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.

该奖项根据 2009 年《美国复苏和再投资法案》（公法 111-5）提供资金。 非技术解释：人们普遍认为，通过充分的监测，可以在火山喷发之前检测到预兆信号。 当岩浆从地幔上升到地壳时，火山喷发之前通常会发生大量地震和火山隆起。 然而，“动乱”的持续时间有多长？变化很大，喷发可以是爆炸性的，也可以是被动的（喷发性的）。 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 8 月圣海伦斯火山的喷发为开发此类方法提供了独特的机会。 此次喷发的岩浆化学成分与 1980 年毁灭性的火山喷发基本相同；然而，这次喷发的气体非常贫乏，因此不会爆炸。 2004 年爆发之前，地震活动仅有几天，并且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）对地面变形的测量来测试这些模型。技术描述：我们将开发与岩浆室和管道耦合的基于物理的模型弹性环境。 随着岩浆上升，压力下降导致挥发性溶出。 这降低了岩浆密度，但增加了粘度和可压缩性。 溶出还促进微晶石结晶；在 MSH 大约 1 公里深度处，岩浆基本上变成了固体塞子，其向上运动受到其边缘的摩擦滑动的阻碍。 腔室压力的变化以及管道壁上的剪切力和法向牵引力用于预测表面变形，这可以与 GPS 数据进行比较。考虑到 GPS 数据和挤压体积的估计，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。 我们将对 2008 年 1 月喷发停止以及此后的地表变形数据进行建模，以更好地限制地壳岩浆室的补给速度。 我们将确定浅层岩浆塞边界上与速率和状态相关的摩擦效应是否可以解释喷发的发生？增加压力克服摩擦阻力，然后摩擦阻力随着滑动而减弱 ?我们还将测试对在圣海伦火山火山口观察到的周期性地面倾斜的可能解释。初步分析表明，倾斜可能是由于管道弯曲附近塞子边缘的剪切造成的，上升的岩浆在挤压到地表之前被重新引导到 1980 年代熔岩穹顶下方的南部。 我们建议，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表明岩塞边缘发生滑移。