Deformation and Seismicity Accompanying Effusive Silicic Eruptions

弧形火山的喷发既有爆炸性的，也有热情洋溢的，虽然喷发的危险性较小，但更易于研究。此外，许多重要的物理和化学过程对这两种类型的活动都是共同的。当岩浆上升时，压力的降低导致挥发性成分的释放，这具有增加熔体粘度和促进晶体生长的双重作用。在过去的十年里，在模拟这些过程方面已经取得了相当大的进展，但令人惊讶的是，很少有人注意到火山导管边界上产生的牵引力与周围弹性介质中的应力和变形之间的耦合。与此同时，常用的火山变形模型仍然高度理想化，而这些理想化在火山喷发的情况下尤其不足。拟议研究的目标是更充分地了解热情洋溢的穹顶建筑喷发的驱动力及其相关的变形和地震活动，特别是重点是圣海伦斯火山目前的喷发。拟议的工作包括开发耦合的岩浆流动和变形模型，以及分析圣海伦斯火山近管道和更广泛尺度的形变和地震数据，以便更好地约束这些模型。最近圣海伦斯火山的重新唤醒表明，我们对驱动火山喷发的过程的理解存在严重限制。正在进行的喷发提出了一些一级问题，包括：圣海伦火山是如何开始喷发的，而前兆活动如此之少？与这次喷发相关的地震活动仅限于地表以上几公里，但岩浆显然是从中地壳上升的。相比之下，在过去的几十年里，震群的深度从2公里延伸到9公里。早期的岩群与岩浆运输有关吗？如果不是，还有什么其他过程可以解释地震活动呢？是什么过程控制着山顶挤压穹顶近场剧烈的瞬时倾斜信号。圣海伦斯？他们对穹顶挤压的力学施加了什么限制？火山喷发后，PBO和美国地质调查局匆忙在火山上部署了GPS仪器。如果我们要利用改进的监测数据，那么我们就必须开始考虑更现实的形变源，考虑从深处的岩浆运动的物理性质及其对周围介质的影响。我们建议开发准解析和有限元模型，将岩浆室和管道系统中的物理化学过程与地表变形和地震活动联系起来。耦合洞室/导管模型的预测将与观测到的随时间变化的形变和喷流通量进行比较，以更好地约束参数，如岩浆室体积和山上的补给率。圣海伦斯。不同的震群孕育模型将被调查并与基于Dieterich的地震活动率理论的观测结果进行比较[Dieterich，Jour.吉普斯。Res，1994]。一种有希望的模型涉及结晶驱动的气体出溶造成的应力循环增加，以及因气体泄漏周期而中断的加压。将分析剧烈的近喷口倾斜周期，以限制这些瞬时变形的来源。最后，我们将检验热模型，以检验2004年第一次喷发的熔岩是1980年代S穹顶喷发的残余岩浆的假设。

Arc volcanoes erupt both explosively and effusively, and while effusive eruptions are less hazardous they are more amenable to study. Furthermore, many important physical and chemical processes are common to both styles of activity. As magma ascends the decrease in pressure results in exsolution of volatile constituents, which has the dual effect of increasing melt viscosity and promoting crystal growth. Considerable progress has been made in the past decade in modeling these processes, yet surprisingly little attention has been given to coupling the resultant tractions on the boundary of volcanic conduits to stress and deformation in the surrounding elastic medium. At the same time commonly used volcano deformation models remain highly idealized, and these idealizations are particularly inadequate in the case of erupting volcanoes. The goal of the proposed research is to more fully understand the driving forces and associated deformation and seismicity of effusive dome-building eruptions, with particular emphasis on the current eruption at Mount St. Helens. The proposed work involves the development of coupled magma flow and deformation models, and the analysis of both near-conduit and broader scale deformation and seismic data at Mount St. Helens in order to better constrain these models.The recent and largely unexpected reawakening of Mount St. Helens demonstrates serious limitations in our understanding of the processes that drive volcanic eruptions. The ongoing eruption has raised a number of first-order questions including: How did St. Helens begin erupting with so little precursory activity? Seismicity associated with the current eruption is limited to the upper few kilometers, yet magma is clearly rising from the mid-crust. In contrast, seismic swarms in preceding decades extended from 2 to 9 km depth. Were the earlier swarms associated with magma transport? If not, what other processes could explain the seismicity? What processes control the dramatic transient tilt signals in the near field of the extruding dome at Mt. St. Helens? What constraints do they place on the mechanics of dome extrusion? Following the onset of the eruption PBO and the USGS rushed to deploy GPS instruments on the volcano. If we are to take advantage of improved monitoring data then it is imperative that we begin to consider more realistic deformation sources which take into account the physical properties of magma movement from depth and its effect on the surrounding medium.We propose to develop both quasi-analytic and Finite Element Method (FEM) models that relate physical-chemical processes in the magma chamber and conduit system to surface deformation and seismicity. Predictions of the coupled chamber/conduit models will be compared to observed time-dependent deformation and effusive flux to better constrain parameters such as magma chamber volume and recharge rate at Mt. St. Helens. Various models of swarm seismogenesis will be investigated and compared to observations based on Dieterich's seismicity rate theory [Dieterich, Jour. Geopys. Res, 1994]. One promising model involves cyclic increase in stress due to crystallization-driven gas exsolution and pressurization interrupted by periods of gas escape. Dramatic near vent tilt cycles will be analyzed to constrain the source of these transient deformations. Finally, we will examine thermal models to test the hypothesis that the first erupted 2004 lavas were residual magmas from the 1980's dome-forming eruptions.