UGA international

Many aspects of the deformation processes that precede destructive earthquakes still remain a mystery. Although laboratory experiments predict that seismic failure is usually preceded by a slow phase of deterioration, these assumptions have yet to be confirmed by convincing observations on natural faults. A number of other fundamental questions are also raised, such as whether these potential precursors could provide an indication of the scale of the earthquake to come.

One of the difficulties researchers face is that it is impossible to directly scan the middle of active faults lying several kilometers underground. The ERC FaultScan project (2019-2024) headed by Florent Brenguier (ISTerre / OSUG - UGA / CNRS / USMB / IRD / IFSTTAR) proposes to develop new approaches to scan the insides of natural fault zones solely by listening to seismic noise and rolling out dense networks of seismic sensors.

An initial study based on an analysis of preliminary data acquired in 2018 in California shows that urban noise, and in particular road and rail traffic noise, usually considered an environmental nuisance, can be turned into a useful signal for scanning seismic faults using ultrasound. Specifically, this study uses the noise made by giant freight trains in South California, the passing of which generates seismic vibrations equivalent to a magnitude 2 earthquake.

The study shows that this apparently random source of seismic signals can be transformed into an impulsive virtual seismic source generating waves that scan the San Andreas fault at depths of several kilometers. This new approach can be used to monitor most of the San Andreas fault system by harnessing California’s rail and road networks.