The 2018 Mw 6.8 Zakynthos (Ionian Sea, Greece) earthquake: seismic source and local tsunami characterization

Abstract

SUMMARY We investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time-series and static coseismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9°E-striking 39°ESE-dipping plane. The rupture history of this model is characterized by a bilateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterization of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source 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

“…While our seismological results support the failure of a splay-thrust fault, we believe that seismological data alone cannot safely discriminate between the two scenarios because of the uncertainty on the depth and dip angle estimations for the aftershocks, as well as the ambiguity in the MT decomposition. However, the scenario of a splay-thrust fault is further supported here by published seismic reflection and bathymetric data ( Figures 1 and 4, Figure S23) that reveal numerous ∼ NNW-SSE trending thrusts that dip 30°-50° to the northeast, beneath Zakynthos and western Peloponnese (Kokkalas et al, 2013;Makris & Papoulia, 2014;Sachpazi et al, 2000;Wardell et al, 2014; this study) and the recording of a minor (10 cm) tsunami along the western coastline of Peloponnese that suggests rupture of the sea-bed (Cirella et al, 2020). It is also supported by the low-dipping (<15°-17°) angle of the plate-interface beneath the epicentral area (e.g., Halpaap et al, 2018;Sachpazi et al, 2000).…”

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

“…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 ]. Slip vectors, oriented to SW, are consistent with the plate motion and the sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate [ 52 ].…”

“…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 ]. Slip vectors, oriented to SW, are consistent with the plate motion and the sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate [ 52 ]. According to a different analysis [ 53 ], the preferred rupture model is the one with a N9°E-striking 39°ESE-dipping plane.…”

Clusty, the waveform-based network similarity clustering toolbox: concept and application to image complex faulting offshore Zakynthos (Greece)

“…Following large-magnitude earthquakes (M w > 6), the geometry of the fault associated with the main rupture, as well as its slip distribution, is often estimated using seismological and geodetic tools (e.g. Koper et al 2011;Yokota et al 2011;Grandin et al 2015;Cirella et al 2020).…”

“…In this study we apply the clustering toolbox to the aftershock sequence of the 25 October 2018 M w 6.9 Zakynthos (Greece) earthquake (Chousianitis & Konca 2019;Cirella et al 2020;Ganas et al 2020;Karakostas et al 2020;Mouslopoulou et al 2020;Sokos et al 2020). The data set includes >2300 events with M ≥ 2.8 recorded at 33 stations from 25/10/2018 to 14/11/2019.…”