The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike-Slip Faulting near Subducting Slab

Abstract

With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform inversion and backprojection of strong-motion data, partly checked by coseismic Global Navigation Satellite System data. We show that the region is under subhorizontal southwest–northeast compression, enabling mixed thrust faulting and strike-slip (SS) faulting. The 2018 mainshock consisted of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral SS, both in the crust. Slip vectors, oriented to southwest, are consistent with plate motion. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and an Mw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations among earthquakes, swarms, and fluid paths in the region.

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

“…(2020), which could be compatible with a splay‐thrust fault, and in contradiction to the low‐dip angle suggested by Sokos et al. (2020) and Ganas et al. (2020), which is more compatible with the plate‐interface.…”

“…Three past earthquakes with sizes and focal mechanisms similar to that recorded in 2018 have ruptured the crust proximal to Zakynthos over the last ∼60 years (in 1959, 1976, and 1997; http://bbnet.gein.noa.gr/HL/). The orientation of their focal mechanisms (Kiratzi & Louvari, 2003;Sokos et al, 2020) is in agreement with the plate-convergence ( Figure 1), indicating that these earthquakes accommodated a fraction of the relative African-Eurasian plate-motion. No events greater than M7 have been recorded in the ZES region instrumentally or historically (Papazachos & Papazachou, 2003).…”

“…The 2018 Zakynthos mainshock ruptured the upper <20 km of the crust as a result of shallow thrust and moderately dipping dextral strike‐slip faulting (Haddad et al., 2020; Sokos et al., 2020). The focal mechanism presents a large nondouble‐couple (non‐DC) component with a large negative compensated linear vector dipole indicating a complex rupture process (Sokos et al., 2020). The Global Centroid MT (Global CMT) project suggests a centroid depth for this event of ∼16 km and a total seismic‐moment release of 2.32 × 10 19 Nm (Dziewonski et al., 1981; Ekström et al., 2012).…”

“…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

“…The rupture propagated unilaterally but mainly towards N-NE and with a down-dip extent up to 38 km, indicating that this event broke the entire underthrust Ionian oceanic crust beneath the megathrust. Others [ 51 ] supported that the earthquake ruptured the Hellenic megathrust plate interface along a N-S striking thrust/oblique-slip fault with a length of ~26 km and low-dip angle (23°) but at a depth of <15 km. On the other hand, a complex pattern was suggested for the 2018 mainshock consisting of two fault segments: a low-dip thrust, and a dominant, moderate-dip, right-lateral strike-slip, both in the crust [ 52 ].…”