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《美国回收与再投资法》（公法111-5）进行的。没有技术解释：通常认为，通过适当的监测，可以在火山喷发之前检测预先信号。 爆发通常是在岩浆从地上升入地壳的地震之前，火山群群和火山的隆起。 但是，“动乱”期间的长度？可以变化很大，喷发可以是爆炸性的，也可以被动（富）。 火山科学的主要目标是基于喷发过程的物理和化学模型以及地震和大地测量监测的物理和化学模型提供更准确的未来行为预测。 2004 - 08年的圣海伦斯山（Mount St. Helens）喷发为开发这种方法提供了独特的机会。 这一事件爆发了岩浆，其化学成分与1980年的毁灭性爆发基本相同。但是，这种喷发非常贫穷，因此非爆炸性。 在2004年发作之前，仅几天的地震活性，没有可检测的地面变形。 我们将开发出严格的基于物理和化学的喷发模型，并针对地面变形的观察到的地震和全球定位系统（GPS）测量。技术描述：我们将开发基于物理的岩浆室和导管的模型，这些模型与弹性环境耦合。 随着岩浆的上升，压力的降低会导致挥发性降低。 这会降低岩浆密度，但增加了粘度和可压缩性。 升级还促进了微矿石结晶。在MSH处的大约1公里处，岩浆本质上成为一个实心塞，其向上运动在其边缘上的摩擦滑动所抵抗。 腔室压力以及导管壁上的剪切和正常牵引的变化用于预测表面变形，可以将其与GPS数据进行比较。 Markov Chain Monte Carlo（MCMC）倒置将用于确定岩浆腔深，形状，体积，初始超压和充电的后验概率分布，鉴于挤出量的GPS数据和估计值。 我们将模拟2008年1月的爆发的停止，并自那时以来表面变形数据，以更好地限制充电速率到地壳岩浆腔中。 我们将确定速率和状态依赖性摩擦对浅岩浆塞边界的影响是否可以解释喷发的发作？增加压力克服了摩擦电阻，然后通过滑动削弱？而且，在喷发开始时运行的一个连续的GPS部位观察到的快速早缩。我们还将测试在圣海伦斯山山口观察到的循环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道附近的塞子缘剪切，在1980年的熔岩圆顶下方，将岩浆重定向到南部，然后再挤入表面。 我们建议对与倾斜相关的准确分析地震进行仔细的分析，将为控制地震发生和挤出的过程提供重要的线索。 例如，倾斜事件期间浅层地震深度的变化可能指向插头边缘的滑移。