Japan: Reconstructing damage in the crust with seismic noise
A team of researchers from the Seismology Group of the Department of Physics "Ettore Pancini," consisting of Dr. Titouan Muzellec, Dr. Grazia De Landro and Professor Aldo Zollo, has reconstructed in detail how the Earth's crust changed after the 6.2-magnitude earthquake that struck the Nagano region of Japan in November 2014.
The study, published in Nature Communications Earth & Environment, shows how the elastic properties of the first kilometers of crust were measurably weakened following the seismic rupture.
The research uses ambient noise interferometry, a technique that exploits natural vibrations generated by the oceans, atmosphere and human activities. Analyzing three months of continuous recordings from 27 seismic stations, the scholars identified a reduction in seismic wave velocity of between 0.5 percent and 1.3 percent after the main shock. This change, while minute, is a direct indicator of rock weakening.
By combining information from hundreds of pairs of stations, the team was able to locate these variations in the crust, reconstructing their spatial distribution. The damage is found to be concentrated in the first two kilometers of depth, near the nucleation zone and areas of maximum slip, i.e., where the fault has experienced the greatest displacements. The correspondence with the areas of most intense shaking recorded by the Japanese accelerometer network reinforces the interpretation of widespread weakening produced by both the main rupture and the strong shaking.
Understanding where and to what extent the crust is damaged is critical not only for monitoring the occurrence of subsequent earthquakes, but also for assessing how the distribution of stresses evolves after a major seismic event. This knowledge helps to more accurately define hazard models and improve the tools with which risk is estimated in active areas.
The study demonstrates the potential of dense seismic networks and advanced processing techniques to transform the Earth's background noise into a high-resolution imaging tool. Without waiting for new earthquakes, it is possible to follow the evolution of the elastic properties of the crust day by day, obtaining valuable information on the mechanical response of fault zones.
These results pave the way for increasingly extensive use of continuous seismic monitoring to identify areas of persistent weakness or post-earthquake recovery processes, contributing to a deeper understanding of how the Earth's crust fractures and heals over time.