Mercury measurements in lake sediments from cores from Clear Lake, California, reveal that high Hg levels (up to 65 ppm) have been characteristic for the sediments for over 10 ka. A strong Hg anomaly occurs in sediments deposited between 10.5 and 8.5 ka, which we relate to a period of relatively rapid tectonic subsidence of part of Clear Lake. We hypothesize that mercury-rich geothermal fluids rose along the activated fractures and faults and were discharged into the lake, causing the anomalously high Hg content of the sediments and leading to deposition of the Sulphur Bank Hg deposit. The total amount of Hg discharged into Clear Lake over the past 15 ka is estimated to be at least 2400 metric tons. Chemostratigraphy of lake sediments in geothermally active areas may hold promise for the detection and dating of major paleoseismic periods.