Seismic imaging of fluid-filled inherited structures of the Northern Thessaly (Greece) seismic gap

e present the first seismic imaging of the crustal volume affected by the March April 2021 Thessaly sequence by applying a 3D seismic tomography to the
aftershocks recorded by an unprecedented number of stations. The results, in
terms of VP, VS, and VP/VS ratio and earthquakes’ location parameters, depict blind
fluid-filled inherited structures within the Northern Thessaly seismic gap. The
tomographic images highlight the basal detachment accommodating the
Pelagonian nappe onto the carbonate of the Gavrovo unit. The high VP/VS
(>1.85) where most of the seismicity occurs increases from SE to NW, showing
possible fluid accumulation in the NW edge of the seismogenic volume that could
have contributed to the sequence evolution. The aftershock relocations correlate
well with the fault planes of the three mainshocks proposed by several geodetic
models, but also show additional possible faults sub-parallel and antithetical to the
main structures, not to be overlooked for future seismic risk mitigation

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