Systematically studying the active faults that released strong earthquakes in the past is the new challenge for seismic hazard assessment. A methodology developed in recent years informs on the slip and number of earthquakes released by a fault. This consists in sampling and analyzing portions of the fault plane in order to determine the rare earth elements (REE) concentrations possibly correlated to seismic cycles and slip history (Carcaillet et al., 2008). Sedimentary limestones are generally REE-poor, while the pedogenic environment (soil) they are in contact with is generally enriched during its development. In the uppermost part of the soil, REE forms organic complexes preventing their leaching and transferring to the deeper part of the soil. Thus, REE+Y enriches the organic richer soil portion (Carcaillet et al., 2008 and references therein). Primary (i.e., limestone) and secondary (i.e., cement) carbonates easily dissolve in the presence of any fluids acidified by dissolved CO2 . Fault planes developing in limestone produce breccias subject to dissolution by rain and circulating (vadose) waters. As explained in detail by Carcaillet et al. (2008), the dissolution of the carbonate along the exposed fault plane produces REE+Y enrichment in the runoff waters. These waters reach the pedogenized colluvial wedge at the base of the fault scarp where REE+Y forms organic complexes and/or are taken up by specific bacteria enriching the topsoil and its fluids. These fluids produce re-precipitation of carbonates on the fault plane. The exchange process stops at the time when the fault plane is exposed due to exhumation (slope erosion processes or surface faulting). We apply this methodology to the Caggiano normal fault (southern Apennines, Italy), cropping out southeast of the Irpinia 1980 earthquake fault (Mw 6.9), which was responsible for both the 1561 and partly the 1857 Basilicata earthquakes (Mw 6.7 and 7.1) (Galli et al., 2006; Bello et al., 2022). We integrate the REE analysis approach with a high-resolution topography analysis along 98 serial topographic profiles to measure vertical separations (Bello et al., 2021) attributable to post Late Glacial Maximum (LGM) faulting. The asymmetric scarp height profiles suggest fault lateral propagation and along-strike variations in the fault evolution. This integrated and multidisciplinary approach highlights the occurrence of seven-to-eleven earthquakes with variable slip between ~40 cm and ~70 cm within post-LGM times. We speculate on the magnitudes of the respective earthquakes obtaining magnitudes to be between 5.5 and 7.0, and most commonly between 6.3 and 6.5. The results suggest a recurrence time between 1.6 and 2.3 ka and a slip rate ranging between 0.6 and 0.9 mm/yr and offer an approach useful to be applied on carbonate fault planes in similar worldwide tectonic.
Studying fault scarps with geochemical and topographic analyzes to understand past earthquakes: an example from the southern Apennines of Italy
Bello S.;Perna M. G.;Consalvo A.;Brozzetti F.;Cirillo D.;Andrenacci C.;Tangari A. C.;Carducci A.;Lavecchia G.;Stoppa F.;
2023-01-01
Abstract
Systematically studying the active faults that released strong earthquakes in the past is the new challenge for seismic hazard assessment. A methodology developed in recent years informs on the slip and number of earthquakes released by a fault. This consists in sampling and analyzing portions of the fault plane in order to determine the rare earth elements (REE) concentrations possibly correlated to seismic cycles and slip history (Carcaillet et al., 2008). Sedimentary limestones are generally REE-poor, while the pedogenic environment (soil) they are in contact with is generally enriched during its development. In the uppermost part of the soil, REE forms organic complexes preventing their leaching and transferring to the deeper part of the soil. Thus, REE+Y enriches the organic richer soil portion (Carcaillet et al., 2008 and references therein). Primary (i.e., limestone) and secondary (i.e., cement) carbonates easily dissolve in the presence of any fluids acidified by dissolved CO2 . Fault planes developing in limestone produce breccias subject to dissolution by rain and circulating (vadose) waters. As explained in detail by Carcaillet et al. (2008), the dissolution of the carbonate along the exposed fault plane produces REE+Y enrichment in the runoff waters. These waters reach the pedogenized colluvial wedge at the base of the fault scarp where REE+Y forms organic complexes and/or are taken up by specific bacteria enriching the topsoil and its fluids. These fluids produce re-precipitation of carbonates on the fault plane. The exchange process stops at the time when the fault plane is exposed due to exhumation (slope erosion processes or surface faulting). We apply this methodology to the Caggiano normal fault (southern Apennines, Italy), cropping out southeast of the Irpinia 1980 earthquake fault (Mw 6.9), which was responsible for both the 1561 and partly the 1857 Basilicata earthquakes (Mw 6.7 and 7.1) (Galli et al., 2006; Bello et al., 2022). We integrate the REE analysis approach with a high-resolution topography analysis along 98 serial topographic profiles to measure vertical separations (Bello et al., 2021) attributable to post Late Glacial Maximum (LGM) faulting. The asymmetric scarp height profiles suggest fault lateral propagation and along-strike variations in the fault evolution. This integrated and multidisciplinary approach highlights the occurrence of seven-to-eleven earthquakes with variable slip between ~40 cm and ~70 cm within post-LGM times. We speculate on the magnitudes of the respective earthquakes obtaining magnitudes to be between 5.5 and 7.0, and most commonly between 6.3 and 6.5. The results suggest a recurrence time between 1.6 and 2.3 ka and a slip rate ranging between 0.6 and 0.9 mm/yr and offer an approach useful to be applied on carbonate fault planes in similar worldwide tectonic.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
