LAKES AS PALEOSEISMIC RECORDS IN THE SEISMICALLY-ACTIVE RIETI PLAIN, CENTRAL ITALY

Small lakes in low relief areas are atypical candidates for studies of paleoseismic records, but can contain seismically induced event layers (seismites) generated through strong ground shaking, sediment transport, hydrological reorganization and/or changes in ground­water chemistry and flow. Lakes Lungo and Ripasottile are shallow lakes (<10 m deep) located in the tectonically active Rieti Basin in the central Apennines, Italy, where large-magnitude earthquakes regularly occur. Sediment cores from these lakes provide paleoseismic indicators for the past ~1000 years. The sedimentological and geochemical features formed by these processes were identified by x-ray fluorescence (XRF) core-scanning, X-ray diffraction (XRD), smear slide analysis, stable isotopic analysis (δ¹³C of organic matter) and bulk geochemical analysis (organic matter percent, calcium carbonate percent, organic carbon percent, total nitrogen, total sulfur). Four event layers were identified in both lakes and correspond with documented large-scale earthquakes in 1298, 1349, 1639, and 1703 CE. The common physical structure was a homogenous bed (homogenite) of re-suspended sediment consisting of a denser base, high magnetic susceptibility (MS) and higher concentration of siliciclastic grains. Organic matter was concentrated towards the top of these beds. Chemical signatures, including peaks in sulfur, strontium, and barium may represent abrupt shifts to a groundwater-dominated system, transient groundwater chemistry changes communicated to the lakes, or changes in groundwater flow and/or spring discharge. Excursions in d¹³C_org may represent disruptions or changes in carbon source. Not all event layers had the same features, a result attributed to differences seismic processes as well as the lake attributes, and anthropogenic modification. The methods may be applied similarly to other low relief lakes in seismically active areas.