Abstract Phreatic activity of Merapi started after nearly 2 years of quiescence following the October–November 2010 eruption which was the largest in more than 100 years. A dozen eruptions identified by visual and/or seismic observations took place between August 2012 and April 2014. We present in this work the results of a detailed analysis of the April 20, 2014 phreatic eruption. We attempted to reconstruct the eruptive process, which lasted for over 30 min. To this end, we determined the wavefield composition by polarization analysis, located high-frequency earthquakes occurring in the initial part of the eruption process and then determined the seismic source migration of low-frequency part of the tremor-like signal [0.3–3 Hz] over time. Source depth of low-frequency signal was obtained by comparing the slowness vector calculated using 3 stations of the seismic antenna with a slowness vector model obtained by ray tracing in the structure, taking into account the topography and a 1D velocity model obtained by spatial auto-correlation analysis. The results allow to distinguish 3 different phases: 1) High-frequency transients interpreted as the result of a sudden decompression caused by the transition of the volcanic fluid to a gaseous phase that occurred approximately 1.5 km deep. This decompression process in the hydrothermal system generated a migration of the low-frequency seismic source from 900 m to 1800 m above sea level; 2) A second decompression process revealed by high-frequency micro-seismicity and associated to the migration of the low-frequency tremor source which is marked first by a descent phase, followed by a sharp ascent until reaching the surface. The evolution of the back-azimuth during the migration process indicates a slight inclination of the conduit, presumably in the orientation of the dome fracture, in the NW-SE direction. This direction is consistent with the alignment of regional tectonic structures and with the directivity of eruption deposits. 3) The seismic source then remains positioned at the altitude of the dome for over 10 min. This phase probably corresponds to the ash emission process. The average migration speed of the low-frequency seismic source from the starting eruptive process to ash emission is about 5 m/s.

中文翻译：

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与印度尼西亚默拉皮火山潜水喷发相关的地震活动迁移

摘要 默拉皮的潜水活动在 2010 年 10 月至 11 月爆发后近 2 年的静止期开始，这是 100 多年来最大的一次。在 2012 年 8 月至 2014 年 4 月期间，通过目视和/或地震观测确定了十几次喷发。我们在这项工作中展示了对 2014 年 4 月 20 日潜水喷发的详细分析结果。我们试图重建持续超过 30 分钟的喷发过程。为此，我们通过极化分析确定了波场组成，定位了喷发初期发生的高频地震，然后确定了类震信号[0.3-3 Hz]低频部分的震源偏移。 ] 随着时间的推移。低频信号的震源深度是通过将地震天线的3个台站计算的时差矢量与结构中射线追踪获得的时差矢量模型进行比较，同时考虑地形和空间自动获得的一维速度模型获得的。相关性分析。结果允许区分 3 个不同的阶段：1) 高频瞬变解释为火山流体转变为气相引起的突然减压的结果，发生在大约 1.5 公里深。热液系统的这种减压过程导致低频震源从海拔 900 m 迁移到 1800 m；2) 高频微地震揭示的第二次减压过程，与低频震源的迁移有关，该过程首先以下降阶段为标志，然后急剧上升直至到达地表。迁移过程中后方位角的演变表明管道在 NW-SE 方向上有轻微的倾斜，大概是在圆顶裂缝的方向上。这一方向与区域构造构造的排列和喷发沉积物的方向性一致。3) 然后震源在圆顶的高度上保持定位超过 10 分钟。该阶段可能对应于灰排放过程。低频震源从开始喷发到出灰的平均迁移速度约为5 m/s。