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Abstract:
In March 2021 an earthquake sequence, including three Mw ~6 normal fault earthquakes on the 3rd, the 4th and the 12th of March, shook Thessaly in central Greece. The epicentres of the three mainshocks propagated northwestward to rupture the northern tip of a large-scale (>100 km) fault relay structure that traverses central Greece. InSAR analysis coupled with field investigations show that only the first and largest earthquake is characterized by a clear rupture that reached the surface along a mapped fault with sharp geomorphic signature. In contrast, the following two large earthquakes were blind, despite the presence of clear pre-existing fault traces near the respective epicentres. Here, we present a kinematic source optimization for which we combine teleseismic waveforms, InSAR and GNSS displacements together with geological constraints to estimate the first-order rupture parameters for location, geometry, slip mechanism and rupture dynamics. This multidisciplinary approach provides a robust source solution, particularly for the first two earthquakes that occurred less than 24 hours apart. The short interval between these events means that with InSAR data the two ruptures were only separately imaged from ascending satellite look direction. In our optimization we consider the individual data errors in data weighting and we perform a fully Bayesian source model uncertainty analysis. Our results reveal the sequential activation of at least 8 individual faults along the relay fault zone, including numerous syn-seismically activated secondary faults. These, together with remapping and analysis of regional faults, reveal the presence of a mature large-scale relay zone.
