geosciences Article Seismicity Patterns Prior to the Thessaly (Mw6.3) Strong Earthquake on 3 March 2021 in Terms of Multiresolution Wavelets and Natural Time Analysis Filippos Vallianatos 1,2,*, Georgios Michas 1,2 and George Hloupis 2,3 1 Institute of Physics of the Earth’s Interior and Geohazards, UNESCO Chair on Solid Earth Physics and Geohazards Risk Reduction, Hellenic Mediterranean University Research Center, 73133 Chania, Greece; [email protected] 2 Section of Geophysics—Geothermics, Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, 15784 Athens, Greece; [email protected] 3 Department of Surveying and Geoinformatics Engineering, University of West Attica, Egaleo Campus, 12244 Athens, Greece * Correspondence: [email protected] or [email protected] Abstract: On 3 March 2021, a strong, shallow earthquake of moment magnitude, Mw6.3, occurred in northern Thessaly (Central Greece). To investigate possible complex correlations in the evolution of seismicity in the broader area of Central Greece before the Mw6.3 event, we apply the methods of multiresolution wavelet analysis (MRWA) and natural time (NT) analysis. The description of seismicity evolution by critical parameters defined by NT analysis, integrated with the results of MRWA as the initiation point for the NT analysis, forms a new framework that may possibly lead to Citation: Vallianatos, F.; Michas, G.; Hloupis, G. Seismicity Patterns Prior new universal principles that describe the generation processes of strong earthquakes. In the present to the Thessaly (Mw6.3) Strong work, we investigate this new framework in the seismicity prior to the Mw6.3 Thessaly earthquake. Earthquake on 3 March 2021 in Terms Initially, we apply MRWA to the interevent time series of the successive regional earthquakes in order of Multiresolution Wavelets and to investigate the approach of the regional seismicity at critical stages and to define the starting point Natural Time Analysis. Geosciences of the natural time domain. Then, we apply the NT analysis, showing that the regional seismicity 2021, 11, 379. https://doi.org/ approached criticality a few days before the occurrence of the Mw6.3 earthquake, when the k1 natural 10.3390/geosciences11090379 time parameter reached the critical value of k1 = 0.070. Academic Editors: Ioannis Keywords: Thessaly earthquake; seismicity patterns; natural time; multiresolution wavelet analysis; Koukouvelas, Riccardo Caputo, criticality Tejpal Singh and Jesus Martinez-Frias Received: 9 August 2021 Accepted: 8 September 2021 1. Introduction Published: 9 September 2021 On 3 March 2021, an MW6.3 earthquake occurred in northern Thessaly (Central Publisher’s Note: MDPI stays neutral Greece), close to the cities of Tyrnavos, Elassona and Larisa (Figure1). The earthquake with regard to jurisdictional claims in occurred in a region that is one of the most seismically active in Greece, mainly character- published maps and institutional affil- ized by normal faulting along NW–SE striking faults, which belong to the Thessaly fault iations. zone [1–10]. Based on the provided focal plane solutions [11], the mainshock was generated by the activation of an NW–SE striking normal fault (Figure1)[ 12]. The mainshock was widely felt in the Thessaly basin and in the surrounding areas, from Athens in the south to the northern borders of Greece. Copyright: © 2021 by the authors. The Thessaly basin has a well-known history of large earthquakes, with mainshocks Licensee MDPI, Basel, Switzerland. presenting typical magnitudes between 6.0 and 7.0 [3–15]. During the last century, eight This article is an open access article major earthquakes, with magnitudes equal to or larger than 6.0, have occurred in this area. distributed under the terms and The M = 7.0, 1954 Sofades earthquake was the most destructive event, resulting in heavy conditions of the Creative Commons damages in the broader southern Thessaly region (Papazachos and Papazachou, 2002; [15]). Attribution (CC BY) license (https:// The M = 6.8, 1957 Velestino earthquake and the M = 6.5, 1980 Almyros earthquake, are two creativecommons.org/licenses/by/ more significant events, with similarly destructive consequences. 