Deformation and Seismicity Accompanying Effusive Silicic Eruptions

伴随硅质喷发的变形和地震活动

• 批准号: 0710844
• 负责人: Paul Segall
• 金额: --
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0710844&HistoricalAwards=false
• 关键词: Deformation; Seismicity; Accompanying; Effusive; Silicic

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.

弧形火山的喷发既具有爆炸性又具有喷发性，虽然喷发的危险性较小，但它们更适合研究。 此外，许多重要的物理和化学过程对于这两种活动都是相同的。 随着岩浆上升，压力下降导致挥发性成分溶出，这具有增加熔体粘度和促进晶体生长的双重作用。 在过去的十年中，在模拟这些过程方面取得了相当大的进展，但令人惊讶的是，很少有人关注火山管道边界上产生的牵引力与周围弹性介质中的应力和变形的耦合。 与此同时，常用的火山变形模型仍然高度理想化，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全面地了解喷发圆顶建筑喷发的驱动力以及相关的变形和地震活动，特别强调当前圣海伦斯火山的喷发。 拟议的工作涉及开发耦合岩浆流和变形模型，以及对圣海伦斯山近管道和更广泛尺度的变形和地震数据进行分析，以便更好地约束这些模型。圣海伦斯群岛表明我们对火山喷发过程的理解存在严重局限性。 正在进行的火山喷发引发了许多首要问题，包括：圣海伦斯火山是如何在前期活动如此之少的情况下开始喷发的？ 与当前喷发相关的地震活动仅限于上部几公里，但岩浆明显从地壳中部上升。 相比之下，前几十年的地震群深度从 2 公里延伸到 9 公里。 早期的群体是否与岩浆输送有关？ 如果不是，还有哪些其他过程可以解释地震活动？ 哪些过程控制圣海伦斯山挤压圆顶近场中剧烈的瞬态倾斜信号？ 他们对圆顶挤压的力学有何限制？ 火山喷发后，PBO 和 USGS 迅速在火山上部署 GPS 仪器。 如果我们要利用改进的监测数据，那么我们必须开始考虑更现实的变形源，其中考虑到岩浆从深度运动的物理特性及其对周围介质的影响。我们建议开发两种准将岩浆房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的解析和有限元方法 (FEM) 模型。 耦合室/管道模型的预测将与观测到的随时间变化的变形和喷流通量进行比较，以更好地约束圣海伦斯山岩浆室体积和补给率等参数。 将研究各种群体地震发生模型，并与基于迪特里奇地震活动率理论的观测结果进行比较 [Dieterich, Jour.地质勘探。研究，1994]。 一种有前途的模型涉及由于结晶驱动的气体逸出和气体逃逸周期中断的加压而导致的应力循环增加。 将分析剧烈的近通风口倾斜循环，以限制这些瞬态变形的来源。 最后，我们将检查热模型来检验以下假设：2004 年首次喷发的熔岩是 1980 年圆顶形成喷发的残留岩浆。