Monthly migration of a tectonic seismic swarm detected by DInSAR: southwest Peloponnese, Greece

In the period 2011 June-October, a tectonic swarm of nearly 1222 earthquakes occurred in the Messenia prefecture at the southwestern region of the Peloponnese Peninsula. The swarm happened in the Messenia's Upper Quaternary basin, 25 km NW of the city of Kalamata, and migrated from NNW towards SSE. The largest earthquakes occurred in 2011 August 14 (M w = 4.8), September 14 (M w = 4.6) and October 10 (M w = 4.7), caused moderate structural damages mainly in old houses in four villages and produced particular unrest to the local population. We have investigated the monthly migration of the swarm using Differential Synthetic Aperture Radar Interferometry (DInSAR), presenting for the first time a very close look at the deformation evolution that may reveal an aseismic slip component of the total movement. The geodetically derived slip distribution for the first 4 months revealed that slip migrated laterally along strike (north to south) and vertically from a deep portion, at ∼2.8 km depth, to a shallow portion, at less than 0.5 km, of the fault plane, and concluded its migration towards the surface with a very shallow M w 4.7 event of 2011 October 10 surprisingly detected by DInSAR.