When the bottom of a lake tells the story of six millennia of earthquakes
Major earthquakes don’t follow any timetable. This conclusion, which overturns the dominant models of the seismic cycle, stems from a decade of research carried out in western Nepal. By analyzing sediments from Lake Rara and other Himalayan lakes, scientists have reconstructed an unprecedented catalog of seismic activity dating back 6,000 years.The principle is based on turbidites, the characteristic layers of debris deposited on lake bottoms during underwater avalanches. When a sufficiently powerful earthquake occurs nearby, it triggers these slides. By taking sediment cores and analyzing their composition, researchers can date and identify past earthquakes with remarkable precision.
The verdict is clear: even devastating earthquakes above magnitude 6.5 occur at random. There are temporary clusters over a few centuries, followed by lulls. But there is no reliable recurring cycle. "Calm periods in no way signal the end of the danger," stresses the research team. "We observe temporary gatherings over a few centuries, then lulls, but no reliable recurring pattern."
Ten years of research under extreme conditions
The study’s lead author, Zakaria Ghazoui-Schaus, conducts his research at ULB’s Geosciences, Environment and Society department, G-TIME laboratory, where he is seconded by the British Antarctic Survey. This work began ten years ago, when Zakaria Ghazoui and Jean Robert Grasso set about deciphering the first results at the Institut des Sciences de la Terre, Université Grenoble Alpes.The meticulous analysis of the data continued with the arrival of Arnaud Watlet, researcher at the Luxembourg Institute of Science and Technology, and Corentin Caudron, professor at ULB. "For the first time in the Himalayas, sediment cores taken from the bottom of lakes have made it possible to reconstruct the history of earthquakes over six millennia", explains the team.
But the sites still needed to be accessed. Lakes Rara and Phoksundo are located in Dolpo, a landlocked region of western Nepal, access to which has long been restricted: today still classified as a special permit zone, it was also severely restricted during the Maoist insurgency (1996-2006).... Some lakes, perched between 4,000 and 5,000 metres above sea level, can only be reached after an approach on foot lasting several days. From Kathmandu, light aircraft fly to regional airstrips such as Jumla or Jomsom. The rest is done on the trails, in successive stages, using convoys of mules to transport the coring equipment.
"Zakaria Ghazoui sums up: "To reconstruct a seismic history spanning several millennia, you first have to go and find natural archives that are rarely accessible. This complex logistics, combined with the altitude, narrow weather windows and the need to work quickly once on site, was the prerequisite for taking samples.
Research at ULB is continuing apace, with the analysis of new sediment cores currently underway. These will shed further light on Nepal’s seismic dynamics and history.
Fifty million people affected
For the inhabitants of the Himalayan arc, from Kashmir to Assam, the stakes are high. The last major earthquake in Nepal in 2015 demonstrated the vulnerability of infrastructure in cities that have become densely populated. Rapid urbanization, the uneven quality of construction and the sometimes faulty application of earthquake-resistant standards all increase the risk.An earthquake is not a disaster in itself: it becomes one when it strikes an exposed area and a vulnerable society. The quality of buildings, the organization of emergency services, the availability of information and social inequalities are all factors that determine the extent of human losses. Studies show that, on a global scale, a disproportionate number of earthquake fatalities are concentrated in countries where levels of corruption are high in relation to economic development.
Risk maps will have to take this fundamental unpredictability into account. Prevention is therefore as much a matter of governance and transparency as it is of geology.
From the Himalayas to Europe: rethinking preparedness
Europe is not the Himalayas, but it does have a wide variety of seismic contexts, requiring a coordinated approach. Belgium records few significant earthquakes, while Italy, Greece and Turkey are among the continent’s most active zones. France, particularly in the Alps and Pyrenees, also presents a real seismic risk.Two lessons need to be drawn. Firstly, the illusion of a programmed risk is misleading. The Lake Rara study shows that major earthquakes occur largely at random over the long term. A long period of calm does not herald a period of safety. This principle of caution applies to all European regions: with short observation series, it’s easy to believe in a cycle that doesn’t exist.
Secondly, cross-border infrastructure networks create interdependencies that transcend national borders. Although Belgium is not exposed to major earthquakes, it is at the heart of European infrastructure, civil protection and international aid networks, making it part of a collective response.
In an interconnected continent, resilience is collective. European countries, despite their differences in seismic exposure, all have an interest in sharing expertise, data and cross-border response protocols.
10.1126/sciadv.adx7747
