Amadeus and Esmeraldas: two marine geophysics campaigns to investigate strong earthquakes off Ecuador and Colombia

Several large earthquakes with magnitude higher than 8 on the Richter scale have already occurred along the margins between the Nazca and South American tectonic plates, under the ocean off Ecuador and Colombia. This region is vulnerable, all the more so because since the 1980s, Ecuador’s oil export terminal is sited within it. More information is needed on this zone of extremely high seismic risk. For this reason, two scientific campaigns, “Amadeus” and “Esmeraldas” were launched on 3 February and will run until 6 April 2005.

These campaigns, conducted by the mixed research unit (UMR) Geosciences-Azur (involving the IRD, CNRS, Universities Pierre et Marie Curie and Nice Sophia-Antipolis) working jointly with the University of Bordeaux, the Marine Technology Unit of the CSIC (Higher Council for Scientific Research) at Barcelona, and its international partners: Canadian (University of Victoria, PGC), Colombian (DIMAR, EAFIT and Caldas Universities, and INGEOMINAS) and Ecuadorian (INOCAR, EPN, Petroproduction) have the objective of studying the natural hazards associated with the large subduction earthquakes, submarine avalanches and tsunamis they trigger. The ocean-going campaigns are being conducted on the IFREMER research ship Atalante.

In 1906, a strong earthquake of 8.8 magnitude shook this part of the world. It was induced by the slipping of the Nazca plate under the North Andean margin. The two plates converge at an average rate of 5.5 cm/year. The slipping is expressed as a fracture zone about 500 km long. Reactivation occurred by earthquakes in 1942, 1958 and 1979, of magnitude 7.7 to 8.2, which provoked large tsunamis and submarine sediment slides.

Data from recent campaigns (between 1998 and 2001) showed the spatial variations of the geological structures and the tectonic regime of the margin, demonstrating the complexity of the fracture system that operates when an earthquake occurs and of the conditions that generate tsunamis. This variability prompted researchers to devise more precisely focused investigations, targeting in particular morphological detail and involving “three-dimensional scanning”.

The AMADEUS campaign (3 February-9 March) should provide a picture of the morphology and geology of active faults and submarine “landslides”. The research team are performing the three-dimensional mapping of seismic fractures of the past, to date and quantify the volumes of sediment caused to slide in response to earthquake activity and to find the recurrence time of the large subduction earthquakes over the past 5000 years.

The research team is using high-resolution multi-beam probes (EM12D, EM1000), a sediment penetrator (3.5 kHz), rapid seismic probes, and will take drill-core samples of sediments which probably hold a record of the large-scale earthquakes of the past (strong earthquakes can generate large mudslides like the one that occurred for example when the recent earthquake shook Algeria.

The ESMERALDAS campaign (15 March-6 April) will give the opportunity to examine, down to 30 to 50 km depth, the structure and geometry of the contact zone between the two plates, at the points where the most massive earthquakes are initiated.

A network of about 60 submarine and terrestrial seismic stations (OBS) will record the sound waves produced by a device towed by the Atalante. The interpretation of the signals thus recorded will enable the researchers to derive a three-dimensional “echogram” of the sector. The network will then be maintained until June 2005 in order to record the zone’s natural seismic activity.

Analysis of these new data will allow a better understanding of past earthquakes, assessment of the probability of new earthquake occurrence along the coast of Ecuador and Colombia, and to make more reliable digital simulation models of the initiation and propagation of tsunami in the area.

“Amadeus will bring a clearer picture of the activity of the faults identified and also, we hope, record in the sediments to the great earthquakes of the past, which could allow us to trace back to well before the 20th Century,” Philippe Charvis, IRD researcher and Director of Geosciences Azur, explained. “Esmeraldas”, he continued, “will provide us with a three-dimensional image of the zone and a means of locating microseisms which signal the accumulation of the various pressures at work.”