Seismic Monitoring and Geomorphic Impacts of the Catastrophic 2018 Baige Landslide Hazard Cascades in the Tibetan Plateau.

Surface hazards can form hazard cascades which expand their reach. However, our understanding of their complex dynamics and ability to mitigate their impacts remain limited. In 2018, two landslides dammed the Jinsha River in the Tibetan plateau and formed landslide‐dammed lakes. Subsequent dam breaches prompted the evacuation of >120,000 people. An early warning system for floods using a regional seismic network has been proposed on the basis of catastrophic floods having been detected ∼100 km away, with seismic energy proportional to discharge. Surprisingly, we find that this catastrophic outburst flood was undetectable beyond a few kilometers, with peak seismic energy preceding peak discharge. We propose that river channel stability also controls seismic energy generation and should be considered for accurate monitoring of catastrophic floods. In contrast, we find that the various processes during dam breach can be well‐characterized seismically further away and provide warning ∼60 min before discharge exceeds monsoon flood levels. We also show that numerical modeling of dam breaches which typically lacks in situ measurements can benefit from incorporating seismic data as constraints. Finally, we show that this event drastically increased sediment fluxes ∼670 km downstream for years and may significantly reduce the capacity of hydropower plants. Our results reveal ways to improve early warning of catastrophic outburst floods and the need to consider surface hazards' long‐term impact when managing infrastructure in mountainous regions. Plain Language Summary: Mountainous regions are prone to surface hazards such as landslides and floods. However, our understanding of how these events evolve and interact is limited. In 2018, two landslides blocked a river in Tibet and created landslide‐dammed lakes. Subsequently, the dam broke and the resulting outburst floods severely damaged downstream infrastructure and led to the evacuation of >120,000 people. In contrast to what previous studies suggest, we find that this catastrophic outburst flood was undetectable beyond a few kilometers, with peak seismic energy preceding peak discharge. We propose that river channel stability also controls seismic energy generation and should be considered for accurate monitoring of catastrophic floods. Conversely, the dam breakage could be detected seismically further away and provide warning ∼60 min before discharge exceeded monsoon flood levels. Finally, we show that this event drastically increased sediment amounts downstream for years and may reduce the capacity of hydropower plants. Our results reveal ways to improve seismic monitoring and early warning of catastrophic floods and the need to consider the long‐term impact of surface hazards when managing downstream infrastructure. Key Points: Catastrophic outburst floods were surprisingly quiet with peak seismic energy preceding peak discharge due to changes in channel stabilityVarious dam breach processes can be well‐characterized seismically, which can constrain numerical modeling and provide earlier warningThese hazard cascades drastically increased sediment fluxes ≥670 km downstream for years and reduced hydropower plant capacity [ABSTRACT FROM AUTHOR]