Subduction zones, where oceanic lithospheric plates plunge into the mantle, are a significant global source of seismic hazards, with earthquakes occurring within these plates presenting one of the biggest threats. At depths of up to 800 km, the occurrence of these earthquakes is surprising, as the pressure and temperature conditions should inhibit rock failure. Researchers have proposed two primary physical mechanisms that could cause seismic activity in 50 to 300 km depth within the subducting plate. The first suggests that fluid-mediated processes weaken the rock, making hydrated regions more seismically active than dry regions. The second mechanism suggests that changes in internal stresses due to variations in the dip angle of the subducting plate cause the seismicity, without necessarily relying on fluid-mediated mechanisms. The southern part of the central Andes in Chile is a remarkable location to investigate the drivers of slab seismic activity, as the subduction angle changes from flat in the north to steep in the south, and it has experienced many large earthquakes, such as the 2010 magnitude 8.8 Maule and 2015 magnitude 8.2 Illapel events.
To better understand the driving mechanisms behind these events, Dr. Constanza Rodriguez Piceda, a former PhD student in the international training group StRATEGy (German Science Foundation) at the Institute of Geosciences, along with colleagues from the GFZ Potsdam and the University of Potsdam, integrated geological, seismological, and geodetic observations. Using seismic tomography, the team mapped the regions of hydration within the oceanic plate and the mantle of the overriding plate and compared the distribution of fluids with the spatial distribution of seismicity. They found that the amount of fluids is highly variable from place to place, changing the way seismic events are spatially distributed in depth. In seismically-active regions with high fluid input, events are related to fluid-mediated processes. Conversely, in areas with low fluid input, the shape of the subducting plate is the primary contributor to seismic localization, with seismicity taking place where the dip angle changes from flat to steep.
Link to article: Rodriguez Piceda, C., Gao, YJ., Cacace, M., Scheck-Wenderoth, M., Bott, J., Strecker, M. & Tilmann, F. The influence of mantle hydration and flexure on slab seismicity in the southern Central Andes. Commun Earth Environ 4, 79 (2023). https://doi.org/10.1038/s43247-023-00729-1