Politis Dimitrios Z, Potirakis Stelios M, Contoyiannis Yiannis F, Biswas Sagardweep, Sasmal Sudipta, Hayakawa Masashi
Department of Electrical and Electronics Engineering, Ancient Olive Grove Campus, University of West Attica, 12244 Egaleo, Greece.
Indian Centre for Space Physics, 43 Chalantika, Garia St. Road, Kolkata 700084, India.
Entropy (Basel). 2021 May 27;23(6):676. doi: 10.3390/e23060676.
In this work we present the statistical and criticality analysis of the very low frequency (VLF) sub-ionospheric propagation data recorded by a VLF/LF radio receiver which has recently been established at the University of West Attica in Athens (Greece). We investigate a very recent, strong (M6.9), and shallow earthquake (EQ) that occurred on 30 October 2020, very close to the northern coast of the island of Samos (Greece). We focus on the reception data from two VLF transmitters, located in Turkey and Israel, on the basis that the EQ's epicenter was located within or very close to the 5th Fresnel zone, respectively, of the corresponding sub-ionospheric propagation path. Firstly, we employed in our study the conventional analyses known as the nighttime fluctuation method (NFM) and the terminator time method (TTM), aiming to reveal any statistical anomalies prior to the EQ's occurrence. These analyses revealed statistical anomalies in the studied sub-ionospheric propagation paths within ~2 weeks and a few days before the EQ's occurrence. Secondly, we performed criticality analysis using two well-established complex systems' time series analysis methods-the natural time (NT) analysis method, and the method of critical fluctuations (MCF). The NT analysis method was applied to the VLF propagation quantities of the NFM, revealing criticality indications over a period of ~2 weeks prior to the Samos EQ, whereas MCF was applied to the raw receiver amplitude data, uncovering the time excerpts of the analyzed time series that present criticality which were closest before the Samos EQ. Interestingly, power-law indications were also found shortly after the EQ's occurrence. However, it is shown that these do not correspond to criticality related to EQ preparation processes. Finally, it is noted that no other complex space-sourced or geophysical phenomenon that could disturb the lower ionosphere did occur during the studied time period or close after, corroborating the view that our results prior to the Samos EQ are likely related to this mainshock.
在这项工作中，我们展示了对甚低频（VLF）电离层下传播数据的统计和临界性分析，这些数据由最近在雅典（希腊）的西阿提卡大学建立的一个甚低频/低频无线电接收机记录。我们研究了2020年10月30日发生的一次非常近、强烈（M6.9）且浅源的地震（EQ），该地震非常靠近希腊萨摩斯岛的北海岸。我们重点关注来自位于土耳其和以色列的两个甚低频发射机的接收数据，基于该地震的震中分别位于相应电离层下传播路径的第5菲涅尔区内或非常靠近该区域。首先，我们在研究中采用了称为夜间波动法（NFM）和终止时间法（TTM）的传统分析方法，旨在揭示地震发生前的任何统计异常。这些分析揭示了在地震发生前约2周和几天内，所研究的电离层下传播路径中的统计异常。其次，我们使用两种成熟的复杂系统时间序列分析方法——自然时间（NT）分析方法和临界波动方法（MCF）进行了临界性分析。NT分析方法应用于NFM的甚低频传播量，揭示了在萨摩斯地震前约2周的时间内的临界性迹象，而MCF应用于原始接收机幅度数据，揭示了分析时间序列中在萨摩斯地震前最接近呈现临界性的时间片段。有趣的是，在地震发生后不久也发现了幂律迹象。然而，结果表明这些并不对应于与地震准备过程相关的临界性。最后，需要指出的是，在研究期间或之后不久，没有发生其他可能干扰低电离层的复杂空间源或地球物理现象，这证实了我们在萨摩斯地震前的结果可能与这次主震有关的观点。
