Source: Tectonics
Identifying long-term seismic activity patterns is crucial for understanding how fault systems evolve, as well as for estimating the probability of future earthquakes. But seismic records date back only hundreds of years—1,000 years at the most—not long enough to fully understand any given fault’s history. Furthermore, because faults can experience times of high activity alternating with quiet periods lasting millennia, seismic forecasts extrapolating from short time spans may greatly over- or underestimate a fault’s rate of activity.
One approach for studying longer-term seismic activity on a fault, chlorine-36 (36Cl) cosmogenic dating, is used to recover histories that can span more than 10,000 years. As slip along a fault progressively exposes rocks, cosmic radiation interacts with carbonate rocks on the fault surface to form atoms of 36Cl, an isotope of chlorine. Concentrations of the isotope reveal approximately how long different rocks have been exposed, a proxy for when earthquakes happened.
Sgambato et al. used 36Cl cosmogenic dating to assess seismic activity over millennia on three faults in Italy’s southern Apennines, where some of the country’s strongest earthquakes have occurred. They then compared the data with other paleoseismic estimates derived from excavating trenches along a fault and tracing markers to measure its displacement. The researchers also calculated slip rates and related annual earthquake probabilities.
The authors found that all three faults experienced periods of both high seismic activity and dormancy in the past 30,000 years and that estimates of earthquake activity from trenching generally agreed with those derived from 36Cl dating. They noted that their results may help show whether these faults are connected to others in the region.
Their research further indicates that slip on a single fault can account for all the regional extension in a given year. This may indicate that strain can be localized to individual faults at certain times. Because their work uncovered a longer record of the clustering of earthquake activity along these faults, it also has implications for seismic hazard forecasting. (Tectonics, https://doi.org/10.1029/2024TC008529, 2025)
—Nathaniel Scharping (@nathanielscharp), Science Writer
