The 19 July 2019 earthquake in Athens, Greece: A delayed major aftershock of the 1999 Mw = 6.0 event, or the activation of a different structure?

•The 2019 earthquake has occurred at the western part of the 1999 aftershocks zone.•The activated structure in 2019 is likely a blind fault.•The eastern part of the 1999 aftershocks involves more than one activated structures.•The 2019 Mw = 5.1 event is the largest to occur in Attica since the 1999 mainshock. Nearly 20 years after the Mw = 6.0 Athens (Greece) earthquake of 7 September 1999, a significant Mw = 5.1 event struck on 19 July 2019 in roughly the same epicentral region, south of Mt Parnitha, ∼20 km NW of the Athens metropolitan area. In this work, we present a seismotectonic study of the 2019 sequence and investigate its relation with the 1999 aftershock zone and fault geometry. We also take advantage of this opportunity to re-examine the 1999 sequence, using modern cross-correlation and double-difference relocation tools. Waveforms recorded by stations of temporary local networks that were installed at a common site in both case studies are taken into account to identify strongly correlated earthquakes, in terms of waveform similarity, that may indicate a significant degree of resemblance in their source parameters. Our results indicate that the 2019 sequence partially overlaps the western segment of the 1999 aftershock zone. A spatial gap in the middle of that zone, attributed to a high coseismic slip patch during the 1999 mainshock, is still present in 2019, while an isolated cluster of the eastern segment was also reactivated. The relocated hypocenters of both sequences are distributed on nearly coinciding planes that strike ∼N113°-118 °E and dip 48−55° towards SSW, apparently offset by 1−2 km at seismogenic depths, though this difference is attributed to location biases. Their updip extensions outcrop at Mt Parnitha, 1−3 km north of the mapped Fili fault segments or their westwards extension. The 2019 sequence is considered to have occurred at the same structure as the western group of the 1999 aftershocks, which is likely a blind fault.