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

Gerassimos A Papadopoulos; Apostolos Agalos; George Minadakis; Ioanna Triantafyllou; Pavlos Krassakis

doi:10.3390/s20195681

Short-Term Foreshocks as Key Information for Mainshock Timing and Rupture: The M_w6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone

Sensors (Basel). 2020 Oct 5;20(19):5681. doi: 10.3390/s20195681.

Authors

Gerassimos A Papadopoulos¹, Apostolos Agalos¹, George Minadakis^{2

3}, Ioanna Triantafyllou⁴, Pavlos Krassakis⁵

Affiliations

¹ International Society for the Prevention & Mitigation of Natural Hazards, 10681 Athens, Greece.
² Department of Bioinformatics, The Cyprus Institute of Neurology & Genetics, 6 International Airport Avenue, Nicosia 2370, P.O. Box 23462, 1683 Nicosia, Cyprus.
³ The Cyprus School of Molecular Medicine, The Cyprus Institute of Neurology & Genetics, 6 International Airport Avenue, 2370 Nicosia, P.O. Box 23462, Nicosia 1683, Cyprus.
⁴ Department of Geology & Geoenvironment, National & Kapodistrian University of Athens, 15784 Athens, Greece.
⁵ Centre for Research and Technology, Hellas (CERTH), 52 Egialias St, 15125 Athens, Greece.

Abstract

Significant seismicity anomalies preceded the 25 October 2018 mainshock (M_w = 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 (M_w = 4.1, M_w = 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.

Keywords: 2018 Zakynthos earthquake; Hellenic Subduction Zone; foreshocks; precursory seismicity; predictive value; rupture process; seismicity cluster; swarm.