Effects of the January 2018 seismic sequence on shear-wave splitting in the upper crust of Marathon (NE Attica, Greece)

Abstract

On the 15th of January 2018, a moderate Mw = 4.2 earthquake occurred in the area of Marathon (NE Attica). This event is of great interest, given that it is located in a region characterized by low seismic activity, even though there are previously identified faults (e.g. Penteli fault). The sequence started 7 days before the mainshock and lasted until the end of the January 2018. In the present study, waveforms from stations of the Hellenic Unified Seismological Network were utilized to explore the seismotectonic regime, using data recorded since 2012. While the focal mechanism of the mainshock is in agreement with known local faults, relocated hypocenters indicate the existence of blind faults. In addition, the proximity of two stations, DION and PTL, to the January 2018 sequence provided a unique opportunity to investigate seismic anisotropy of the upper crust in the study area, for the first time. Analysis of local events between 2012 and 2018 led to the revelation of an anisotropic layer beneath Marathon. However, a complex anisotropy regime is identified. The polarization direction of the Sfast in DION presents stable characteristics and is oriented according to the regional maximum horizontal stress component. However, PTL exhibits rotation to a direction perpendicular to the one that conforms to stress orientation, possibly associated with local faults of similar direction. Time-delays through the whole period are uncommonly low, for such studies, in both stations. Nevertheless, this changes for the January 2018 sequence in DION. Higher values of time-delays permit the establishment of trends that could be attributed to stress accumulation and relaxation, related to the Mw = 4.2 event, in agreement with the Anisotropic Poro-Elasticity model.