Earthquakes in Slow Motion
A new scientific study, cosigned by three researchers from École normale supérieure - PSL (France), Caltech (United States) and École Polytechnique (France), could teach us a lot about the most devastating earthquakes. Their article, published in Nature Magazine, finds that so-called "slow slip" or "silent" earthquakes observed in Cascadia in the northwest region of the United States behave more similarly to regular earthquakes than previously thought.
Legend : At the Cascades Range, the Pacific Ocean floor slides beneath the North American tectonic plate. Credit : Eric Vantroeyen, Sylvain Michel, Adriano Gualandi et Jean-Philippe Avouac
"Slow slip" or "silent" earthquakes are a type of earthquake with no quake. The paradox is not apparent since the ground does not move as in the case of a classic earthquake, but very slowly, so gradually that the event does not cause any damage. Where a large amount of energy is typically released in one minute during large tremors, it takes several weeks of slow slippage to produce the same energy. These seismic events are interesting for three reasons: their "slow slip" characteristic makes it possible to track the way they appear and spread, they reoccur much more frequently than major earthquakes, and they are especially good counterparts of the most devastating major earthquakes.
This crucial discovery is published in an article in Nature Magazine cosigned by Sylvain Michel (École normale supérieure - PSL, Department of Geosciences (ENS/CNRS), France), Adriano Gualandi, (Caltech, Pasadena, USA, currently a visiting researcher in the Gaspard Monge Program at the Solid Mechanics Laboratory (LMS - École Polytechnique/CNRS)) and Jean-Philippe Avouac (Caltech, Pasadena, USA, currently a visiting professor in the Gaspard Monge Program at the Solid Mechanics Laboratory (LMS - École Polytechnique/CNRS)): "In this study, we show that the technique that we have developed allow detecting and imaging "slow slip" events even of very low amplitude. We have therefore been able to highlight that, contrary to previous speculation, they follow the same scaling laws as regular earthquakes and are therefore good counterparts," explains Jean-Philippe Avouac.
This discovery is based on open-access data from 352 GPS stations located along the Cascadia Subduction Zone in the states of Washington and Oregon in the northwest region of the United States. From the slow travel recorded from these stations, it was possible to follow the slow slippage events for over a decade. More than 40 events were documented in detail and were found to be very similar to regular earthquakes, albeit much slower. They follow in particular the same scaling laws.
This similarity opens up interesting prospects for studying earthquakes as "slow slip" events occur much more frequently than large earthquakes. This will allow us to test different hypotheses on the mechanisms of earthquake occurrence as well as to test strategies for alerting us of the imminent occurrence of a major seismic event.
Reference : « Similar scaling laws for earthquakes and Cascadia slow-slip events ». Sylvain Michel, Adriano Gualandi, Jean-Philippe Avouac. Nature (2019)