Constraining particle size-dependent plume sedimentation from the 17 June 1996 eruption of Ruapehu Volcano, New Zealand, using geophysical inversions
Constraining particle size-dependent plume sedimentation from the 17 June 1996 eruption of Ruapehu Volcano, New Zealand, using geophysical inversions
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DOI：
10.1002/2013jb010387
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发表时间：
2014-03-01
影响因子：
3.9
通讯作者：
Rosenberg, M. D.
中科院分区：
文献类型：
--
作者：
Klawonn, M.;Frazer, L. N.;Rosenberg, M. D.
弱亚旋-旋羽流经常对人口和航空造成危害，但迄今为止，这些载有颗粒的羽流的许多关键参数都没有受到很好的约束。通过反演火山灰沉降物的扩散过程，本研究恢复了沿着约束良好的弱羽流轨迹的颗粒大小相关的质量分布。我们使用的例子，1996年6月17日在新西兰的Ruapehu喷发，并根据我们的计算质量每单位面积火山灰测量和粒度分布在118个样本位置。颗粒下降时间和采样收集时间的比较，以及在喷发期间的观测结果表明，小于250 μ m的颗粒可能作为聚集体沉降。为了简单起见，我们假设所有这些细颗粒都是以恒定尺寸和密度的聚集体落下的，而我们假设大颗粒是以终速落下的单个颗粒。大颗粒（d >= 250 μ m）和细颗粒（d < 250 μ m）沿羽流轨迹的质量沉降沿着呈现明显的趋势，这可能是由于两种不同的沉降行为（聚集沉降与单颗粒沉降）造成的。此外，我们计算得到的颗粒大小分布在弱羽沿着其轴，并发现粒子模式从最初的1 φ模式转移到2.5 φ模式10公里的喷口，并占主导地位的2.5至3 φ模式10-180公里的喷口，在那里的羽流到达海岸线，我们没有进一步的字段限制。计算的羽流内的粒子分布提供了新的约束条件的质量输运过程中弱羽流和改进以前的模型。单颗粒沉降和聚集体沉降之间的明显衰减趋势可以作为一种新的工具，以确定其他喷发的聚集体的颗粒大小。
Weak subplinian-plinian plumes pose frequent hazards to populations and aviation, yet many key parameters of these particle-laden plumes are, to date, poorly constrained. This study recovers the particle size-dependent mass distribution along the trajectory of a well-constrained weak plume by inverting the dispersion process of tephra fallout. We use the example of the 17 June 1996 Ruapehu eruption in New Zealand and base our computations on mass per unit area tephra measurements and grain size distributions at 118 sample locations. Comparisons of particle fall times and time of sampling collection, as well as observations during the eruption, reveal that particles smaller than 250 mu m likely settled as aggregates. For simplicity we assume that all of these fine particles fell as aggregates of constant size and density, whereas we assume that large particles fell as individual particles at their terminal velocity. Mass fallout along the plume trajectory follows distinct trends between larger particles (d >= 250 mu m) and the fine population (d < 250 mu m) that are likely due to the two different settling behaviors (aggregate settling versus single-particle settling). In addition, we computed the resulting particle size distribution within the weak plume along its axis and find that the particle mode shifts from an initial 1 phi mode to a 2.5 phi mode 10 km from the vent and is dominated by a 2.5 to 3 phi mode 10-180 km from vent, where the plume reaches the coastline and we do not have further field constraints. The computed particle distributions inside the plume provide new constraints on the mass transport processes within weak plumes and improve previous models. The distinct decay trends between single-particle settling and aggregate settling may serve as a new tool to identify particle sizes that fell as aggregates for other eruptions.