High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound
High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound
复制标题
瓦努阿图亚苏尔火山的高频率超长周期地震活动：震源机制以及与地表爆炸和次声的解耦
DOI：
10.1093/gji/ggab533
复制
发表时间：
2022
影响因子：
2.8
通讯作者：
Garaebiti, Esline
中科院分区：
文献类型：
--
作者：
Matoza, Robin S.;Chouet, Bernard A.;Jolly, Arthur D.;Dawson, Phillip B.;Fitzgerald, Rebecca H.;Kennedy, Ben M.;Fee, David;Iezzi, Alexandra M.;Kilgour, Geoff N.;Garaebiti, Esline
瓦努阿图亚苏尔火山是一个持续活动的开放喷口的玄武岩-安山岩层锥，具有持久和长期的喷发活动。我们介绍了亚苏尔地震声场实验的结果，提供了2016年7月27日至8月3日局部密集的宽带地震和次声波网络覆盖。我们用部署在陨石坑和无人驾驶飞机系统(UAS)的摄像机的一致视频数据证实了我们的地声观测结果。这些波形包含大量信号，反映了亚苏尔迅速发生和持续的爆炸活动。Yasur爆炸的典型次声特征是典型的短持续时间且通常不对称的爆炸波形，其特征是急剧的压缩起爆和宽带频率成分。S的主要地震信号是周期为∼2-10的大量重复甚长周期信号。极长周期地震事件是“高比率”的，在整个数据集内几乎连续重复发生，事件间隔时间较短(∼20-60 S)。我们观察到地震VLP和声源同步的可变性。次声波形清楚地描绘了爆炸事件，地震VLP是其下层。然而，强地震VLP也发生在只有微弱次声表达的情况下。对地震VLP的多重分析揭示了地震-声源解耦的系统性进展。在有和没有地面爆炸和次声的情况下，相同的主要地震VLP倍增都会发生，这些转变发生在我们的实地行动期间的几天时间尺度上。随后，我们使用模板匹配、堆叠和全波形反转来成像主要的VLP多重线的来源机制。主要的VLP多重叠层的反演指向由双裂纹(加力)或管-裂纹(加力)机制组成的复合源。推导出的机制对应于山顶喷口正下方的点源，质心深度在地形以下900-1000米范围内的∼。所有机制均显示为北东向裂隙，向西北方向倾斜相对较浅，表明亚苏尔以下稳定的构造地质特征控制了VLP源、质心和机制。我们在导致Yasur爆炸的管道中的气体段塞上升的框架下解释了结果。VLP的机理和次声时间(如果存在)可以用一个浅缓冲的自上而下的模型来解释，在该模型中，段塞上升在到达浅部底部之前是相对抗震的。这一浅层段塞的破裂触发了向下传播的压力扰动，并在管道底部耦合(VLP质心)。如果浅部是开放的，爆炸就会传播到表面，产生次声。在没有地面爆炸和弱或没有次声的情况下，(相同的倍增)∼900-1000米深度的主要VLP与地面爆炸和次声的解耦强烈地表明终端段塞上升的缓冲。这种缓冲可以通过在喷口或直接位于喷口下方的各种条件来实现，例如富含晶体的岩浆的高粘度层、来自回填的碎屑盖、泡沫层或这些条件的组合。我们的实验捕捉到的YASUR VLP具有与地表过程分离的震源深度和机制，并且随着时间的推移是稳定的。
Yasur volcano, Vanuatu is a continuously active open-vent basaltic-andesite stratocone with persistent and long-lived eruptive activity. We present results from a seismo-acoustic field experiment at Yasur, providing locally dense broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. We corroborate our seismo-acoustic observations with coincident video data from cameras deployed at the crater and on an unoccupied aircraft system (UAS). The waveforms contain a profusion of signals reflecting Yasur’s rapidly occurring and persistent explosive activity. The typical infrasonic signature of Yasur explosions is a classic short-duration and often asymmetric explosion waveform characterized by a sharp compressive onset and wideband frequency content. The dominant seismic signals are numerous repetitive very-long-period (VLP) signals with periods of ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). We observe variability in the synchronization of seismic VLP and acoustic sources. Explosion events clearly delineated by infrasonic waveforms are underlain by seismic VLPs. However, strong seismic VLPs also occur with only a weak infrasonic expression. Multiplet analysis of the seismic VLPs reveals a systematic progression in the seismo-acoustic source decoupling. The same dominant seismic VLP multiplet occurs with and without surficial explosions and infrasound, and these transitions occur over a timescale of a few days during our field campaign. We subsequently employ template matching, stacking, and full-waveform inversion to image the source mechanism of the dominant VLP multiplet. Inversion of the dominant VLP multiplet stack points to a composite source consisting of either a dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The derived mechanisms correspond to a point-source directly beneath the summit vents with centroid depths in the range ∼900–1000 m below topography. All mechanisms suggest a northeast trending crack dipping relatively shallowly to the northwest and indicate a VLP source centroid and mechanism controlled by a stable structural geologic feature beneath Yasur. We interpret the results in the framework of gas slug ascent through the conduit responsible for Yasur explosions. The VLP mechanism and timing with infrasound (when present) are explained by a shallow-buffered top-down model in which slug ascent is relatively aseismic until reaching the base of a shallow section. Slug disruption in this shallow zone triggers a pressure disturbance that propagates downward and couples at the conduit base (VLP centroid). If the shallow section is open, an explosion propagates to the surface, producing infrasound. In the case of (the same multiplet) VLPs occurring without surficial explosions and weak or no infrasound, the decoupling of the dominant VLPs at ∼900–1000 m depth from surficial explosions and infrasound strongly indicates buffering of the terminal slug ascent. This buffering could be achieved by a variety of conditions at or directly beneath the vents, such as a high-viscosity layer of crystal-rich magma, a debris cap from backfill, a foam layer, or a combination of these. The dominant VLP at Yasur captured by our experiment has a source depth and mechanism separated from surface processes and is stable over time.