No abstract

Periods of unrest at caldera-forming volcanic systems characterized by increased rates of seismicity and deformation are well documented. Some can be linked to eventual eruptive activity, while others are followed by a return to quiescence. Here we use a 20 year record of interferometric synthetic aperture radar (InSAR) and GPS measurements from Santorini volcano to further our understanding of geodetic signals at a caldera-forming volcano during the periods of both quiescence and unrest, with measurements spanning a phase of quiescence and slow subsidence (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), followed by a phase of unrest (January 2011 to April 2012) with caldera-wide inflation and seismicity. Mean InSAR velocity maps from 1993-2010 indicate an average subsidence rate of~6 mm/yr over the southern half of the intracaldera island Nea Kameni. This subsidence can be accounted for by a combination of thermal contraction of the 1866-1870 lava flows and load-induced relaxation of the substrate. For the period of unrest, we use a joint inversion technique to convert InSAR measurements from three separate satellite tracks and GPS observations from 10 continuous sites into a time series of subsurface volume change. The optimal location of the inflating source is consistent with previous studies, situated north of Nea Kameni at a depth of~4 km. However, the time series reveals two distinct pressure pulses. The first pulse corresponds to a volume change (ΔV) within the shallow magma chamber of (11.56 ± 0.14) × 10 6 m 3 , and the second pulse has a ΔV of (9.73 ± 0.10) × 10 6 m 3 . The relationship between the timing of these pulses and microseismicity observations suggests that these pulses may be driven by two separate batches of magma supplied to a shallow reservoir. We find no evidence suggesting a change in source location between the two pulses. The decline in the rates of volume change at the end of both pulses and the observed lag of the deformation signal behind cumulative seismicity, suggest a viscoelastic response. We use a simple model to show that two separate pulses of magma intruding into a shallow magma chamber surrounded by a viscoelastic shell can account for the observed temporal variation in cumulative volume change and seismicity throughout the period of unrest. Given the similarities between the geodetic signals observed here and at other systems, this viscoelastic model has potential use for understanding behavior at other caldera systems.

Recent studies have indicated that for the first time since 1950, intense geophysical activity is occurring at the Santorini volcano. Here, we present and discuss the surface deformation associated with this activity, spanning from January 2011 to February 2012. Analysis of satellite interferometry data was performed using two well‐established techniques, namely, Persistent Scatterer Interferometry (PSI) and Small Baseline Subset (SBAS), producing dense line‐of‐sight (LOS) ground deformation maps. The displacement field was compared with GPS observations from 10 continuous sites installed on Santorini. Results show a clear and large inflation signal, up to 150 mm/yr in the LOS direction, with a radial pattern outward from the center of the caldera. We model the deformation inferred from GPS and InSAR using a Mogi source located north of the Nea Kameni island, at a depth between 3.3 km and 6.3 km and with a volume change rate in the range of 12 million m3 to 24 million m3 per year. The latest InSAR and GPS data suggest that the intense geophysical activity has started to diminish since the end of February 2012.

The EUREF Permanent Network Densification is a collaborative effort of 26 European GNSS analysis centers providing series of daily or weekly station position estimates of dense national and regional GNSS networks, in order to combine them into one homogenized set of station positions and velocities. During the combination, the station meta-data, including station names, DOMES numbers, and position offset definitions were carefully homogenized, position outliers were efficiently eliminated, and the results were cross-checked for any remaining inconsistencies. The results cover the period from March 1999 to January 2017 (GPS week 1000-1933) and include 31 networks with positions and velocities for 3192 stations, well covering Europe. The positions and velocities are expressed in ITRF2014 and ETRF2014 reference frames based on the Minimum Constraint approach using a selected set of ITRF2014 reference stations. The position alignment with the ITRF2014 is at the level of 1.5, 1.2, and 3.2 mm RMS for the East, North, Up components, respectively, while the velocity RMS values are 0.17, 0.14, and 0.38 mm/year for the East, North, and Up components, respectively. The high quality of the combined solution is also reflected by the 1.1, 1.1, and 3.5 mm weighted RMS values for the East, North, and Up components, respectively.

