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

Abstract

Significant seismicity anomalies preceded the 25 October 2018 mainshock (Mw = 6.8), NW Hellenic Arc: a transient intermediate-term (~2 yrs) swarm and a short-term (last 6 months) cluster with typical time-size-space foreshock patterns: activity increase, b-value drop, foreshocks move towards mainshock epicenter. The anomalies were identified with both a standard earthquake catalogue and a catalogue relocated with the Non-Linear Location (NLLoc) algorithm. Teleseismic P-waveforms inversion showed oblique-slip rupture with strike 10°, dip 24°, length ~70 km, faulting depth ~24 km, velocity 3.2 km/s, duration 18 s, slip 1.8 m within the asperity, seismic moment 2.0 × 1026 dyne*cm. The two largest imminent foreshocks (Mw = 4.1, Mw = 4.8) occurred very close to the mainshock hypocenter. The asperity bounded up-dip by the foreshocks area and at the north by the foreshocks/swarm area. The accelerated foreshocks very likely promoted slip accumulation contributing to unlocking the asperity and breaking with the mainshock. The rupture initially propagated northwards, but after 6 s stopped at the north bound and turned southwards. Most early aftershocks concentrated in the foreshocks/swarm area. This distribution was controlled not only by stress transfer from the mainshock but also by pre-existing stress. In the frame of a program for regular monitoring and near real-time identification of seismicity anomalies, foreshock patterns would be detectable at least three months prior the mainshock, thus demonstrating the significant predictive value of foreshocks.

Global Precedent-Based Extrapolation Estimate of the M8+ Earthquake Hazard (According to USGS Data as of 1 June 2021)

“…Another seven (41%) M7+ earthquakes have partial predictability. They occurred in the identified clusters of activity (see the green circles in Figure A17) but go beyond the permissible errors of predictive extrapolations: M7.5 earthquake on 25 We believe that the ratios of 41%, 41% and 8% between full and partial predictability and its absence is a sufficient basis for starting to test precedent-based extrapolation estimations of seismic hazards in real time. The main clusters of hazardous activity as of 1 June 2021 are listed in Table A13.…”

“…In this paper, only activation sequences and (among them) foreshock sequences are considered. In relation to the time of the mainshock, there are [7,25] close (from several hours to several days), short-term (up to 5 to 6 months) and long-term (several years) foreshocks among the foreshocks. We examine all the listed types of foreshocks without exception.…”

Earthquakes in the Ionian Sea, Greece, Documented from Little-Known Historical Sources: AD 1513–1900

“…The most recent case was a large earthquake that occurred to the south of Zakynthos on 25 October 2018 (M w 6.84) [1,3,4,16,17]. Strong earthquakes occurring near Zakynthos are usually associated with precursory foreshocks [18] or swarm-type activity [19,20]. Such seismic clusters have been observed in association with historical strong earthquakes as well, like the 1893 sequence, which is examined later.…”