When the Earth is breathing : How terrestrial tides awaken submarine faults?
A recent study by IFREMER reveals that terrestrial tides, caused by the gravitational pull of the Moon and the Sun, can reactivate submarine faults. This reactivation could trigger significant methane emissions — a potent greenhouse gas — into the ocean and subsequently the atmosphere. It could also influence the seismic activity of faults, potentially transforming our understanding of the origins of certain earthquakes.
Terrestrial tides are slow movements of the Earth's crust resulting, like ocean tides, from the gravitational pull of the Moon and the Sun. A study by IFREMER, published in the Journal of Geophysical Research: Solid Earth, shows that these terrestrial tides may influence the behavior of submarine faults and impact the pressure of fluids trapped within sedimentary layers along these faults.
42 days of fieldwork in the Black Sea
A seismic fault is a rupture surface between two rock or sediment blocks, along which slow or rapid movements may occur, potentially inducing an earthquake.
As part of the ANR project Blame and the european projet DOORS, a team of researchers from IFREMER sought to understand how faults are activated by studying a section of the Romanian margin in the Black Sea. In 2021, they boarded the Pourquoi pas? vessel, part of the French Oceanographic Fleet, operated by Ifremer and its subsidiary Genavir, for the Ghass2 expedition.
Scientists deployed sensors, specifically piezometers developed by IFREMER, to measure variations in temperature and pressure within the first few meters of submarine sediments across two faults separated by 790 meters. This monitoring took place in two phases : an initial 15-day period in September 2021 during the Ghass2 campaign, and a second phase between October 2021 and May 2023. At the end of the campaign, the scientists decided to leave the piezometers on the two most interesting sites for 18 months of continuous observation.
During the first observation phase, abnormal variations in temperature and pressure of submarine fluids suggested active methane degassing on both faults, perfectly synchronized with terrestrial tide cycles—approximately every 12 hours.
« The deformation of the lithosphere caused by terrestrial tides, combined with fluid pressure, increases the pressure along the fault to a critical threshold. Once this threshold is exceeded, the fault reactivates, releasing part of the gas, in this case, methane. »
Vincent Riboulot, Second author of the study and marine geologist at IFREMER
However, during the longer second observation phase, researchers found no evidence of this interaction. Why such a contrast?
When fluid pressure inside the fault is high, as observed during the first study phase, a slight increase in pressure induced by terrestrial tides can suffice to reactivate the fault and release gases into the water column. Conversely, if the pressure is low because the fluid recharge process is incomplete, the fault is less sensitive to terrestrial tides.
« The gas that was release during the first observation period at the study site caused a drop in pressure. The fault then became insensitive to tides, explaining why no degassing was observed during the second period. »
Vincent Riboulot, Second author of the study and marine geologist at IFREMER
A new approach to monitoring earthquakes
These findings are not limited to the faults of the Black Sea; they could transform our understanding of certain earthquakes worldwide. By identifying the role of fluid overpressure in fault reactivation, this study opens new avenues for monitoring seismic risks, as well as for tracking methane emissions — a potent greenhouse gas — that could influence climate change if released from the water column into the atmosphere.