The Apennine Mountains dominate the Italian peninsula, spanning 1,200 kilometers and reaching peaks as high as 2,912 meters. In the northern part of the range lies the Altotiberina fault, and in a recent study spanning 4.5 years, scientists from Italy’s Istituto Nazionale di Geofisica e Vulcanologia (INGV) created a detailed catalog of the seismic activity in the region that is giving them the best look yet at the fault’s behavior and seismic potential.
The Altotiberina fault is a normal fault, meaning the two overlapping slabs of Earth’s crust are being pulled apart, with the hanging wall sliding down the face of the footwall. This movement can occur in either abrupt slips or gradual “creeping.” The Altotiberina fault is also categorized as low angle, meaning the angle formed by the fault line with the horizontal plane is small—about 15°–20° in this case. Often, the anatomy of faults like Altotiberina is dominated not by large occasional earthquakes but by consistent clusters of tiny ones, a concept known as microseismicity.
To get a more complete picture of what was going at the fault, Valoroso et al. used a dense network of seismic and geodetic sensors to record data on the crust’s movement in the region from 2010 to 2014 at depths between 4 and 16 kilometers. The networks belong to The Altotiberina Near Fault Observatory, a modern multidisciplinary research and monitoring infrastructure managed by the INGV. They detected more than 37,000 quakes with a magnitude less than 3.9, occurring at a very consistent rate of approximately 2.2 events per day. The enormous trove of data provided the most detailed record to date of how the fault is evolving over time and in space.
In particular, the authors found 97 clusters of small repeating earthquakes. These miniquakes tended to occur in pairs, and the time interval between them seems to predict the rate at which the hanging wall slides down the footwall. Using these results, the authors suggest that this consistent creeping may drive the behavior of the fault at large, which has previously been calculated to be slipping at a rate of 1.7 millimeters per year. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014607, 2017)
—David Shultz, Freelance Writer