Rare earth elements record past earthquakes on exhumed limestone fault planes

Abstract : Evaluating the timing and size of past earthquakes is a constant challenge in palaeoseismology. Here, we show that rare earth element (REE) concentrations can be used to constrain the number and slips of major past earthquakes. We focus on one of the rare faults worldwide - the Magnola normal fault, Italy - whose Holocene earthquake history is precisely known. We analysed 42 samples from along the 10 m that form the surface of the well-preserved seismically exhumed fault scarp and seven samples extracted from the fault plane section buried in the ground. We show that the first metre of soil contaminates the scarp rocks, producing a marked concentration 'peak' in REE. Similar peaks are recognized on the exhumed scarp, at positions that coincide with the transitions between the known past ruptures. We conclude that the REE peaks locate palaeo-ground levels and are excellent markers for the large earthquakes that exhumed the scarp.