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

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。非技术解释：人们普遍认为，通过充分的监测，可以在火山爆发之前探测到前兆信号。火山爆发前往往伴随着地震群，岩浆从地下涌出时火山也会膨胀。S地幔进入地壳。然而，“动荡”时期的长短？可能变化很大，喷发可能是爆炸性的，也可能是被动的（渗出的）。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年至2008年的圣海伦火山喷发为开发这种方法提供了一个独特的机会。这次喷发的岩浆的化学成分与1980年那次毁灭性的喷发基本相同；然而，这次喷发的气体非常少，因此不会爆炸。2004年的地震发生前只有几天的地震活动，没有探测到地面变形。我们将建立严格的物理和化学喷发模型，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。技术描述：我们将开发与弹性环境相耦合的岩浆房和管道的物理模型。当岩浆上升时，压力的降低导致挥发性析出。这降低了岩浆密度，但增加了粘度和可压缩性。脱溶也促进微晶的结晶；在MSH大约1千米深的地方，岩浆基本上变成了一个固体塞，它的向上运动受到边缘摩擦滑动的阻碍。利用腔室压力的变化以及管道壁上的剪切和法向牵引力来预测地表变形，并将其与GPS数据进行比较。根据GPS数据和估算的挤压量，将使用马尔可夫链蒙特卡罗（MCMC）反演来确定岩浆室深度、形状、体积、初始超压和补给的后验概率分布。我们将模拟2008年1月火山喷发停止的情况，以及从那时起的地表变形数据，以便更好地限制地壳岩浆库的补给速率。我们将确定是否速率和状态依赖于摩擦效应的边界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，摩擦阻力随着滑动而减弱。以及在喷发开始时一个连续的GPS站点观测到的快速早期通货紧缩。我们还将测试在圣海伦火山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，倾斜可能是由于导管弯曲处附近的塞子边缘的剪切作用，在那里上升的岩浆被重新定向到1980？在喷发到地表之前，它会形成一个熔岩穹窿。我们认为，仔细分析与倾斜相关的精确定位地震将为控制地震发生和挤压过程提供重要线索。例如，在倾斜事件期间，浅层地震深度的变化可能指向塞子边缘的迁移滑动。

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.