The fluid-saturated clayey sediments that characterize the most superficial part of subduction zones can favour tsunamigenic earthquakes. This is the result, published in the journal Nature Communications, obtained by a research team led by the National Institute of Geophysics and Volcanology (INGV) of Rome which involved the University of Pisa, the University College London and the Department of Geosciences of the University of Padua.

The study is based on the analysis of samples from the Pāpaku thrust, a tsunamigenic fault located in the Hikurangi subduction zone (New Zealand), and collected during the Integrated Ocean Drilling Program (IODP) Expedition 375 in 2019. As explained by Dr. Stefano Aretusini, researcher at INGV and first author of the paper, “by analysing in the laboratory the behavior of the samples taken in the Hikurangi subduction zone, we discovered that the clays present tend to have a low resistance to seismic movement due to the pressurized water they retain inside".

More specifically, the experiments were carried out at the High Pressure and High Temperature (HP-HT) Laboratory of the INGV using SHIVA (Slow to HIgh Velocity Apparatus), a system designed to reproduce the "engine" of earthquakes in the laboratory and financed by the European Research Council on a project directed by Prof. Giulio Di Toro from the Department of Geosciences and co-author of the paper. “SHIVA is an extremely versatile experimental apparatus designed to study friction in rocks and other materials”, explains Prof. Di Toro, “and it is the result of a more than ten-year collaboration between our Department and the National Institute of Geophysics and Volcanology. Together with RoSA (Low to High Velocity Rotary Shear Apparatus), a second experimental machine installed in our Department and able to reproduce the high temperatures and pressures inside the Earth's crust where earthquakes ‘originate’ and propagate, these two machines are unique in the world to study the mechanics of earthquakes”.

In the future, adds Dr. Aretusini, “we will analyze with the same method other types of materials sampled during the mission, to try to understand which of them can favour earthquake slip once they reach seismogenic depths into the subduction zone”.

1. According to the widely accepted model (Bilek and Lay, 2002), subduction zone megathrust faults are made of areas which can (unstable, dark grey areas in figure) or cannot (stable, white areas in figure) nucleate earthquakes. In the unstable areas, the yellow star indicates where tsunamigenic earthquakes might nucleate.
2. Zoom at the front of a propagating earthquake: while the seismic rupture (grey disk) goes through the megathrust fault from left to right at a speed of kilometres per second, just behind it, the relative movement of the fault bounding rocks (red line) occurs at meters per second. In the middle, the fault opposes a resistance (τ, black line, 3-30 MPa), that evolves in response to the water pressure (P, blue line, 1-10 MPa). As water pressure increases, the fault “floats” so that the resistance decreases.