Subevents of long‐period seismicity: Implications for hydrothermal dynamics during the 2004–2008 eruption of Mount St. Helens
Subevents of long‐period seismicity: Implications for hydrothermal dynamics during the 2004–2008 eruption of Mount St. Helens
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长周期地震活动的子事件：2004-2008 年圣海伦斯火山喷发期间热液动力学的影响
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
B. Chouet
中科院分区：
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作者：
R. Matoza;B. Chouet
[1]2004-2008年圣海伦斯火山（MSH）爆发期间地震活动最引人注目的方面之一是在持续时间段内重复长周期（LP）或“鼓声”事件发生的精确规律性。然而，这种精确的规律性并不总是被观察到，有时LP事件的时间发生变得更加随机。此外，伴随着2004-2008年MSH喷发期间占主导地位的LP类事件，有一个近连续的，随机发生的一系列较小的地震事件。这些子事件并不总是简单的小振幅版本的主导LP类事件，而是出现，而不是从一个单独的随机过程，只有松散耦合到主LP源机制。我们分析了子事件的间隔时间和振幅分布，使用波形互相关来从子事件中分离LP事件。我们还讨论了地震震颤，伴随着2005年3月8日在MSH潜水爆炸事件。这种震颤由爆炸过程中触发的LP和子事件的快速连续组成，此外还有持续脱气产生的宽带噪声。紧接着，地震活动恢复到爆炸前的发生模式。这种触发与蒸汽从系统中迅速喷出以及随后恢复到爆炸前的地震活动有关，表明这两种地震事件类型都起源于地下热液系统的一个区域，该区域（1）与2005年3月8日潜水爆炸的储层接触，但（2）没有被爆炸事件破坏或排出。最后，我们讨论了可能的热力学条件，在加压热液裂缝，可能会引起地震活动。典型LP事件的压降估计值一般不足以将浅热液裂缝中的纯水扰动到不稳定状态。然而，溶解的挥发物，如CO2，可能会导致一个更不稳定的系统，增加地震的潜在的热液裂纹受到快速热通量。因此，2004-2008年MSH下方热液和岩浆系统的相互作用似乎能够解释广泛的观测现象，包括子事件，LP事件，较大（Md > 2）事件和潜水爆炸。
[1] One of the most striking aspects of seismicity during the 2004–2008 eruption of Mount St. Helens (MSH) was the precise regularity in occurrence of repetitive long-period (LP) or “drumbeat” events over sustained time periods. However, this precise regularity was not always observed, and at times the temporal occurrence of LP events became more random. In addition, accompanying the dominant LP class of events during the 2004–2008 MSH eruption, there was a near-continuous, randomly occurring series of smaller seismic events. These subevents are not always simply small-amplitude versions of the dominant LP class of events but appear instead to result from a separate random process only loosely coupled to the main LP source mechanism. We present an analysis of the interevent time and amplitude distributions of the subevents, using waveform cross correlation to separate LP events from the subevents. We also discuss seismic tremor that accompanied the 8 March 2005 phreatic explosion event at MSH. This tremor consists of a rapid succession of LPs and subevents triggered during the explosion, in addition to broadband noise from the sustained degassing. Immediately afterward, seismicity returned to the pre-explosion occurrence pattern. This triggering in relation to the rapid ejection of steam from the system, and subsequent return to pre-explosion seismicity, suggests that both seismic event types originated in a region of the subsurface hydrothermal system that was (1) in contact with the reservoir feeding the 8 March 2005 phreatic explosion but (2) not destroyed or drained by the explosion event. Finally, we discuss possible thermodynamic conditions in a pressurized hydrothermal crack that could give rise to seismicity. Pressure drop estimates for typical LP events are not generally large enough to perturb pure water in a shallow hydrothermal crack into an unstable state. However, dissolved volatiles such as CO2 may lead to a more unstable system, increasing the seismogenic potential of a hydrothermal crack subject to rapid heat flux. The interaction of hydrothermal and magmatic systems beneath MSH in 2004–2008 thus appears able to explain a wide range of observed phenomena, including subevents, LP events, larger (Md > 2) events, and phreatic explosions.