The Evolution of Preseismic Patterns Related to the Central Crete (Mw6.0) Strong Earthquake on 27 September 2021 Revealed by Multiresolution Wavelets and Natural Time Analysis

Abstract

On 27 September 2021, a shallow earthquake with focal depth of 10 km and moment magnitude Mw6.0 occurred onshore in central Crete (Greece). The evolution of possible preseismic patterns in the area of central Crete before the Mw6.0 event was investigated by applying the method of multiresolution wavelet analysis (MRWA), along with that of natural time (NT). The monitoring of preseismic patterns by critical parameters defined by NT analysis, integrated with the results of MRWA as the initiation point for the NT analysis, forms a promising framework that may lead to new universal principles that describe the evolution patterns before strong earthquakes. Initially, we apply MRWA to the interevent time series of the successive regional earthquakes in order to investigate the approach of the regional seismicity towards critical stages and to define the starting point of the natural time domain. Then, using the results of MRWA, we apply the NT analysis, showing that the regional seismicity approached criticality for a prolonged period of ~40 days before the occurrence of the Mw6.0 earthquake, when the κ1 natural time parameter reached the critical value of κ1 = 0.070, as suggested by the NT method.

The 27 September 2021 Earthquake in Central Crete (Greece)—Detailed Analysis of the Earthquake Sequence and Indications for Contemporary Arc-Parallel Extension to the Hellenic Arc

“…The earthquake of 27 September 2021, 06:17:21 UTC, occurred near Arkalochori, Central Crete, ~25 km SSE of Heraklion city [25,26]. Its focal mechanism is characterized by an SSW-NNE to SW-NE-trending, nearly dip-slip normal faulting.…”

Investigation of the Thiva 2020–2021 Earthquake Sequence Using Seismological Data and Space Techniques

“…It is also worth noting that the evolution of a seismic swarm is not always similar to the one of the present study. Recently, on 27 September 2021, while the Thiva swarm was still active, a destructive earthquake (M w = 6.0) occurred in Arkalochori in central Crete [31,32]. In Arkalochori, the seismic activity was initiated about 4 months before the mainshock, in early June 2021, as an earthquake swarm.…”

“…The broad Thiva area is characterized by normal faulting (Figure 1). It is worth noting that recent strong and destructive earthquakes in Greece have occurred on normal faults, such as the Lesvos 2017 [5,17,18], Kos 2017 [5,19,20], Samos 2020 [21][22][23][24], Thessaly 2021 [25][26][27][28][29][30], and Arkalochori (Crete) 2021 [31,32] earthquakes. Fault segmentation is common near Thiva, with the presence of a multitude of smaller normal faults all around the area.…”

Satellite-Observed Thermal Anomalies and Deformation Patterns Associated to the 2021, Central Crete Seismic Sequence

“…The absence of thermal anomalies during October could indicate that after the earthquake occurrence, the stress field relaxed ( [63] and references therein) and thus reduced the above-described processes. A crucial note is that the appearance of thermal anomalies was in the time period where the system presented critical dynamics [25].…”

“…The epicenter of the seismic event was located at latitude 35.15 N and longitude 25.27 E, while the focal depth was about 10 km and associated with the Kastelli Fault Zone [22] whose mean recurrence interval has been estimated at approximately 800 years [23]. Since the beginning of June, almost 4 months earlier, a significant number of foreshocks had beenrecorded in the broader area, while a rich aftershock sequence was observed in the months after the main shock occurrence [21,22,24,25]. On 24 July, a strong preseismic event appeared with magnitude Mw4.8.…”