A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on the study of fracto-electromagnetic emissions (fracto-EME) at the MHz band, the ultra-low frequency (ULF) magnetic field variations (<3 Hz) and the subionospheric very low frequency (VLF) propagation anomalies. We present recent analysis results for these electromagnetic signals using two independent methods which are known for their ability to uncover critical dynamics, the recently proposed method of critical fluctuations (MCF) and the natural time (NT) analysis method. Our results show that all three considered electromagnetic signals present critical characteristics from a few weeks up to a few days before the main shock occurrence. On the other hand, signatures for the departure from the critical (highly symmetrical) state towards a low symmetry state, a state during which there is high localization of the EQ preparatory process, have been identified in specific cases for the MHz fracto-EME as well as for the ULF magnetic field variations. Based on a multidisciplinary analysis, a four-stage model of EQ dynamics by means of fracto-EME in the MHz and kHz bands has recently been proposed. The hypothesis that the precursors considered in this article emerge during the spatially extensive phase of EQ preparation, which corresponds to the first stage of the abovementioned four-stage model, as well as their relation with the foreshock seismic activity are discussed.

在过去的几十年中，文献已经报道了可能与地震（EQ）准备过程有关的各种各样的电磁现象。他们的研究中一个有趣的方面是对相关可观测对象的时间序列分析，旨在研究任何嵌入式动力学。在这篇评论文章中，我们重点研究MHz频带上的分形电磁发射（fracto-EME），超低频（ULF）磁场变化（<3 Hz）和亚电离层极低频（VLF）传播异常。我们使用两种独立的方法来提供这些电磁信号的最新分析结果，这些方法以发现临界动力学的能力而闻名，这是最近提出的临界波动（MCF）方法和自然时间（NT）分析方法。我们的结果表明，从主冲击发生前的几周到几天，所有这三种考虑到的电磁信号都呈现出关键特性。另一方面，在特定情况下，对于MHz fracto-EME，已经确定了从临界（高度对称）状态向低对称状态（即EQ准备过程高度本地化的状态）偏离的特征，如下所示：以及ULF磁场变化。在多学科分析的基础上，最近提出了一种通过分形EME在MHz和kHz频段进行EQ动力学的四阶段模型。假设本文中考虑的前体在EQ制备的空间扩展阶段出现，该阶段对应于上述四阶段模型的第一阶段，