Abstract Volcanic debris avalanches are extremely destructive phenomena, with the potential to travel many kilometers from their source region, either as rockslides or as mass flows. Given that they may even be triggered at inactive volcanoes, their hazard is often underestimated. Understanding the dynamics of such mass movements is essential for evaluating and mitigating hazards. A number of case studies have been carried out around the world, but there is still a need for further studies of flow-dominated avalanches, which remain poorly constrained. These studies would have high educational value, providing striking examples to teach decision makers and at-risk populations about the hazard. In this study, we investigate the 7500 cal. year B.P. Lastarria debris avalanche. It is a 7 km-long deposit, with exceptional preservation of both the flow structures and the collapse scar. Detailed fieldwork, morphometric mapping of over 600 surface features, and numerical modelling was carried out to constrain the avalanche's trigger and flow parameters. Numerical models and field scaling relationships are in good agreement, suggesting maximum velocities of 210 to 270 km h−1, negligible basal friction, low cohesion (50 kPa) and an intermediate friction coefficient. Structures are dominantly oriented parallel to transport direction, suggesting minimal influence from a smooth paleotopography. Lastarria provides an example of a shallow flank failure, initiated along stratigraphic planes, that allowed low strength pyroclastic strata to disaggregate rapidly and then quickly accelerate to flow as a granular material at high velocity beyond the base of the volcano. Overall, Lastarria provides excellent constraints on granular avalanche initiation and flow, which are valuable for hazard assessments and for the study of less well-preserved flow deposits elsewhere. The question of which precursory signs may warn of such a flank failure remains open, and is important to address in future studies.

中文翻译：

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中安第斯山脉 Lastarria 碎片雪崩的动力学和物理参数

摘要 火山碎屑雪崩是极具破坏性的现象，有可能以岩崩或质量流的形式从源区传播数公里。鉴于它们甚至可能在不活动的火山上被触发，它们的危害往往被低估。了解此类群众运动的动态对于评估和减轻危害至关重要。世界各地已经进行了许多案例研究，但仍然需要对流动主导的雪崩进行进一步研究，这些雪崩仍然受到很好的约束。这些研究具有很高的教育价值，提供了显着的例子来向决策者和高危人群传授有关危害的知识。在这项研究中，我们调查了 7500 cal。BP Lastarria 碎片雪崩。这是一个 7 公里长的矿床，流动结构和塌陷疤痕都得到了特别的保护。进行了详细的实地工作、600 多个表面特征的形态测量映射和数值建模，以限制雪崩的触发和流动参数。数值模型和场标度关系非常吻合，表明最大速度为 210 到 270 km h-1，基本摩擦可以忽略不计，低内聚力 (50 kPa) 和中等摩擦系数。结构主要平行于运输方向，表明光滑古地形的影响最小。Lastarria 提供了一个沿地层平面开始的浅侧翼破坏的例子，这使得低强度的火山碎屑岩地层迅速分解，然后迅速加速以粒状物质的形式高速流动到火山底部之外。总体而言，Lastarria 对颗粒雪崩的发生和流动提供了极好的限制，这对于危险评估和其他地方保存较差的流动沉积物的研究很有价值。哪些先兆迹象可能会警告这种侧翼故障的问题仍然悬而未决，并且在未来的研究中很重要。