The Role of Earth Tides in Reactivating Shallow Faults and Triggering Seafloor Methane Emissions

The role of solid Earth tide in fault reactivation has significant implications for understanding earthquake triggering, carbon sequestration, and the global carbon budget. Despite extensive research on this topic, the relationship between Earth tide and fault reactivation remains unclear. In this study, we investigate the influence of solid Earth tide on the reactivation of sub-seabed faults, which may lead to the release of methane. We monitored the sub-seabed temperature and pore-fluid pressure at two sites on a fault system located in the Black Sea. Two sets of data obtained from distinct periods revealed inconsistent results. For the first set of data and despite the distance between the two sites (∼790 m), the responses in terms of temperature and pore pressure changes were synchronous (September 2021). We showed from these data that the presence of over-pressured fluid promotes fault reactivation under Earth tide cycles, resulting in synchronized degassing events. For the second set of data recorded from the same two sites (September 2021–May 2023), we did not identify any concomitance between Earth tides and the monitored parameters. Our analyses show that discrepancies in observations could be related to the fluid discharge/recharge process. The fluid discharge observed during the first period resulted in a decrease in excess pore-pressure, making the fault insensitive to Earth tides during the subsequent recharge period. Our data also sheds light on conflicting literature results, suggesting that the interaction between faults and Earth tides primarily depends on fluid pore pressure, a parameter rarely measured.