantic Ocean coastlines

“…Again, sea-bottom shape showing numerous submarine canyons located along the margins are also able to focus waves energy toward specific locations aside the canyons' extremity as pro- posed by Roger and Hébert (2008) or Ioualalen et al (2010) for the Balearic and Atacames promontories respectively and more specifically by Divyalakshmi et al (2011) for the southeast coast of India and Iglesias et al (2013) onto synthetic models.…”