Distinct Lake Sedimentary Imprints of Earthquakes, Floods and Human Activities in the Xiaojiang Fault Zone: Towards a Quantitative Paleoseismograph in the Southeastern Tibetan Plateau

Abstract

Frequent occurrence of large earthquakes in the southeastern Tibetan Plateau (TP) has posed serious threats to social security and ecological environment. Continuously deposited lake sediments that widely distributed in the active and complicated fault zones have been recently showing great potential for paleoseismic reconstruction. However, extreme climatic events and intense human activities may make the seismic signal unrecognizable in lake sediments. In this study, high-resolution analyses of sedimentary structure, grain-size distribution, dry density, magnetic susceptibility, elemental composition, and carbon and nitrogen contents, as well as absolutely radioactive dating were conducted on seven representative sediment cores from the depocenter, nearshore and inlet areas of Yangzong Lake, a typical fault lake in the Xiaojiang Fault zone, southeastern TP. These new data were calibrated by historical documents of earthquakes, floods and human activities, suggesting that seismically induced mass-transport deposits (MTDs, i.e., turbidites) were massive and/or amalgamated (earthquake doublet), became fining and thickening towards the lake center (without changing lake morphology), and occasionally exhibited soft sediment deformation structures (SSDs). An extremely strong earthquake could cause coseismic subsidence of the lake basin and destruct the local hydrological system. In contrast, flood deposits were thinner with horizontal beddings, had higher terrestrial organic matter, and distributed locally in the lake inlet area. Human activities-induced sediments were inversely graded, had horizontal beddings and no erosive base, and exhibited exceptionally high contents of heavy metals. In addition, macroseismic investigations and statistical results from intensity prediction equations (IPEs) provided a conservative threshold of ~ 8 Modified Mercalli Intensities (MMI) for triggering turbidites, and a ~ 10 MMI for inducing coseismic subsidence and hydrological destruction. This study was among the first attempts to establish a quantitative lacustrine paleoseismograph in the southeastern TP, and the results would greatly improve the valid assessment of geohazard risks.

Keywords: Turbidite, carbon sequestration, Hydrological destruction, Lake paleoseismology, Xiaojiang Fault, Anthropocene