4.0/). Geosciences 2021, 11, 379. https://doi.org/10.3390/geosciences11090379 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 379 2 of 14 Strong arguments show that the earthquake generation process can be considered Geosciences 2021, 11, x FOR PEER REVIEW 2 of 14 as a critical point phenomenon that culminates with a large event as some critical point is approached [16–25]. New findings regarding the complex dynamics that characterize various geodynamic phenomena illustrate stimulating features in the framework of new earthquake,concepts, asare that two of more non-extensive significant events statistical, with physics similarly [22 –destructive25], multiresolution consequences. wavelets analysis [26–28] and of the novel time domain, termed as natural time [29–39]. Figure 1. Earthquake epicenters of the aftershock sequence of the 3 March 2021, Thessaly earthquake Figure(yellow 1. star),Earthquake from 3 Marchepicenters 2021 of to the 18 Aprilaftershock 2021 (M sequence≥ 2.0). Theof the focal 3 March mechanisms 2021, Thessaly of the mainshock earth- quakeand the(yellow largest star), aftershock from 3 areMarch also 2021 indicated, to 18 alongApril with2021 the(M regional≥ 2.0). The faults focal shown mechanisms with black of solidthe mainshocklines (for detailsand the see largest the text). aftershock are also indicated, along with the regional faults shown with black solid lines (for details see the text). The concept of natural time (NT) has been introduced recently to analyze possible pre-seismicStrong arguments signals [29 show,30,35 that]. The the analysis earthquake of various generation complex process systems can be in theconsidered NT domain as a enablescritical point the optimal phenomenon extraction that of culminates signal information with a large by reducing event as the some uncertainties critical point related is approachedto the conventional [16–25]. New time, findings as well asregarding the identification the complex of long-range dynamics correlationsthat characterize in the variousevolution geodynamic of the system, phenomena even in illustrate the presence stim ofulating “heavy features tails” [in40 the]. The framework usefulness of ofnew NT concepts,analysis as has that been of discussednon-extensive in a numberstatistical of physics applications [22–25], to knownmultiresolution critical phenomena, wavelets analysissuch as [26–28] fracturing, and earthquakes,of the novel time the 2-Ddomain, Ising termed model andas natural 3-D turbulent time [29–39]. flow [24,35], and referencesThe concept therein, of andnatural it has time been (NT) tested has experimentallybeen introduced in recently fracturing to experimentsanalyze possible in the pre-seismiclaboratory signals by analyzing [29,30,35]. acoustic The analysis emissions of timevarious series complex [23,41]. systems in the NT domain enablesFurthermore, the optimal extraction wavelet-based of signal methods information have been by reducing introduced the touncertainties characterize related fractal tosignals the conventional [42–45] and time, to overcome as well effectsas the iden associatedtification with of non-stationaritieslong-range correlations [46,47], in a verythe evolutionfrequent of effect the insystem, the time even dynamics in the presence of an earthquake of “heavy sequence. tails” [40]. The usefulness of NT analysisThe has goal been of thediscussed present in work a number is to test of andapplications evaluate to the known seismicity critical patterns phenomena, in terms suchof MRWA as fracturing, and NT earthquakes, analyses, as the applied 2-D Ising in the model evolution and 3-D of seismicity turbulent priorflow [24,35], to the recent and referencesMw6.3 Thessaly therein, strongand it has event. been The tested recent experimentally upgrading ofin thefracturing regional experiments seismological in the net- laboratoryworks [48 by] provides analyzing an acoustic accurate emissions catalogue time of microseismicity series [23,41]. in the area and enables the applicationFurthermore, of such wavelet-based methodologies. methods The earthquake have been catalogs introduced used herein to characterize are taken fromfractal the signalsHellenic [42–45] Unified and Seismological to overcome Networkeffects associated (HUSN) (withhttp://eida.gein.noa.gr/ non-stationarities [46,47],, last accessed a very frequenton 27 May effect 2021 in), the where time instruments dynamics of belonging an earthquake to the sequence. HL (National Observatory of Athens, The goal of the present work is to test and evaluate the seismicity patterns in terms of MRWA and NT analyses, as applied in the evolution of seismicity prior to the recent Mw6.3 Thessaly strong event. The recent upgrading of the regional seismological net- works [48] provides an accurate catalogue of microseismicity in the area and enables the Geosciences 2021, 11, x FOR PEER REVIEW 3 of 14 application of such methodologies. The earthquake catalogs used herein are taken from Geosciences 2021, 11, 379 the Hellenic Unified Seismological Network (HUSN) (http://eida.gein.noa.gr/, last3 ofac- 14 cessed on 27 May 2021), where instruments belonging to the HL (National Observatory of Athens, Institute of Geodynamics, 1997) [49] and HT (Aristotle University of Thessa- lonikiInstitute Seismological of Geodynamics, Network, 1997) 1981) [49]