Table4_Forecasting and communicating the dispersion and fallout of ash during volcanic eruptions: lessons from the September 20, 2020 eruptive pulse at Sangay volcano, Ecuador.XLSX

Volcanic ash is a hazard to human health and activities. Accurate and timely forecasts coupled with effective communication about the dispersion and fallout of volcanic ash during explosive events are essential to reduce impacts on local communities and limit economic losses. In this study, we present the first detailed description of an eruptive pulse at Sangay volcano and its eruption source parameters. The eruptive pulse on September 20, 2020, started at about 9:20 UTC and lasted between 90 and 100 min, producing an eruptive column that rapidly separated into (1) a higher (15.2 km above sea level, asl), gas-rich cloud moving east-southeast and (2) a lower (12.2 km asl), ash-rich cloud moving west and causing ash fallout up to 280 km from the volcano. Field data collected immediately after the event allow estimating the volume of bulk tephra to be between 1.5 and 5.0 × 106 m3, corresponding to a volcanic explosivity index of 2. The eruptive pulse, identified as violent Strombolian, emitted andesitic ash that was more mafic than products ejected by Sangay volcano in recent decades. Component analysis and glass chemistry of juvenile particles support the hypothesis that this event excavated deeper into the upper conduit compared to typical Strombolian activity at Sangay volcano, while grain-size analysis allows reconstruction of the total grain-size distribution of the fallout deposit. The discrepancies between the ash fallout simulations performed with the Ash3D online tool and the actual deposit are mainly the result of inaccurate pre- and syn-eruptive configurations and highlight the importance of additional processes such as aggregation. Communication products issued during the event included (1) several standard short reports, (2) volcano observatory notices for aviation, (3) social media posts, and (4) a special report providing the results of the ash fallout simulation. Although communication was effective with the authorities and the connected population, an effort must be made to reach the most vulnerable isolated communities for future events. This study shows that forecasting ash dispersion and fallout during volcanic eruptions can guide early warnings and trigger humanitarian actions, and should become a standard in volcano observatories worldwide.

火山灰对人类健康与生产活动均构成显著威胁。在火山爆发事件中，精准及时的预报结合针对火山灰扩散与沉降的有效信息发布，是减轻对当地社区影响、遏制经济损失的核心举措。本研究首次详细阐述了桑盖（Sangay）火山的一次喷发脉冲事件及其喷发源参数。2020年9月20日的此次喷发脉冲于协调世界时（UTC）约9:20启动，持续时长90至100分钟，形成的喷发柱快速分为两部分：(1) 海拔15.2千米的富气云团向东南偏东方向移动；(2) 海拔12.2千米的富灰云团向西移动，并在距火山最远280千米的区域引发火山灰沉降。事件发生后即刻收集的野外实测数据显示，整体火山碎屑体积介于1.5×10⁶至5.0×10⁶立方米之间，对应火山爆发指数（Volcanic Explosivity Index，VEI）为2级。此次喷发脉冲被判定为强烈斯特龙博利式（Strombolian）喷发，喷出的安山质火山灰比近数十年来桑盖火山喷发的产物更富含铁镁质。对新生碎屑颗粒的组分分析与玻璃化学成分特征佐证了这一假说：相较于桑盖火山典型的斯特龙博利式活动，此次事件对上部岩浆通道的侵蚀深度更深；而粒度分析则可重建沉降沉积物的完整粒度分布。利用Ash3D在线工具开展的火山灰沉降模拟结果与实际沉积层之间存在偏差，这主要源于喷发前与喷发过程中配置参数的不准确，并凸显了诸如颗粒聚集等额外过程的重要性。本次事件期间发布的沟通产品包括：(1) 多份标准简报；(2) 面向航空业的火山观测站通告；(3) 社交媒体帖文；(4) 一份详述火山灰沉降模拟结果的专项报告。尽管与官方部门及触达人群的沟通成效良好，但未来仍需加大工作力度，以覆盖最易受影响的偏远孤立社区。本研究表明，火山喷发期间针对火山灰扩散与沉降的预报工作可指导早期预警并触发人道主义救援行动，该流程应成为全球各火山观测站的标准工作规范。