Constraints on the Size, Overpressure, and Volatile Content of the Mount St. Helens Magma System from Geodetic and Dome-Growth Measurements During the 2004-2006+ Eruption
Constraints on the Size, Overpressure, and Volatile Content of the Mount St. Helens Magma System from Geodetic and Dome-Growth Measurements During the 2004-2006+ Eruption
复制标题
2004-2006 年火山喷发期间大地测量和穹顶生长测量对圣海伦斯火山岩浆系统的大小、超压和挥发性含量的限制
DOI：
10.3133/pp175022
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发表时间：
2008
期刊：
影响因子：
3.7
通讯作者：
J. E. Quick
中科院分区：
文献类型：
--
作者：
L. Mastin;E. Roeloffs;N. Beeler;J. E. Quick
在华盛顿的圣海伦斯火山持续喷发期间，熔岩以2004年10月的约7-9 m 3 /s的速度连续喷出，到2005年1 - 2月下降到1-2 m 3 /s。根据大地测量数据估计，岩浆库的体积损失为1.6-2.7× 107 m3，仅为2005年底喷发体积7.5× 107 m3的百分之几十。在本文中，我们使用大地模型来约束岩浆库的大小和深度。我们还问是否挤压体积和大地收缩体积之间的关系是一致的排水水库的可压缩岩浆内的线性弹性寄主岩石。最后，我们比较了挤压和大地紧缩与理想化的模型，这样一个水库的时间历程。关键参数包括喷发体积Ve、穹隆密度ρ e、储集层体积VC、储集层初始超压ρ ex 0、喷发压降Δp、储集层压缩系数κ C <$（1/VC）（dVC/ dp）、岩浆密度ρ M和岩浆压缩系数κ M <$（1/ρ M）（dρ M /dp）。地震速度和储层几何形状建议κ C = 2×10 - 11 Pa-1，但力学考虑建议κ C =7-15×10 - 11 Pa - 1。大地测量数据与顶部深5±1 km、底部深~10-20+ km的椭球源最匹配。在没有补给的情况下，岩浆库体积dVC的减少与喷发体积V的关系为Ve/dVC =（ρ M /ρ e）（1+κ M /κ C）。当κ C =~7-15×10 - 11 Pa-1，ρ M = ρ e时，Ve和dV C的估计值表明κ M =1.4-3.0×10 - 10 Pa-1。对应于储层中的岩浆气体含量为Vg =0至1.8体积%。如果我们假设渗出率与储层压力呈线性关系，并且储层的补给率恒定，则渗出率应随时间呈指数下降，直至达到等于补给率的值。这种形式的最佳拟合曲线表明，在喷发的前500天的补给率为1.2-1.3立方米/秒。最佳拟合常数包括乘积V C p ex 0（κ C + κ M），使得可以使用由喷发体积与大地收缩体积的比率约束的κ C和κ M值来约束储层体积。另一方面，如果我们假设压力-渗出率为对数关系，再补给率为常数，则圆顶体积-时间曲线应遵循修正的对数关系，在给定时间的总喷发体积与V C Δp（κ C + κ M）成正比。当κ C =7-15×10 - 11 Pa-1时，测井曲线和指数曲线的结果表明，当Δp或pex 0小于~30 MPa时，储集层体积至少为几立方千米。从考虑（1）岩浆可压缩性，（2）熔岩穹丘抑制渗出的重量和（3）补给率的时间变化的数值计算中获得了类似的结果。这些结果是一致的概念，水库的体积至少是几倍以上的最大的全新世火山喷发的圣海伦斯（4公里3致密岩石当量+体积为3.4万年的YN喷发）。指数和对数模型预测的历史水库减压不完全匹配的位移数据在GPS站JRO 1。例如，这两种模型都没有预测到JRO在喷发的第一个月会发生快速的径向向内运动。这种运动，随后长期的线性紧缩，表明喷发的岩浆已被取代的比例越来越大的补给，但补给率仍然略低于目前（2006年初）的渗出率。
During the ongoing eruption at Mount St. Helens, Washington, lava has extruded continuously at a rate that decreased from ~7-9 m 3 /s in October 2004 to 1-2 m 3 /s by December 2005. The volume loss in the magma reservoir estimated from the geodetic data, 1.6-2.7×10 7 m 3 , is only a few tens of percent of the 7.5×10 7 m 3 volume that had erupted by the end of 2005. In this paper we use geodetic models to constrain the size and depth of the magma reservoir. We also ask whether the relations between extruded volume and geodetic deflation volume are consistent with drainage of a reservoir of compressible magma within a linearly elastic host rock. Finally, we compare the time histories of extrusion and geodetic deflation with idealized models of such a reservoir. Critical parameters include erupted volume V e , dome density ρ e , reservoir volume V C , initial reservoir overpressure ρ ex 0 , pressure drop during the eruption Δp, reservoir compressibility κ C ≡(1/V C )(dV C / dp), magma density ρ M , and magma compressibility κ M ≡ (1/ρ M )(dρ M /dp). Seismic velocity and reservoir geometry suggest κ C ≈2×10 -11 Pa -1 , but mechanical considerations suggest κ C =7-15×10 -11 Pa - 1 . The geodetic data are best fit with an ellipsoidal source whose top is 5±1 km deep and whose base is ~10-20+ km deep. In the absence of recharge, the decrease in magma-reservoir volume dV C is theoretically related to the erupted volume V by V e /dV C =(ρ M /ρ e )(1+κ M /κ C ). For κ C =~7-15×10 -11 Pa -1 and ρ M ≈ρ e , estimates of V e and dV C suggest that κ M =1.4-3.0×10 -10 Pa -1 . corresponding to a magmatic gas content in the reservoir of v g =0 to 1.8 percent by volume. If we assume that effusion rate is linearly related to reservoir pressure and that the recharge rate into the reservoir is constant, the effusion rate should decrease exponentially with time to a value that equals the recharge rate. Best-fit curves of this form suggest recharge rates of 1.2-1.3 m 3 /s over the first 500 days of the eruption. The best-fit constants include the product V C p ex 0 (κ C + κ M ), making it possible to constrain reservoir volume using values of κ C and κ M constrained from ratios of erupted volume to geodetic deflation volume. If, on the other hand, we assume a logarithmic pressure-effusion rate relation and a constant recharge rate, the dome volume-time curve should follow a modified logarithmic relation, with the total erupted volume at a given time proportional to V C Δp (κ C + κ M ). Using κ C =7-15×10 -11 Pa -1 , results from log and exponential curves suggest a reservoir volume of at least several cubic kilometers if Δp or p ex 0 is less than ~30 MPa. Similar results are obtained from numerical calculations that consider temporal changes in (1) magma compressibility, (2) the weight of the lava dome suppressing effusion, and (3) recharge rate. These results are consistent with the notion that the reservoir volume is at least a few times larger than the largest Holocene eruption of Mount St. Helens (4 km 3 dense-rock-equivalent + volume for the 3.4-ka Yn eruption). Both the exponential and logarithmic models predict a history of reservoir decompression that imperfectly matches displacement data at GPS station JRO1. Neither model, for example, predicts the rapid radially inward movement at JRO during the first month of the eruption. Such movement, followed by long-term linear deflation, suggests that erupted magma has been replaced in increasing proportions by recharge, but that the recharge rate remains somewhat less than the current (early 2006) effusion rate.