**1. Introduction**

Santorini volcano is considered to belong in caldera-forming systems undergoing long-term periods of quiescence, of approximately 20,000 years (Figure 1). Although its last big eruption dates back 3600 years, its volcanic activity up to the most recent eruption in 1950 [1] was intertwined with the building of the intra-caldera islets of Palea and Nea Kameni. The latest volcano's reactivation was followed by the restless period of 2011, which did not culminate in an eruption. Increased microseismic activity [2] and significant ground uplift [3] reaching 14 cm from March 2011 to March 2012 at Cape Skaros (Figure 1), and 9 cm at Nea Kameni islet [4], underlined the seismo-volcanic unrest. Most studies based on geodetic data interpret the episode with a single inflation source due to magma intrusion, at the northern part of the caldera, estimated within 3–4 km depth [4–8]. An alternative model based on GPS data [9], proposes two inflationary magmatic sources that relocate in depth and geographic location throughout the unrest. The evolution of the shallow magma body beneath Santorini appears to have been regulated by the episodic rapid magma supply from a deeper magmatic system, while a significant volume of magma was intruded in short pulses between January 2011 and April 2012 [6,10]. Since the ending of the 2011–12 unrest, the volcano has presented no further activity, as confirmed by geodetic measurements [4,11] and seismicity [2]. Similar behavior, with extended periods of quiescence, interrupted by short non-eruptive activity, is also observed in other caldera volcanic systems [12–14].

**Figure 1.** Location map of Santorini volcano and seismicity distribution for the period 2011–17 [2]. Earthquake foci in gray color represent the unrest period between January 2011 and May 2012 (ML ≤ 3.3), whereas epicenters in red correspond to the post-unrest period from September 2012 to August 2017 (ML ≤ 2.3). KFZ and CFZ represent the Kameni and Columbos Fault Zones, respectively. CS indicates Cape Skaros undergoing the maximum uplift (14 cm) during the 2011–12 unrest episode [4].

Recent advances in geodetic imaging techniques and the systematic availability of Synthetic Aperture Radar (SAR) data from spaceborne missions provided the necessary tools and data, in order to monitor the deformation field of Santorini volcano over the post-unrest period, and to re-evaluate the deformation mechanism during the unrest. MT-InSAR techniques were employed to generate and analyze the SAR deformation time series. A comprehensive analysis of multi-sensor SAR acquisitions of Copernicus Sentinel-1, Radarsat-2 and TerraSAR-X satellite missions was performed to investigate the evolution of ground deformation from 2012 to 2017. The Copernicus Sentinel-1 and Radarsat-2 measurements were further analyzed in an inversion framework to estimate the source parameters that can best resolve the observed displacements, while TerraSAR-X data provided an additional source of information to verify the deformation pattern. Finally, insights on the post-unrest response of the volcano and the interrelationship between the post-unrest interval and the unrest event were investigated, while ERS-1 and -2 and ENVISAT data from 1992 to 2010 [3,4] were additionally used to interpret the similarity between the pre- and post-unrest volcano's deformation.