S U M M A R YSeismological, GPS and historical data suggest that most of the 40 mm yr −1 convergence at the Hellenic Subduction Zone is accommodated through aseismic creep, with earthquakes of M W 7 rupturing isolated locked patches of the subduction interface. The size and location of these locked patches are poorly constrained despite their importance for assessment of seismic hazard. We present continuous GPS time-series covering the 2008 M W 6.9 Methoni earthquake, the largest earthquake on the subduction interface since 1960. Post-seismic displacements from this earthquake at onshore GPS sites are comparable in magnitude with the coseismic displacements; elastic-dislocation modelling shows that they are consistent with afterslip on the subduction interface, suggesting that much of this part of the interface is able to slip aseismically and is not locked and accumulating elastic strain. In the Hellenic and other subduction zones, the relationship between earthquakes on the subduction interface and observed long-term coastal uplift is poorly understood. We use cGPS-measured coseismic offsets and seismological body-waveform modelling to constrain centroid locations and depths for the 2008 Methoni M W 6.9 and 2013 Crete M W 6.5 earthquakes, showing that the subduction interface reaches the base of the seismogenic layer SW of the coast of Greece. These earthquakes caused subsidence of the coast in regions where the presence of Pliocene-Quaternary marine terraces indicates recent uplift, so we conclude that deformation associated with the earthquake cycle on the subduction interface is not the dominant control on vertical motions of the coastline. It is likely that minor uplift on a short length scale (∼15 km) occurs in the footwalls of normal faults. We suggest, however, that most of the observed Plio-Quaternary coastal uplift in SW Greece is the result of thickening of the overriding crust of the Aegean by reverse faulting or distributed shortening in the accretionary wedge, by underplating of sediment of the Mediterranean seafloor, or a combination of these mechanisms.

The NOANET network is a continuously operating high-rate Global Navigation Satellite System (GNSS) network in Greece whose primary role is to enhance and support geophysical research employing GNSS data. This network is operated by the Institute of Geodynamics of the National Observatory of Athens and currently (September 2020) consists of 26 stations, most of which are located close to major seismogenic structures of Greece to optimally measure tectonic and seismically induced motions. All NOANET receivers are configured to record and collect data with a sampling rate of 1 Hz, although some of them also collect data every 5 Hz on their ring buffer. The network is committed to free and open data sharing within the scientific community, and the collected data are made available via the NOANET data repository and distribution point for all interested parties with no limitations. Integrity, validity, and quality checks of the acquired data are performed using a variety of software tools along with in-house developed programs to supervise the network performance and detect ill-formed data and/or awkward station behavior. In addition, the conventional low-rate GNSS observation data of all NOANET stations are routinely processed on a daily basis to supervise their performance through their position time series. Since the beginning of its establishment, the NOANET network has recorded a variety of deformation signals, and a large number of published papers have used GNSS data from stations that are part of NOANET to constrain, model, and interpret the nature of the associated geophysical phenomena.

Integral field spectroscopy (IFS) provides a unique capability to spectroscopically study extended sources over a 2D field of view, but it also requires new techniques and tools. In this paper, we present an automatic code, Spectroscopic Analysis Tool for intEgraL fieLd unIt daTacubEs, satellite, designed to fully explore such capability in the characterization of extended objects, such as planetary nebulae, H ii regions, galaxies, etc. satellite carries out 1D and 2D spectroscopic analysis through a number of pseudo-slits that simulate slit spectrometry, as well as emission line imaging. The 1D analysis permits direct comparison of the integral field unit (IFU) data with previous studies based on long-slit spectroscopy, while the 2D analysis allows the exploration of physical properties in both spatial directions. Interstellar extinction, electron temperatures and densities, ionic abundances from collisionally excited lines, total elemental abundances and ionization correction factors are computed employing the pyneb package. A Monte Carlo approach is implemented in the code to compute the uncertainties for all the physical parameters. satellite provides a powerful tool to extract physical information from IFS observations in an automatic and user configurable way. The capabilities and performance of satellite are demonstrated by means of a comparison between the results obtained from the Multi Unit Spectroscopic Explorer (MUSE) data of the planetary nebula NGC 7009 with the results obtained from long-slit and IFU data available in the literature. The satellite characterization of NGC 6778 based on MUSE data is also presented.

Dionysos Satellite Observatory and Higher Geodesy Laboratory have been in operation since the 60s and their main objective is to fulfill academic and research needs, determined through the ongoing scientific and technological advance in the field of geodesy. They are involved in all scientific domains related to the determination of earth’s size and figure, as well as its temporal variations. Their field of expertise is Satellite Geodesy, (spanning a wide range of applications like reference systems, tectonic geodesy, etc.), as well as the study of the geoid and earth's gravity field.