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.

弧形火山既爆炸性地爆发，又有味道，尽管爆发的危险不大，但它们更容易研究。 此外，两种活动风格都共有许多重要的物理和化学过程。 随着岩浆升高，压力的降低会导致挥发性成分的实现，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，在对这些过程进行建模方面取得了长足的进步，但令人惊讶的是，几乎没有引起人们对在火山导管边界上耦合所得段落的压力和周围弹性介质中的压力和变形的关注。 同时，常用的火山变形模型仍然高度理想化，在爆发火山的情况下，这些理想化尤其不足。 拟议的研究的目的是更充分地了解激进的圆顶建造爆发的驱动力以及相关的变形和地震性，特别强调了目前在圣海伦斯山的喷发。 提出的工作涉及耦合的岩浆流量和变形模型的发展，以及在圣海伦斯山的近部和更广泛的规模变形和更广泛的尺度变形和地震数据的分析，以便更好地限制这些模型的近期且在很大程度上出乎意料的恢复。山圣海伦斯山表明，在我们对驱动火山爆发的过程中的理解中表明了严重的限制。 正在进行的喷发提出了许多一阶问题，包括：圣海伦斯如何开始爆发如此少的前期活动？ 与当前喷发相关的地震性仅限于上公里，但岩浆显然从中壳中升起。 相比之下，前几十年的地震群从2 km深度延伸。 较早的群与岩浆运输有关吗？ 如果没有，其他哪些过程可以解释地震性？ 什么过程控制了圣海伦斯山挤出圆顶近场的戏剧性瞬态倾斜信号？ 他们对圆顶挤出机制有哪些限制？ 爆发PBO发作和USGS急于在火山上部署GPS仪器。 如果我们要利用改进的监视数据，那么我们必须开始考虑更现实的变形源，这些变形源考虑了岩浆运动的物理特性，从深度及其对周围介质的影响。我们建议在岩浆室和孔子隔间和孔子系统中既有物理化学过程又有有限元方法和有限元方法（FEM）模型，并进行表面上的良性和Secormations和Sesiss降低。 将耦合室/导管模型的预测与观察到的时间依赖性变形和向上的通量进行比较，以更好地约束参数，例如圣海伦斯山的岩浆腔室体积和充值速率。 将研究各种模型的群地震发生，并将基于Dieterich的地震率理论[Dieterich，Jour。地板。 Res，1994]。 一个有前途的模型涉及由于结晶驱动的气体脱离而导致的应力增加，并且被气体逃逸周期中断了加压。 将分析戏剧性的近距离倾斜周期，以限制这些瞬时变形的来源。 最后，我们将检查热模型，以测试以下假设：2004年第一次爆发的熔岩是1980年代形成圆顶爆发的残留岩浆。