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Identifying past earthquakes on an active normal fault (Magnola, Italy) from the chemical analysis of its exhumed carbonate fault plane

Abstract : A normal fault scarp exhumed by repeated strong earthquakes is made of a series of rupture zones that were exposed, thus weathered, over significantly different time spans. We show that such differential weathering can be detected in the chemical content of the fault scarp rocks, and its signature used as a base to decipher the past earthquake history of the fault. We focus on the Magnola normal fault, Central Italy, whose Holocene seismic slip history has already been determined by Palumbo et al. (ESPL, 225, 163–176, 2004) from in situ 36Cl cosmic ray exposure dating of the fault limestone scarp surface. Five major earthquakes were found to have occurred over the last 12 ka, with slips of 1.5–3 m and recurrence times of 0.7–3.1 ka. We analyze the major and trace element concentrations of 15 carbonate samples collected from base to top of the 10 m-high Magnola Holocene scarp, next to the previous sampling done by Palumbo et al. [Palumbo, L., Benedetti, L., Bourlès, D., Cinque, A., Finkel, R., 2004. Slip history of the Magnola fault (Apennines, Central Italy) from 36Cl surface exposure dating: evidence for strong earthquake over Holocene. Earth Planet. Sci. Lett. 225, 163–176.]. We find that most element concentrations decrease upscarp at a rate averaging 5%/m. This decrease is attributed to leaching and re-precipitation of purer calcite that increase with exposure time. Superimposed to the overall leaching, concentration peaks are found at the transition zones separating the earthquake ruptures. These concentration peaks likely result from enrichment of the scarp sections that remained stuck in the 30–50 cm-thick impurity-rich upper soil during the quiescence periods that separated the earthquakes. Because the rare earth elements (REE) are among those most significantly enriched at the earthquake transition zones, they are the best chemical markers of past large seismic events. We finally propose a first-order model that reproduces adequately the observations. Our preliminary work thus suggests that geochemical analyses of preserved, seismically exhumed limestone fault scarp rocks may help deciphering the seismic slip history of a fault.
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Submitted on : Tuesday, October 21, 2008 - 12:27:23 PM
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J. Carcaillet, I. Manighetti, C. Chauvel, A. Schlagenhauf, J. Nicole. Identifying past earthquakes on an active normal fault (Magnola, Italy) from the chemical analysis of its exhumed carbonate fault plane. Earth and Planetary Science Letters, Elsevier, 2008, 271 (1-4), pp.145 à 158. ⟨10.1016/j.epsl.2008.03.059⟩. ⟨insu-00332636⟩



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