There are unsolved crimes. And unexplained natural disasters. The largest tsunami to have struck the Mediterranean coasts in the past two centuries was among these “cold cases”—until Frédérique Leclerc decided to examine, using a robot at a depth of 1,200 meters, the base of an underwater fault escarpment. A veil of mystery surrounded the events of 9 July 1956, when, at 6 a.m., a gigantic 20-meter-high wave, preceded by a significant earthquake, crashed onto the shore of the Greek island of Amorgos and swept across the Aegean Sea.
How did this lecturer at the Géoazur laboratory of Côte-d’Azur University find herself in the Cyclades archipelago, searching, amid the rugged underwater terrain, for traces of a seven-decade-old earthquake? Likely through encounters. First, at the middle school in her hometown of Senlis (Oise), with a life and earth sciences teacher who took her 8th-grade students to visit the volcanoes of Auvergne every year. Then, in 2008 during a bachelor’s internship at the Volcanological and Seismological Observatory of Martinique, with Nathalie Feuillet from the Paris Institute of Earth Physics.
Impressed by the “maturity” and “autonomy” of this brilliant student from Paris-Diderot University, the geophysicist took her under her wing and, the following year, brought her on expeditions across the Lesser Antilles volcanic arc.
“In this region near a subduction zone, the North American and South American plates dive beneath the Caribbean plate, creating deformations in the Earth’s crust that uplift or submerge islands or activate fault systems, some of which occasionally break, generating earthquakes,” explains the tectonicist, miming the movement of lithospheric blocks with her hands. On that first oceanographic campaign, the goal was to map one of the structures suspected to have caused the magnitude 6.3 earthquake in 2004 that caused significant damage in the Saintes archipelago (Guadeloupe) and produced a small tsunami.
Detailed views
The voyage was revelatory. The sun, dolphins, flying fish glinting in turquoise waters, daily life aboard the vessel N/O Suroît… everything fascinated the young researcher. Particularly the operation of bathymetric and reflection seismic instruments used to locate faults. Over sporadic movements spanning hundreds of thousands of years, these faults had carved escarpments hundreds of meters high on the seafloor and altered the layering of lava and sediments deposited there.
“With sufficiently high-resolution imaging, it is possible to document their geometry,” she explains. This was achieved during a 2010 campaign on the Roseau Fault, responsible for the largest disaster in Guadeloupe in a century.
Following a period of hesitation, during which Leclerc volunteered to observe Hawaii’s Mauna Loa volcano and discovered an interest in human sciences—from running blogs to publishing a popular science book and involving historians and sociologists in her Amorgos project—she often found ways to combine her research with these broader perspectives.
All these experiences naturally led to a thesis dedicated to estimating the magnitude of the largest potential earthquakes in the Saintes region, the movement of tectonic structures crossing it, and the vertical displacement of some islands. This without validating models suggesting that in 2004 the Roseau Fault slipped 10 kilometers and moved vertically by 1 meter. “Changes this small in seafloor topography were invisible in images obtained from the vessels,” recounts Leclerc, describing her frustration.
Chance lent a hand. In 2013, an incident aboard the Pourquoi-pas ? forced geologist Javier Escartin to cancel an Atlantic campaign. Stranded in Pointe-à-Pitre with a robot, the Ifremer Victor 6,000, and an autonomous underwater vehicle, the CNRS research director at the École normale supérieure decided to observe the Roseau Fault and called the doctoral student. She flew to Guadeloupe the day after next.
It was a stroke of luck! In a few dives at 1,000 meters, at the locations she indicated, the machines delivered detailed views of the escarpment base. A flat surface of freshly exposed light-colored rocks—the slip plane of the earthquake—was visible. This tectonic structure’s discovery at this location was of the utmost importance. This kind of seafloor uplift, commonly observed on continents after major earthquakes, had never been linked to an underwater seismic event. Active faults are largely unknown in this environment.
The images immediately drew interest from the marine geoscience community. Another campaign, “SubSaintes,” validated the method in 2017. “But it was Frédérique Leclerc who originated it. Through enthusiasm and hard work, she became the mission leader in 2013,” assures Javier Escartin, who has continued to support her since.
After two years of postdoctoral research in Singapore, with missions off Sumatra and Sumba Island, Leclerc joined Géoazur in 2016. Discussing with a Greek colleague, she began applying methods developed in the Antilles to the Amorgos event. This magnitude 7.6 earthquake, one of the largest recorded in Europe, killed at least 54 people in the Cyclades in 1956. Witnesses reported 30-meter waves on some coasts and 2–3 meters on others. Yet nothing fully explained the event.
Indeed, if the earthquake’s focus was 45 kilometers deep, as suggested by scarce seismometer records, ground movements transmitted to the water column could not have flooded the coasts with a 30-meter tide. And if it was 25 kilometers, should a tsunami even be considered? Could underwater avalanches triggered by the tremors have locally produced some observed effects? Impossible to resolve without examining the vast fault network discovered in 2015 between Santorini and Amorgos.
Traces of recent displacement
The Covid-19 pandemic disrupted the French oceanographic fleet schedule, and for this expedition Leclerc had to rely on the coastal vessel L’Europe, from Ifremer, equipped with the autonomous vehicle Idefx and the Ariane robot. Three campaigns followed, in 2022, 2023, and April 2025, dedicated to mapping, surveying, and observing seafloor reliefs. As a result, at the base of the Amorgos Fault escarpment, traces of recent seafloor displacement were identified—smooth, light-colored surfaces 9 to 16 meters high, consistent with an earthquake of the 1956 magnitude.
At 38, Frédérique Leclerc has launched a new research project with colleagues from the French Alternative Energies and Atomic Energy Commission and the National Observatory of Athens to understand the subsequent tsunami. Using new land and sea investigations, archival research, and modeling, the exploration of past tsunamis has begun, and a wave of discoveries is expected.
