Coseismic and aseismic normal fault slip in Central Greece from InSAR time series

Normal faults in a rate-and-state friction model release seismic energy in distinct, instantaneous seismic events (i.e. earthquakes) between steady state periods. However, recent geodetic work in the Gulf of Corinth, Greece suggests that some seismogenic normal faults can also undergo transient aseismic slip events above steady state deformation rates in interseismic periods. Integrating the full range of fault slip behaviours into fault evolution frameworks is required to better constrain how normal faults accommodate and release strain with implications for rift development and seismic hazard. Therefore, further detailed observation of both coseismic and aseismic slip behaviours across normal faults at all time scales are needed.

In this analysis, we exploit open-source, vertical ground motion data from the European Ground Motion Service (EGMS), derived from five-years of Interferometric Synthetic Aperture Radar (InSAR) measurements, to evaluate uplift and subsidence in areas of active tectonics. While vertical ground motion data likely reflects a range of geological, hydrological and anthropogenic processes, isolating tectonic signals allows quantification of fault motion on annual to decadal time scales using the Europe-wide dataset. Therefore, this data bridges an important time-scale gap between event-specific InSAR studies and geological assessments and provides regional context to ground motion. Here, we use time series spanning 2019 to 2023 to assess vertical ground motion across normal faults in Central Greece that have, and have not, hosted large earthquakes in this period.

Spatio-temporal ground motion analysis is conducted for the March 2021, Mw > 6 earthquakes in the Larissa Basin (Thessaly). The cascading rupture style of the earthquakes and aftershocks is resolved in EGMS time series, and geometries of uplift and subsidence are plotted to define rupture parameters and fault plane projections. High coseismic uplift to subsidence ratios of 1:6 – 1:9 reflect the tight structural controls on this earthquake sequence. In contrast to Larissa, EGMS time series across the Coastal Fault System of the North Gulf of Evia imply aseismic normal fault slip. Differential vertical ground motion is recorded across both the Kamena Vourla and Arkitsa fault segments with little to no associated seismicity. Time-averaged throw rates of 2 - 3 mm/yr are measured at an uplift to subsidence ratio of 1:2. These throw rates exceed the long-term, geodetic extension rates across the North Gulf of Evia suggesting that the faults are moving in a transient period of elevated aseismic slip between 2019 and 2023. The nearby Atalanti Fault, which hosted two Mw > 6.4 earthquakes in 1894, shows no differential ground motion across its plane reflecting that the fault is in a locked state. The observed variable shallow crustal behaviour of normal faults implies long-term, geologically derived throw rates on normal faults likely combine transient periods of elevated aseismic slip, coseismic slip, and steady state strain accommodation.