IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Poster presentation 1835
The tsunami of Djidjelli (Eastern Algeria) of August 22 th, 1856: the
seismotectonic context and its modelling
Prof. Yelles-Chaouche Abdelkarim
Seismology C.R.A.A.G. IASPEI
J. Dverchre, A. Domzig, B. Mercier De Lpinay, N. Babonneau, H. Hbert, J.Roger, A.
Kherroubi, D. Graindorge, R. Bracne, A. Cattaneo, V.Gaullier, B. Savoye, P. Leroy,
R. Ait Ouali
The Djidjelli tsunami of August 22 th, 1856, at 10 p.m. is the first event well known in . It occurred after
the earthquake which hit the region and destroyed the city of Djidjelli, a small city located 300 km east
of Algiers (the capital of ). One day before, a first seismic shock happened in the same area causing
damages. The tsunami was well described by authors as Roth (1950) or Ambraseys (1982) who
reported many details of the effects on the Algerian coast. These authors indicated that a seawave of 2
m or 3m was generated by the earthquake. It flooded the lower part of the town and propagated until
the Balearic island. Effects of the tsunami were felt all around the western Mediterranean region.
Although the tsunami was attributed to an offshore seismic event with a location of about 15 miles from
the coast, its seismotectonic context remained unknown until the recent Maradja2 survey carried out in
November 2005. This survey conducted by the French R/V The Suroit allowed to map the bathymetry of
the eastern margin of from Dellys to Annaba . From the data collected, a morphotectonic map of the
eastern region of has been obtained. Off Djidjelli, four tectonic active structures are outlined along the
foot of the slope. They correspond to a set of en echelon faults with an E-W orientation. From the
seismic profiles, these structures correspond to thrust ramps dipping towards the south. This pattern
explains the occurrence of the double shock and the occurrence of the tsunami on the August 22th.This
later could have also triggered marine landslides. Some instabilities are still observed along the margin.
Using numerical modelling of the tsunami waves due to the earthquake only, we discuss the effect of
tsunami due to the source inferred, compared to the available observations, especially in the Balearic
islands. The Djidjelli and the last Boumerdes May 21st, 2003 tsunamis are considered as the most
important ones which occurred on the Algerian margin. Their occurrence demonstrates that seismicity
along the Algerian margin could represent a major threat for the western Mediterranean region. The
installation of an alert system in this region through the NEAMTWS project will be an important tool to
mitigate the risk.
Keywords: djidjelli, tsunami, en echelon faults