Earthquakes that occur within the dipping oceanic plate of subduction zones are a major source of global seismic hazards. The depths of these earthquakes can reach up to 800 km, which is remarkable because pressure and temperature conditions at such great depths should favour ductile over brittle failure of rock. In general, the exact physical mechanism responsible for intermediate-depth seismicity (50‑300 km) is widely debated, with some studies attributing the failure to fluid-mediated processes that can weaken the rock. Consequently, regions with higher levels of hydration are expected to be more seismically active than dry regions. Other studies suggest that seismicity is due to changes in the internal stresses caused by variations in the dip angle of the subducting plate as it plunges into the deep mantle, without necessarily relying on fluid-mediated mechanisms. One exceptional area to take a closer look into the drivers of slab seismic activity is the southern part of the central Andes in western South-America. Here the subduction angle changes from flat (<5°) in the north to steep (30°) in the south, and it has been the site of a considerable amounts of great earthquakes, such as the 2010 M8.8 Maule and 2015 M8.2 Illapel events. Another remarkable feature of this region is that slab seismicity is detected up to 300 km away from the oceanic trench.
To better understand the driving mechanisms behind these events, Constanza Rodriguez Piceda, a former PhD student in the DFG graduate school StRATEGy together with researchers from the Section 4.5 “Basin Modeling, from Section 2.4 “Seismology” and from the University of Potsdam, integrated geological, seismological, and geodetic observations. They used seismic tomography to map the regions of hydration within the oceanic plate and the mantle of the overriding plate and compared it with the distribution of seismicity. They found that the amounts of fluid are highly variable from place to place and that this affects the spatial distribution of seismicity, with hydrated regions being more seismically active. In regions of low fluid input, however, 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. Overall, this study combined thermomechanical and geological approaches to help narrow down the range of possible explanations for the seismic behaviour in the area.
Original publication:
Rodriguez Piceda, C., Gao, Y., Cacace, M., Scheck-Wenderoth, M., Bott [Sippel], J., Strecker, M., Tilmann, F. (2023): The influence of mantle hydration and flexure on slab seismicity in the southern Central Andes. - Communications Earth & Environment, 4, 79.
https://doi.org/10.1038/s43247-023-00729-1