Potential laharic risks associated with the Millennium Eruption products of Tianchi volcano, NE China

Abstract

Tianchi volcano has experienced continuous eruptions, including the Millennium Eruption (ME) event that produced a volcanic explosivity index of 6–7 in approximately 946 CE, which is among the most hazardous eruptions in human history. We take notice that the voluminous ME pyroclastics provided highly erodible sediments. Post eruption and secondary lahars took place frequently and could travelled up to approximately 500 ​km along the three main rivers that sourced from the Tianchi volcano. Thus, the post eruption or secondary lahar threaten people in this area. To assess the secondary lahar risk in the specified area, a comprehensive analysis is required focusing on several aspects: the triggering mechanisms, interaction between ME pyroclastic materials and the local topography, and the population that could be affected by lahars. This study investigates and reviews the distributions and field data (thickness, structure, and grain size) of ME pyroclastics from different origins (fallout, PDCs, and lahar). The frequent re-depositioin of ME pyroclastic materials, particularly in the form of debris flow, poses significant hazards after ME. However, the mobilization of ME pyroclastic materials and water supply are various in catchments. Secondary lahar hazards can be triggered by various factors, including extreme runoff (flood), failure of temporary block lake, and disturbances in caldera lake. The ME pyroclastic deposits changed topography, which reflecting by drainage density in different drainage basins, and consequently have various responses to rainfall and runoff. Thus, we distinguish different potential mechanisms of secondary lahars in each of the drainage basins. Finally, residential sites scatter on and near to the post ME lahar deposits were plotted on a geological map, the number of population are estimated based on demographic census data, revealing that more than 462,000 residents are threatened by secondary lahar hazards. According to various hazard triggers and road network system, communities in the four drainage basins face complications regarding evacuation during the occurrence of a lahar, especially those located on the border.