Supplementary Movies: High-rate very-long-period seismicity at Yasur volcano, Vanuatu: Source mechanism and decoupling from surficial explosions and infrasound

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 broadband seismic and infrasonic network
coverage from 27 July to 3 August 2016. 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 to 10 sec. The VLP
seismic events are "high-rate", reoccurring near-continuously
throughout the dataset with short interevent times (~20 to 60 sec). 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 time-scale
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 ~900-1,000 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
~900-1,000 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.

瓦努阿图的亚苏尔火山是一座持续活动的开放式通道玄武安山质复式火山，长期维持着持续性喷发活动。本研究呈现了亚苏尔火山的地震-声学野外实验结果：该实验于2016年7月27日至8月3日期间，搭建了局域高密度宽频带地震与次声监测台网。我们通过部署在火山口及无人驾驶航空器系统（Unoccupied Aircraft System, UAS）上的摄像机获取的同步视频数据，对本次地震-声学观测结果进行了验证。
地震与次声波形中包含大量信号，清晰反映出亚苏尔火山快速频发且持续的喷发活动。亚苏尔火山喷发的典型次声信号特征为经典的短时长、常呈非对称的喷发波形，其显著标志为尖锐的压缩起始相位与宽频带频率成分。研究区主要地震信号为大量重复出现的超长周期（Very-Long-Period, VLP）信号，其周期约为2至10秒。该类VLP地震事件属于“高发生率”类型，在整个数据集内近乎持续重复出现，事件间隔时间较短（约20至60秒）。
我们观测到地震VLP信号与次声声源的同步性存在变化：由次声波形清晰识别的喷发事件，其下方均对应有地震VLP信号；但也存在强地震VLP信号仅伴随微弱次声响应的情况。对地震VLP信号的多重事件分析揭示了地震-声学源解耦的系统性演化规律：同一组主要地震VLP重复事件簇，既可伴随地表喷发与次声信号出现，也可在无此类现象时出现，且这类转换在本次野外实验期间以数日为时间尺度发生。
随后我们采用模板匹配、波形叠加与全波形反演方法，对该组主要VLP重复事件簇的震源机制进行成像分析。对该组VLP重复事件簇的叠加波形进行反演，结果显示其复合震源机制可归为双裂隙（附加力）或管裂隙（附加力）模型。反演得到的震源机制对应位于火山口下方的点源，其质心深度处于地表以下约900至1000米范围内。所有震源机制均显示存在一条走向北东、向北西缓倾的裂隙，同时表明VLP震源的质心与机制受控于亚苏尔火山下方一处稳定的构造地质特征。
我们以气体弹体沿通道上升的框架来解释本次研究结果，该过程正是亚苏尔火山喷发的成因。带有次声信号（若存在时）的VLP震源机制与发震时间，可通过浅层缓冲自上而下模型得到解释：气体弹体上升过程相对无地震活动，直至抵达浅层通道底部；气体弹体在该浅层区域内的破裂会引发压力扰动，该扰动向下传播并在通道底部（即VLP震源质心）处耦合产生地震信号。若浅层通道处于开放状态，喷发将传播至地表并产生次声信号。当该组VLP信号出现但无地表喷发、仅伴随微弱或无次声信号时，深度约900至1000米处的主要VLP信号与地表喷发及次声信号之间的解耦现象，强烈表明气体弹体在上升末期受到了缓冲阻滞。这种缓冲阻滞可由火山口及其下方的多种条件实现，例如富含晶体的高黏度岩浆层、回填形成的碎屑盖层、泡沫层，或上述多种条件的组合。
本次实验捕捉到的亚苏尔火山主要VLP信号，其震源深度与机制均独立于地表过程，且随时间保持稳定。