Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals
Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals
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根据超长周期信号波形反演确定的墨西哥波波卡特佩特尔火山瓦尔坎脱气源机制
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发表时间：
2005
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通讯作者：
A. Arciniega‐Ceballos
中科院分区：
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作者：
B. Chouet;P. Dawson;A. Arciniega‐Ceballos
[1]通过最小化嵌入均匀介质中的点源计算数据与合成数据之间的残差，分析了Popocatepetl火山喷发伴随的15-70 s波段甚长周期（VLP）信号的源机制。我们的反演，其中考虑到火山地形的波形的两个爆发（2000年4月23日和5月23日）代表温和的火山活动，以及再现。震源中心位于山顶火山口西周边下方1500 m处，模拟震源由一个浅倾斜裂缝（向东倾斜10°的岩床）和一个陡倾斜裂缝（向东北倾斜83°的岩脉）相交而成，其表面延伸部分将火山口一分为二。两种裂缝经历了相似的充气、放气和再充气的顺序，反映了3-5 min时间间隔内的增压、降压和再增压的循环。最大的力矩释放发生在岩床中，显示出最大体积变化500-1000 m3，压降3-5 MPa，并且恢复压力的幅度等于压降幅度的1.2倍。相比之下，堤坝中的最大体积变化较小（200-300 m3），相应的压降为1-2 MPa，压力恢复等于压降。伴随着这些体积源是单力分量的幅度为108 N，符合熔体平流响应压力瞬变。源的体积分量的源时间历史表明，显著的质量运动开始在窗台内，并在几秒钟内触发在堤防的质量运动响应。这样的源行为是一致的开放的通道，从缓慢加压的岩床驱动岩浆结晶的被压抑的气体逃逸。这条通道的打开和火山气体的快速排出导致了压力下降。压力恢复的岩浆填充岩床是由气体扩散从所产生的过饱和熔体成气泡。假设在40 MPa的环境压力下有一个硬币形裂纹，观察到的压力和体积变化可以用以下属性建模：裂纹半径（100 m）、裂纹孔径（5 m）、气泡数密度（1010-1012 m-3）、初始气泡半径（10 - 6 m）、最终气泡半径（10 - 5 m）和熔体中气体浓度的净减少（0.01重量%）。
[1] The source mechanism of very long period (VLP) signals accompanying volcanic degassing bursts at Popocatepetl is analyzed in the 15–70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two eruptions (23 April and 23 May 2000) representative of mild Vulcanian activity are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500 m below the western perimeter of the summit crater, and the modeled source is composed of a shallow dipping crack (sill with easterly dip of 10°) intersecting a steeply dipping crack (northeast striking dike dipping 83° northwest), whose surface extension bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation, reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3–5 min. The largest moment release occurs in the sill, showing a maximum volume change of 500–1000 m3, pressure drop of 3–5 MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is less (200–300 m3), with a corresponding pressure drop of 1–2 MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources are single-force components with magnitudes of 108 N, consistent with melt advection in response to pressure transients. The source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds. Such source behavior is consistent with the opening of a pathway for escape of pent-up gases from slow pressurization of the sill driven by magma crystallization. The opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40 MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius (100 m), crack aperture (5 m), bubble number density (1010–1012 m−3), initial bubble radius (10−6 m), final bubble radius (∼10−5 m), and net decrease of gas concentration in the melt (0.01 wt %).