Clayey sediments from the Hikurangi subduction margin in New Zealand analyzed with a new method. The area in the past was the site of tsunamis and earthquakes
The clayey materials of the faults present in the subduction areas, that is, where a tectonic plate slides under another plate, retain a “water buffer” inside them. This means that they favor earthquakes potentially capable of causing tsunamis. This is the result of the study “Fluid pressurization and earthquake propagation in the Hikurangi subduction zone” conducted thanks to the collaboration between the Italian Istituto Nazionale di Geofisica e Vulcanologia, the Universities of Pisa and Padua, and the University College London, on some samples from the Hikurangi area in New Zealand. The work was published in Nature Communications.
“In subduction zones”, explains Stefano Aretusini, researcher at INGV and first author of the study, “the seismic sliding that occurs at shallow crustal depths can lead to the generation of tsunamis and earthquakes. Due to the experimental difficulties in deforming the materials present in these areas, the physical processes that reduce the resistance of the thrust to which the fault is subjected are poorly understood. Analyzing in the laboratory the behavior of the samples taken in the Hikurangi subduction zone”, continues the researcher, “we discovered that the clays present tend to have a low resistance to seismic movement due to the pressurized water they retain inside”.
To study the behavior of these clays coming from the fault, the researchers conducted experiments on the samples collected during the international drilling mission “Integrated Ocean Drilling Program 375” carried out in 2018 off the North Island of New Zealand, attended by Professor Francesca Meneghini of the University of Pisa, second author of the published work.
In detail the samples of the rocks present inside the fault have been pulverized. The powders have been tested in the High Pressure and High Temperature (HP-HT) Laboratory of the INGV through a sophisticated apparatus, SHIVA (Slow to High Velocity Apparatus), funded by the European Research Council on a project by Giulio Di Toro, from University of Padua and co-author of this study, and reproduces the “engine” of earthquakes (the fault) allowing to observe what happens to the interior of the earth’s crust, with the deformations that the rock undergoes under high pressures.
Inside SHIVA, the powders have been analyzed using a new method to retain the water inside them while they are deformed at the typical speeds of earthquakes”.
Through the control tests conducted on a material whose characteristics are known, a Carrara marble powder, the researchers came to the conclusion that these clays favor the seismic slip of the fault precisely because of their ability to retain water, a characteristic that makes them ‘weak’.
“When I decided to participate in the oceanographic expedition”, says Francesca Meneghini, “I immediately contacted my colleagues from INGV and the University of Padua, with whom I have been collaborating for years. I was sure it was a unique opportunity to test the new experimental technique developed at the Institute and make a further contribution to our knowledge of seismic phenomena”.
“The subsequent developments of this research”, concludes Stefano Aretusini, “will be to analyze with the same method also other types of materials sampled during the mission to try to understand which of them can favor earthquake slip once they are transported into the subduction zone”.