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.
Highlights
Significant increases of seismic activity in space and time, termed seismicity clusters, were recognized long ago
Seismicity analysis performed for the entire time interval from 1 January 2014 until the 2018 strong earthquake occurrence and within a radius of R = 30 km around its epicenter shows that four states of seismicity can be distinguished (Figure 4)
Since most aftershocks have small source depths and taking into account that the mainshock fault dips towards the east, we suggest that aftershocks distribution very likely has been controlled by the stress released by the mainshock rupture process and by pre-existing stress in the swarm/foreshock area
Summary
Significant increases of seismic activity in space and time, termed seismicity clusters, were recognized long ago. Pioneering research performed in the 1960s described three main types of clusters: foreshocks, swarms, aftershocks [1,2]. Foreshocks precede a forthcoming stronger earthquake in the same area and in a short time, e.g., in a few days or up to a few months [3,4,5,6,7,8]. Such a type of cluster is usually termed short-term foreshock sequence. Several mechanisms have been proposed for the foreshock generation, e.g., [9,10,11,12,13], they are considered as a promising precursor for the prediction of mainshocks, e.g., [14,15,16]
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