Extreme Hydrometeorological Events, a Challenge for Gravimetric and Seismology Networks

Extreme events will become more common due to global change, requiring enhanced monitoring and pushing conventional observation networks to their limits. This encourages us to combine all the possible sources of information to obtain a complete picture of extreme events and their evolution. This commentary builds on an example of the July 2021 catastrophic floods that hit northwest Europe, for which the use of seismometer and gravimeter captures complementary data and brings a new understanding of the event and its dynamics. A sudden increase in seismic noise coincides with the testimony reporting on a “tsunami” downstream of the geophysical station. Concurrently, the gravimeter showed increasing saturation of the weathered zone, showing less and less water accumulation and increasing runoff. When rain re‐intensified after a 3‐hr break, the subsoil's saturation state induced an accelerated runoff increase, as revealed by the river flow, in a much stronger way than during the rainy episodes just before. We show that the gravimeter detected the saturation of the catchment subsoil and soil in real‐time. When the rain re‐intensified, this saturation resulted in a sudden, devastating and deadly flood. Our study opens up the possibility of integrating real‐time gravity in early warning systems for such events. Plain Language Summary Global change will require increased Earth monitoring and push conventional observation networks to their limits. This challenge encourages us to be open to all the possible sources of information to obtain a complete picture of extreme events and their evolution. This commentary illustrates this using seismometer and gravimeter complementary observations that bring a new understanding of Belgium and Germany's July 2021 catastrophic floods. A sudden increase in seismic noise coincides with the testimony reporting on a “tsunami” downstream of the geophysical station. At the same time, the gravimeter, which can measure the gravitational attraction of groundwater, showed that the soil was becoming increasingly saturated and could store less and less water. Given this saturation, when rain re‐intensified after a 3‐hr break, the ground could not absorb water anymore. The saturation induced an accelerated increase in water running off the land surface, as revealed by the river flow, causing a sudden, devastating and deadly flood. Our study opens up the possibility of using real‐time gravity in early warning systems for such events. Key Poi