Intraslab Deformation and Rupture of the Entire Subducting Crust During the 25 October 2018 Mw 6.8 Zakynthos Earthquake

Abstract

We constrained the rupture process of the 25 October 2018 M w 6.8 Zakynthos earthquake by jointly inverting near-fault strong motion records, static, and high-rate GPS displacements along with teleseismic waveforms. The inverted slip distribution is characterized by two asperities with peak slip amplitude of 1.4 m, which is located at 17 km depth. The rupture essentially propagated unilaterally in the downdip direction after the initiation and to the north-northeast toward Zakynthos island. The earthquake produced a deep rupture whose downdip extent approached 38 km, indicating that this event broke the entire underthrust Ionian oceanic crust beneath the megathrust and penetrated into the top few kilometers of the solid uppermost mantle. The extent of the rupture implied the presence of a low coefficient of dynamic friction. The mainshock triggered a counterclockwise rotation of the stress field with respect to the long-term regional stress. Aftershocks occurred at shallower depths than the coseismic slip zone.Plain Language Summary On 25 October 2018 a major earthquake occurred 45 km SSW of Zakynthos Island at Western Greece. The region where the earthquake struck is one of the most active seismic zones in the country. It is where the active Hellenic subduction, that is, the interface along which the Nubia plate subducts under the Aegean plate, reaches the Cephalonia Transform Fault. We jointly used seismic and geodetic data sets to study the 2018 Zakynthos earthquake and revealed that it was not a typical subduction or a transform fault event; it was something different. By constraining the rupture process of the earthquake, we showed that this event did not slip the megathrust surface of the subduction zone but instead fractured the entire underthrust Ionian oceanic crust as it descends beneath the Aegean plate suggesting intraslab deformation and probably tearing of the subducting slab. Furthermore, the main shock induced a counterclockwise rotation of the stress field with respect to the long-term regional stress.

Rupture Process of the 2020 Mw7.0 Samos Earthquake and its Effect on Surrounding Active Faults

“…The high‐rate data were processed with TRACK v1.31 (Herring et al., 2018) by performing epoch‐by‐epoch kinematic analysis following the processing and filtering approach described in Chousianitis et al. (2016) and Chousianitis and Konca (2019). The derived GPS waveforms are displayed in Figure .…”

Earthquake Swarms, Slow Slip and Fault Interactions at the Western‐End of the Hellenic Subduction System Precede the Mw 6.9 Zakynthos Earthquake, Greece

“…On October 25, 2018, a M w 6.9 earthquake ruptured the western termination of the Hellenic Subduction System (HSS) across a zone where plate‐motion transitions from mainly thrust to mainly strike‐slip faulting (Royden & Papanikolaou, 2011), providing a valuable new case study of a well‐monitored earthquake that ruptures a subduction termination (Figure 1). This earthquake, that occurred southwest of the island of Zakynthos (Chousianitis & Konca, 2019; Sokos et al., 2020) (Figure 1), was preceded by a 5‐year‐long tectonic instability in the broader epicentral area of the M w 6.9 event and an intense aftershock sequence (Figures 2 and 3). Here, we report on a multidisciplinary data set of seismological, geodetic, seismic reflection, and bathymetric information that, collectively, capture the earthquake and fault kinematics within this earthquake sequence (hereafter refer to as the Zakynthos Earthquake Sequence [ZES]), prior to and after the main event.…”

Short-Term Foreshocks as Key Information for Mainshock Timing and Rupture: The Mw6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone

“…Higher-angle reverse faults (dip angles > 30°), likely splaying off from the plate interface, are observed at shallower depths with respect to the interplate earthquakes [ 45 , 46 ] and have been observed as playing a major role in the seismogenesis along the outer Hellenic fore-arc, e.g., [ 47 , 48 ]. The predominant compressive stress regime observed in the study area to the west and southwest of Peloponnese changes to strike-slip in the area to the NW of Zakynthos, Kefalonia and Lefkada Islands along the so-called Kefalonia Fault Zone, e.g., [ 49 ] ( Figure 1 a).…”