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。非技术性解释：人们普遍认为，只要有足够的监测，就有可能在火山喷发前发现前兆信号。火山喷发之前通常会发生成群的地震和火山隆起，因为岩浆从地球上升起--S地幔进入地壳。然而，动乱的时期有多长？可能会有很大的不同，而且皮疹可能是爆炸性的，也可能是被动的(渗出性)。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。2004-08年的圣海伦斯火山喷发为发展这种方法提供了一个独特的机会。这一幕喷发的岩浆化学成分基本上与1980年毁灭性的喷发相同；然而，这次喷发非常缺乏气体，因此不具爆炸性。在2004年地震爆发之前，只有几天的地震活动，没有检测到地面变形。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)地面变形的测量结果对这些模型进行测试。技术描述：我们将开发基于物理的岩浆室和管道模型，这些模型与弹性环境相耦合。当岩浆上升时，压力的降低导致挥发性出溶。这降低了岩浆密度，但增加了粘度和可压缩性。喷出溶液还促进微晶石结晶；在MSH大约1公里深的地方，岩浆实质上成为一个固体塞子，其上升运动受到其边缘摩擦滑动的阻碍。腔体压力的变化以及管壁上的剪切力和法向拉力被用来预测表面变形，可以与GPS数据进行比较。在给定GPS数据和挤压体积估计的情况下，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。我们将模拟2008年1月火山喷发的停止，以及自那时以来的地表形变数据，以更好地限制进入地壳岩浆室的补给速度。我们将确定浅层岩浆塞边界上的速度和状态相关的摩擦效应是否可以解释喷发的开始？增加压力会克服摩擦阻力，然后摩擦阻力会随着滑动而减弱？我们还将测试在圣海伦斯火山口观察到的周期性地面倾斜的可能解释。初步分析认为，这种倾斜可能是由于导管弯曲处附近岩塞边缘的剪切作用所致，在那里上升的岩浆在1980年S熔岩穹顶之下被重新导向南下，然后挤压到地表。我们认为，仔细分析与倾斜相关的准确定位的地震将为控制地震成因和挤压的过程提供重要线索。例如，倾斜事件期间浅层地震深度的变化可能指向岩塞边缘的迁移滑移。

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.