The 2021 Greece Central Crete ML 5.8 Earthquake: An Example of Coalescent Fault Segments Reconstructed from InSAR and GNSS Data

Abstract

The ML 5.8 earthquake that hit the island of Crete on 27 September 2021 is analysed with InSAR (Interferometry from Synthetic Aperture Radar) and GNSS (Global Navigation Satellite System) data. The purpose of this work is to create a model with sufficient detail for the geophysical processes that take place in several kilometres below the earth’s surface and improve our ability to observe active tectonic processes using geodetic and seismic data. InSAR coseismic displacements maps show negative values along the LOS of ~18 cm for the ascending orbit and ~20 cm for the descending one. Similarly, the GNSS data of three permanent stations were used in PPK (Post Processing Kinematic) mode to (i) estimate the coseismic shifts, highlighting the same range of values as the InSAR, (ii) model the deformation of the ground associated with the main shock, and (iii) validate InSAR results by combining GNSS and InSAR data. This allowed us to constrain the geometric characteristics of the seismogenic fault and the slip distribution on it. Our model, which stands on a joint inversion of the InSAR and GNSS data, highlights a major rupture surface striking 214°, dipping 50° NW and extending at depth from 2.5 km down to 12 km. The kinematics is almost dip-slip normal (rake −106°), while a maximum slip of ~1.0 m occurred at a depth of ca. 6 km. The crucial though indirect role of inherited tectonic structures affecting the seismogenic crustal volume is also discussed suggesting their influence on the surrounding stress field and their capacity to dynamically merge distinct fault segments.

An Updated Earthquake Catalogue in Crete Derived by the Development of Local 1D Velocity Models and Hypocentre Relocation

“…The primary difference is that the models of [39,64] produced substantially shallower events than the events that belong in the Arkalochori seismic sequence in Central Crete [63]. In particular, the stronger events of that sequence (Mw = 6.0 and Mw = 5.3) were located at very shallow depths (H < 5 km), which looks improbable in terms of earthquake physics and the area's history [65][66][67]. The local 1D models in the first two areas west of the study area (Antikythera-Chania, Rethymno) have a relatively simplified structure since they are composed of four and three layers, respectively.…”

“…The 27 September 2021 mainshock (Mw = 6.0) was preceded by 300 events of M L ≥ 2.2 in the area between the Galatas and Avli normal faults in the area S-SE of Heraklion city. The epicentre of the mainshock was located less than 5 km to the SE of Arkalochori (lat: 35.1433 • N, long: 25.2688 • E) at a depth of ~9.5 km, slightly different from that of [63][64][65]. The denser cluster of events is located west of the mainshock in an approximately 15 km long area associated with the foreshocks (June-September 2021).…”