Detection of Low-Frequency Earthquakes in Broadband Random Time Sequences: Are They Independent Events?
Detection of Low-Frequency Earthquakes in Broadband Random Time Sequences: Are They Independent Events?
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DOI：
10.1029/2019jb017643
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发表时间：
2019-08-01
影响因子：
3.9
通讯作者：
Ide, Satoshi
中科院分区：
文献类型：
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作者：
Ide, Satoshi
低频地震（lfe）主要是利用匹配滤波技术在相对较窄的频带内使用脉冲模板波形从连续地震记录中检测出来的。但是，该方法也可以从某些随机时间序列中检测事件，而不需要明确的孤立事件。本文利用宽带慢震模型、布朗慢震（BSE）模型和完全随机噪声序列的合成力矩加速度进行简单的数值模拟，证明了这一事实。匹配滤波技术根据阈值确定两个时间序列中的信号，其中包括相对孤立的脉冲状波动。这些波形相对于信号时序叠加，在两个时间序列中显示出与假设模板相似的明显脉冲，这突出表明我们可能会将原始时间序列误解为包含许多孤立的脉冲事件。BSE模型与随机噪声在远低于分析频带的频率上存在一个重要的区别，堆叠的BSE序列在非常低的频率上包含相干信号，这些信号在噪声中是不可见的。在Cascadia俯冲带的实际观测中也有类似的低频相干信号，表明这些LFE信号与一些慢滑动相一致。因此，由于信号处理的原因，所谓的lfe可能是一种误解，或者至少它们是冰山一角，这些信号形成了一个非常宽带的缓慢地震样滑动过程的组成部分，可能发生在亚秒到多年的时间尺度上。
Low-frequency earthquakes (LFEs) are detected primarily from continuous seismograms using a matched-filter technique with an impulsive template waveform in a relatively narrow frequency band. However, this method can also detect events from some kinds of random time sequences without clearly isolated events. Here this fact is demonstrated via simple numerical simulations using the synthetic moment accelerations from a model of broadband slow earthquake, the Brownian slow earthquake (BSE) model, and a totally random noise sequence. The matched-filter technique identifies time sections including relatively isolated pulse-like fluctuations, as signals in both time sequences, depending on the threshold. These waveforms, stacked relative to the signal timing, show a clear impulse similar to the assumed template in both time sequences, which highlights that we might potentially misinterpret an original time sequence as containing many isolated pulse-like events. An important difference exists between the BSE model and random noise at frequencies much lower than the analyzed frequency band, with the stacked BSE sequence containing coherent signals at very low frequencies, which are not visible in the noise. Real observations in the Cascadia subduction zone also contain similar coherent signals at low frequencies, suggesting that these LFE signals are coincident with some slow slip. Therefore, so-called LFEs might potentially be a misinterpretation due to signal processing, or at least they are the tip of the iceberg, with these signals forming a component of a very broadband slow-earthquake-like slip process that possibly occurs over sub-second to multi-year timescales.