Earthquakes are among the most dangerous and destructive natural hazards. Better understanding earthquake triggering by tectonics and by external processes is crucial for a more realistic assessment of earthquake hazards, especially in densely populated regions such as the west coast of Taiwan.
In this manuscript, an international collaboration led by Philippe Steer, an assistant professor at the Université de Rennes 1, show that a single intense erosional event can transiently change the seismicity of an active mountain belt. The researchers focus their study on the seismicity in Taiwan, one of the most tectonically active mountain range on Earth. They investigated how the seismic activity in Taiwan changed after typhoon Morakot, which delivered 3 m of rainfall in 3 days, causing numerous landslides and one of the most intense erosion ever recorded. P. Steer et al. demonstrated, by a careful statistical analysis, that there are significantly more small-magnitude and shallow earthquakes during the 2.5 years after typhoon Morakot than before, and that this change occurs only in the area showing extensive mass wasting.
The collaboration explain this change of seismicity by an increase of crustal stresses at shallow depth (<15 km), related to surface erosion. Indeed, the load represented by the weight of the rocks moved by the numerous landslides and then transported away by rivers can be significant. The progressive removal of this load changes the state of stresses in the shallow part of the crust in a way that favours seismicity on thrust faults, which are common features in mountain ranges such as Taiwan. This offers a physical link to explain how the erosion event induced by typhoon Morakot has resulted in more numerous shallow and small-magnitude earthquakes in Taiwan.
If numerous studies have shown that earthquakes can impact erosion and landscapes, this new result is, according to the authors' knowledge, the first direct evidence of the ongoing influence of erosion on seismicity, and in turn on tectonics. For the first time, the interactions between tectonics and surface processes are evidenced at the time scale of elementary surface and tectonic processes, i.e. at the time scale of earthquakes, typhoons and floods.
Earthquake statistics changed by typhoon-driven erosion
Philippe Steer, Louise Jeandet, Nadaya Cubas, Odin Marc, Patrick Meunier, Martine Simoes, Rodolphe Cattin, J. Bruce H. Shyu, Maxime Mouyen, Wen-Tzong Liang, Thomas Theunissen, Shou-Hao Chiang and Niels Hovius
Scientific Reports volume 10, Article number: 10899 (2020) | doi: 10.1038/s41598-020-67865-y