Slow-Deforming Orogens Revisited: Multi-Decadal Postseismic Transients and Implications for Earthquake Forecasts in Italy

Regions with relatively low tectonic deformation rates, such as the Apennines in Italy, are commonly assumed to exhibit stationary geodetic velocities indicative of purely long-term, plate-tectonic-driven strain accumulation. However, moderate earthquakes (m_w ≥ 5.9) can induce viscoelastic transients lasting multiple decades. These transients can bias strain-rate estimates by superimposing postseismic signals onto the long-term tectonic trend, thereby inflating geodesy-based seismic hazard forecasts.

In this study, we integrate GNSS velocity solutions and stress-orientation data generating strain-rate models for Italy. We then convert the strain-rate field into earthquake-rate forecasts by assuming a Tapered Gutenberg–Richter distribution. To test the stationarity assumption, we compare these forecasts against both (i) an extensively documented earthquake catalog since 1780 (m_w ≥ 5.9) and (ii) a synthetic catalog constructed on mapped seismogenic sources. The correlation between epicenters and forecast “hotspots” is strongest for earthquakes in the last century, whereas older events exhibit systematically weaker alignment. This temporal pattern suggests that recent moderate-to-strong events are still driving postseismic deformation today.

A case study of the 2009 Mw 6.3 L’Aquila earthquake further demonstrates that multi-decadal viscoelastic relaxation can maintain elevated strain rates for at least 30–60 years. Because crustal extension in the Apennines is generally only a few millimeters per year, even a transient signal of 0.3–0.6 mm/yr is enough to skew hazard estimates if interpreted as steady deformation. Consequently, our results call for a refined approach in seismic forecasting—one that rigorously accounts for “ghost transients” before translating geodetic measurements into hazard models.

Overall, our study highlights the need to reconcile short- and medium-term postseismic processes with long-term tectonic loading in slow-deforming regions (Carafa et al., 2024). Incorporating better rheological constraints and denser geodetic networks can help isolate these persistent transients, ultimately leading to more accurate seismic risk assessments and improved mitigation strategies.

 

Carafa, M.M.C., Bird, P., Verdecchia, A., Taroni, M., Doglioni , C. Empirical evidence for multi-decadal transients affecting geodetic velocity fields and derived seismicity forecasts in Italy. Sci Rep 14, 19941 (2024). https://doi.org/10.1038/s41598-024-70816-6