Transient aseismic vertical deformation across the steeply-dipping Pisia-Skinos normal fault (Gulf of Corinth, Greece)

Geodetically-derived deformation rates are sometimes used to infer seismic hazard, implicitly assuming that short-term (annual-decadal) deformation is representative of longer-term deformation. This is despite geological observations indicating that deformation/slip rates are variable over a range of timescales. Using geodetic data from 2016-2021, we observe an up to 7-fold increase in vertical deformation rate in mid-2019 across the Pisia-Skinos normal fault in Greece. We hypothesise that this deformation is aseismic as there is no temporally correlated increase in the earthquake activity (M>1). We explore four possible physical mechanisms, and our preferred hypothesis is that the transient deformation is caused by centimetre-scale slip in the upper 5km of the Pisia fault zone. This is the first observation of shallow tectonic (i.e. not related to human activities) aseismic deformation on a normal fault globally. Our results suggest that continental normal faults can exhibit variable deformation over shorter timescales than previously observed, and thus care should be taken when utilising geodetic rates to quantify seismic hazard.