ORIGINAL ARTICLE A study on off-fault aftershock pattern at N-Adria microplate Gianni Bressan & Carla Barnaba & Andrea Magrin & Giuliana Rossi Received: 13 November 2017 /Accepted: 13 February 2018 # Springer Science+Business Media B.V., part of Springer Nature 2018 Abstract The spatial features of the aftershock se- quences triggered by three moderate magnitude events with coda-duration magnitudes 4.1, 5.1 and 5.6, which occurred in Northeastern Italy and Western Slovenia, were investigated. The fractal dimension and the orienta- tions of the planar features fitting the hypocentral data have been inferred. The spatial organization is articulated through two temporal phases. The first phase is charac- terized by the decreasing of the fractal dimension and by vertically oriented planes fitting the hypocentral foci. The second phase is marked by an increase of the fractal dimension and by the activation of different planes, with more widespread orientation. The aftershock temporal distribution is analysed with a model based on a static fatigue process. The process is favoured by the decrease of the overburden pressure, the sharp variations of the mechanical properties of the medium and the unclamping effect resulting from positive normal stress changes caused by the mainshock stress step. Keywords Off-fault aftershock . Aftershock sequences . Fractal dimension . PCA analysis . Static fatigue 1 Introduction The aftershocks mainly occur in zones of increased stress induced by the mainshock. Their spatial pattern, however, can be complex. Das and Henry (2003) claimed that in most cases, the aftershocks are clustered in the areas at the edge of the fault where the mainshock causes a stress increase. Furthermore, aftershocks are often located on the mainshock fault in areas which slipped less or where sharp changes from high to low slip occur. The case of off-fault aftershocks appears distinctive. The analysis of seismic sequences induced by events of moderate magnitude occurred in Northeast- ern Italy, and Western Slovenia (Bressan et al. 2007), in many cases, does not show preferential alignments along the mainshock fault plane or indicates that the spatial pattern of the aftershocks can be resolved on a discrete plane. Das and Henry (2003) pointed out that the off-fault aftershocks can occur only on preexisting favourably oriented weak planes close to failure. Das and Scholz (1981) formulated a theory about the after- shock mechanism, based on the phenomenon of subcrit- ical crack growth using the concepts of linear elastic fracture mechanics. Scholz (1990) emphasized that the aftershocks are delayed fractures, caused by a static fatigue process involving the slow growth of cracks, along or in the neighbourhood of the mainshock fault plane. The dy- namic stress step caused by the mainshock loads the neighbouring zones to stresses much higher than their long-term strength. These zones, characterized by a time-dependent strength, fail by static fatigue with a J Seismol https://doi.org/10.1007/s10950-018-9737-x G. Bressan : C. Barnaba : A. Magrin (*) OGS, Centro Ricerche Sismologiche, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Via Treviso 55, 33100 Udine, Italy e-mail: amagrin@inogs.it G. Rossi OGS, Centro Ricerche Sismologiche, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Borgo Grotta Gigante, 42/c, 34010 Trieste, Italy