Modeling recent behavior of Mt. St. Helens: extrusion dynamics, deformation, and seismicity

对圣海伦斯山的近期行为进行建模：挤压动力学、变形和地震活动

• 批准号: 0910708
• 负责人: Paul Segall
• 金额: $ 14.46万
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-09-01 至 2011-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0910708&HistoricalAwards=false
• 关键词: Modeling; recent; behavior; Mt.; St.

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Non-technical Explanation: It is generally believed that with adequate monitoring it is possible to detect premonitory signals prior to volcanic eruptions. Eruptions are often preceded by swarms of earthquakes and bulging of the volcano as magma rises from the earth?s mantle into the crust. However, the length of the period of` ?unrest? can vary greatly, and eruptions can be either explosive or passive (effusive). Major goals of volcano science are to provide more accurate forecasts of future behavior based on physical and chemical models of the eruptive process combined with seismic and geodetic monitoring. The 2004-08 eruption of Mount St. Helens provides a unique opportunity to develop such methods. This episode erupted magma with essentially the same chemical composition as the devastating 1980 eruption; however this eruption was very gas poor and thus non-explosive. The 2004 onset was preceded by only a few days of seismic activity and no detectable ground deformation. We will develop rigorous physical and chemical based models of the eruption and test these against observed seismic and Global Positioning System (GPS) measurements of ground deformation.Technical Description: We will develop physically-based models of magma chambers and conduits that are coupled to the elastic surroundings. As magma ascends the decrease in pressure results in volatile exsolution. This decreases magma density, but increases viscosity and compressibility. Exsolution also promotes microlite crystallization; at roughly 1 km depth at MSH the magma becomes essentially a solid plug, the upward motion of which is resisted by frictional sliding on its margins. Changes in chamber pressure as well as shear and normal tractions on the conduit walls are used to predict surface deformation, which can be compared to GPS data. Markov Chain Monte Carlo (MCMC) inversions will be used to determine posterior probability distributions for magma chamber depth, shape, volume, initial overpressure, and recharge, given the GPS data and estimates of extrusion volume. We will model the cessation of the eruption in January of 2008, and surface deformation data since that time, to better constrain the rate of recharge into the crustal magma chamber. We will determine whether rate and state dependent friction effects on the boundary of the shallow magma plug can explain both the onset of the eruption ? increasing pressure overcomes frictional resistance which then weakens with sliding ? and also the rapid early deflation observed at the one continuous GPS site operating at the eruption onset.We will also test possible explanations of cyclic ground tilt observed in the crater of Mount St. Helens. Preliminary analysis suggests that the tilts may be due to shear on the margin of the plug near the bend in the conduit, where ascending magma is redirected to the south beneath the 1980?s lava dome before extruding onto the surface. We suggest that careful analysis of accurately located earthquakes associated with the tilts will provide important clues to the processes controlling both seismogenesis and extrusion. For example, variations in the depths of shallow earthquakes during tilt events might point toward migrating slip on the margin of the plug.

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。非技术性解释：一般认为，通过适当的监测，可以在火山爆发前检测到前兆信号。 火山爆发之前，经常会有成群的地震和岩浆从地面上升起时火山的膨胀。的地幔进入地壳。 然而，"期间“的长度？动乱？火山喷发的形式可以是爆炸性的，也可以是被动的（喷发）。 火山科学的主要目标是根据喷发过程的物理和化学模型，结合地震和大地测量监测，对未来的行为提供更准确的预测。 2004- 2008年圣海伦火山的爆发为发展这种方法提供了一个独特的机会。 这一幕喷发的岩浆与1980年毁灭性的喷发基本上具有相同的化学成分;然而，这次喷发非常缺乏气体，因此不具有爆炸性。 2004年爆发之前只有几天的地震活动，没有可检测到的地面变形。 我们将开发严格的基于物理和化学的喷发模型，并根据观测到的地震和全球定位系统（GPS）测量的地面变形来测试这些模型。技术说明：我们将开发基于物理的岩浆房和管道模型，这些模型与弹性环境相耦合。 当岩浆上升时，压力降低导致挥发性出溶。 这降低了岩浆的密度，但增加了粘度和可压缩性。 出溶作用也促进微晶结晶;在MSH大约1公里深处，岩浆基本上变成了一个固体塞，其向上运动受到其边缘摩擦滑动的阻力。 腔室压力以及管道壁上的剪切和法向牵引力的变化用于预测表面变形，这可以与GPS数据进行比较。马尔可夫链蒙特卡罗（MCMC）反演将用于确定岩浆房深度，形状，体积，初始超压和补给的后验概率分布，给定的GPS数据和挤出量的估计。 我们将对2008年1月喷发的停止以及自那时以来的地表变形数据进行建模，以更好地限制地壳岩浆房的补给率。 我们将确定是否率和状态依赖摩擦的影响边界上的浅岩浆塞可以解释爆发的开始？增加的压力克服了摩擦阻力，然后随着滑动而减弱？以及在火山爆发开始时连续运行的GPS观测站观测到的快速早期收缩。我们还将测试对圣海伦斯火山口观测到的周期性地面倾斜的可能解释。初步分析表明，倾斜可能是由于剪切附近的弯曲在管道，上升的岩浆被重新定向到南方的1980？火山喷发前的熔岩圆顶。 我们认为，仔细分析与倾斜相关的准确定位的地震将提供重要的线索，控制地震成因和挤出的过程。 例如，倾斜事件期间浅地震深度的变化可能表明塞子边缘的迁移滑动。