Researchers from the Peruvian Geophysics Institute, IRD and the University of Nice Sophia Antipolis have discovered a new type of slow earthquake off the coast of Peru and, in Nature Geosciences, put forward a tectonic stress relaxation model to enable better estimation of the seismic hazard.
It is well known that the stresses caused in subduction zones cause earthquakes that can sometimes be powerful, such as the Chile earthquake in 2010.
Until recently, researchers thought that active faults followed seismic cycles with periods where stresses accumulated resulting in an earthquake when the stress was released, before the forces built up again in the fault and the cycle started again as the plates moved. About fifteen years ago, satellite methods used to measure deformations in the earth’s crust highlighted a different form of plate relaxation where no significant seismic activity occurs, known as aseismic slip transients. Until now, two types of aseismic slip had been described: slow slip during the inter-seismic phase and post-seismic slip after major earthquakes.
The team of geophysicists specialised in the Andes have now discovered a new mechanism in northern Peru. In this area, subduction of the Nazca oceanic plate under the South American continent, at a rate of approximately 6 cm/year, is mainly an aseismic movement. The phenomenon observed by the researchers comprises a simultaneous sequence of moderate seismic activity (magnitude 5.8 – 6.0) and slow slips. The slip is formed and develops immediately after two moderate earthquakes of 6.0 and 5.8 magnitude respectively. This phenomenon is also different from the aseismic transient processes observed because the size of the slip is not connected to the scale of the earthquake that causes it. In fact, a 5.8 magnitude earthquake can cause a larger slip than a 6.6 magnitude equivalent. This is the first time that a mixed seismic/aseismic process has been observed, with the two forms of slip appearing to interact during the sequence.
The process was brought to light using geodesic measurements and an original seismological analysis. It illustrates a new form of stress relaxation in subduction zones.
Better knowledge of the friction properties along the main active faults and of the processes via which the accumulated stresses are released is required to develop predictive models. These models will allow for better seismic risk management.