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首页> 外文期刊>Bulletin of Volcanology: Journal of the International Association of Volcanology and Chemistry of the Earth s Interior >GPR investigation of tephra fallout, Cerro Negro volcano, Nicaragua: A method for constraining parameters used in tephra sedimentation models
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GPR investigation of tephra fallout, Cerro Negro volcano, Nicaragua: A method for constraining parameters used in tephra sedimentation models

机译：尼加拉瓜的塞罗·内格罗火山，特非拉沉积物的GPR调查：一种用于限制特非拉沉积模型的参数的方法

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摘要

Most tephra fallout models rely on the advection-diffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection-diffusion equation to tephra sedimentation using data collected on the proximal (350 to ~1,200 m from the vent) to medial (greater than ~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high-resolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection-diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2-3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700-1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of ~500 m ~2 s ~(-1) estimated for the deposits, confirming that the advection-diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to ~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than ~2 km; and (3) a medial zone, in which particles fall from ~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud-a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of ~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.
机译：大多数特非拉尘埃沉降模型都依靠对流扩散方程来预测沉积，进而预测火山灾害。在这里，我们使用从玄武质煤渣锥的近端（距通风口约350至1200 m）到内侧（距通风口约1200 m以上）收集的数据测试了对流扩散方程在特弗拉沉积中的应用，塞罗内格罗火山，位于尼加拉瓜。通过结合内侧区域的矿坑数据和在近泄气口和近侧区域收集的高分辨率地面穿透雷达（GPR）数据，我们对该火山的特发拉沉积过程的了解得到了极大的改善。如果应用对流扩散方程，则单个提夫拉沉积物的厚度应具有高斯侧风剖面和指数变化，且距通风孔的距离较远。在塞罗内格罗（Cerro Negro），稳定的贸易风加上短暂的相对较低能量爆发（VEI 2-3）形成相对简单的沉积物。 GPR数据是在距通风口700-1,600 m处沿三个侧风剖面收集的;样品坑被用来估算距喷口最远13 km的1992 tephra矿床的厚度。在近端GPR剖面中确定的层位显示出高斯分布，具有较高的统计置信度，沉积物的扩散系数估计为〜500 m〜2 s〜（-1），这证实了对流扩散方程能够模拟沉积近端区域。通风口顺风的变薄趋势从圆锥体底部（350 m）到通风口〜1200 m呈指数下降。超过此距离，会在GPR和样品坑数据集中识别出沉积层增厚。合并的数据揭示了三种沉积方式：（1）圆锥体本身附近的通风孔区域，沉积后沉淀物在颗粒流中重新移动; （2）颗粒从不到2 km的高度掉落的近端区域; （3）中间区域，其中颗粒从约4至7 km下降，并且沉积物比基于沉积物近端区域变薄趋势的预期厚。太白云层厚度过大，定义了从近端到中部沉积相的过渡，表明从以羽状边缘的沉降为主的沉积转变为以浮床喷发云的沉降为主的沉积-这是以前大量发现的沉积特征爆发。我们将这种变化解释为代表扩散定律的变化，该变化发生在〜400 s的总粒子下落时间（数值模型的下落时间阈值）上。因此，在塞罗内格罗（Cerro Negro）收集的详细GPR剖面图和矿坑数据有助于验证当前的特弗拉沉积数值模型。

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