Nicholas Voulgaris

Moderate‐to‐large earthquakes in rifts may occur on leading boundary faults or inner antithetic faults. Here we show a rare case of the 2020–2021 seismic sequence in the Corinth rift, that culminated in the shallow rupture of the antithetic fault, neither preceded nor followed by the leading fault rupture. The hypocenter of the largest shock (Mw 5.3 of 17 February 2021) was located at ∼8 km depth. However, seismic waveform data, supported by satellite‐geodetic and tide gauge measurements, pointed to rupture at shallow depth (∼3 km), where no earthquakes were previously observed. We show that the earthquake most probably ruptured two orthogonal, conjugate fault segments: a weak nucleation phase occurred in the microseismically highly active sub‐horizontal detachment layer, followed – a few seconds later – by a larger, shallow moment release on a high‐angle, south‐dipping normal fault. The latter is the Mornos offshore fault, antithetic to the leading, north‐dipping Psathopyrgos fault. Our study presents the first instrumental/observational evidence of a very shallow Mw 5+ event in this rift – and one of the few reported worldwide. The depth limit of the main shallow slip patch coincides with the expected crossing of the Mornos fault with the Psathopyrgos fault, stressing the importance of fault segmentation and rooting inherited from the rift history. This unusual shallow slip in a depth range with little background seismicity and few aftershocks needs to be further investigated by dynamic modeling as a possible prototype of hazardous events in rift environments.