The earthquake intervent time distribution along the Hellenic subduction Zone.

Η ελληνική ζώνη υποβύθισης είναι η πιο σεισμική περιοχή της Ευρώπης. Η κατανομή Weibullέχει χρησιμοποιηθεί σαν μοντέλο για την περιγραφή των χρονικών διαστημάτων μεταξύ διαδοχικών σεισμών σε συγκεκριμένες ρηξιγενείς ζώνες (Rikitake, 1976; Rikitake, 1991), καθώς και για την περιγραφή της σεισμικότητας μίας ευρύτερης περιοχής (Hasumietal., 2009). Taδεδομένα που αναλύουμε σε αυτή την εργασία αφορούν την ελληνική ζώνη υποβύθισης και καλύπτουν την περίοδο 1976-2009. Χρησιμοποιούμε τις σεισμικές πηγές επιφανειακών σεισμών όπως αυτές ορίζονται από τους PapaioannouandPapazachos(2000), καθώς και τονκατάλογο σεισμών των Makropoulosetal. (2012) για την Ελλάδα και τις παρακείμενες περιοχές. Η εφαρμογή της κατανομής Weibullστα χρονικά διαστήματα μεταξύ διαδοχικών σεισμών αναλύεται και περιγράφεται σε αυτή την εργασία.The Hellenic Subduction Zone (HSZ) is the most seismically active region in Europe (Becker and Meier, 2010). The evolution of such an active region is characterized by complex phenomenology and is expressed through seismicity. Seismicity temporal patterns remain as one of the most important topics in earth sciences. The Weibull distribution has been used as a recurrence time model for large earthquakes (Rikitake, 1976; Rikitake, 1991). Moreover,Hasumi et al. (2009) used the Weibulllog Weibull distribution for the study of the interoccurrence times of earthquakes in Japan.The dataset formed in this study concerns the seismic belt of the HSZ during the period 1976-2009. We use the external seismic sources of shallow earthquakes in the Aegean and the surrounding area (Papaioannou and Papazachos, 2000) along with the updated and extended earthquake catalogue for Greece and adjacent areas (Makropoulos et al., 2012).The application of the Weibull distribution to the interevent times of the formed dataset is analyzed and discussed

Fluid geochemistry investigations on the volcanic system of methana

An extensive geochemical survey on the fluids released by the volcanic/geothermal system of Methana was undertaken. Characterization of the gases was made on the basis of the chemical and isotopic (He and C) analysis of 14 samples. CO2 soil gas concentration and fluxes were measured on the whole peninsula at more than 100 sampling sites. 31 samples of thermal and cold groundwaters were also sampled and analysed to characterize the geochemistry of aquifers. Anomalies referable to the geothermal system, besides at known thermal manifesta-tions, were also recognized at some anomalous degassing soil site and in some cold groundwater. These anomalies were always spatially correlated to the main active tectonic system of the area. The total CO2 output of the volcanic system has been preliminary estimated in about 0.2 kg s-1. Although this value is low compared to other volcanic systems, anomalous CO2 degassing at Methana may pose gas hazard problems. Such volcanic risk, although restricted to limited areas, cannot be neglected and further studies have to be undertaken for its better assessmen

Diffuse and focused carbon dioxide and methane emissions from the Sousaki geothermal system, Greece

We report first data on chemical composition of the gas emitted by the geothermal system of Sousaki, Greece. Gas manifestations display typical geothermal gas composition with CO2 as the main component and CH4 and H2S as minor species. Soil gas composition derives from the mixing of two end-members (atmospheric air and geothermal gas). Soil CO2 fluxes range from<2 to 33,400 g m2 d1. The estimated diffuse output of hydrothermal CO2, estimated for an area of 0.015 km2, is about 630 g s1, while a tentative estimation of CH4 diffuse output gave a value of about 1.15 g s

Dynamic Multifractality in Earthquake Time Series: Insights from the Corinth Rift, Greece

Earthquake time series are widely used to characterize the main features of seismicity and to provide useful insights into the dynamics of the seismogenic system. Properties such as intermittency and non-stationary clustering are common in earthquake time series such that multifractal concepts seem essential to describe the temporal clustering variability. Here we use a multifractal approach to study the time dynamics of the recent earthquake activity in the Corinth rift. The results indicate the degree of heterogeneous clustering and correlations acting at all time scales that suggest non-Poissonian behavior. Additionally, the multifractal analysis in different time periods showed that the degree of multifractality exhibits strong variations with time, which are associated with the dynamic evolution of the earthquake activity in the rift and the transition between periods of high and low seismicity

GEOCHEMICAL CHARACTERIZATION OF THERMAL AND COLD GROUNDWATERS OF METHANA PENINSULA (PELOPONNESUS, GREECE)

A comprehensive hydrogeochemical study of the cold and thermal groundwaters of the presently quiescent volcanic system of Methana was undertaken collecting 59 natural water samples during the period 2004-2007. Methana is a peninsula whose climatology and hydrology can be compared to the nearby small islands of the Aegean Sea. Similarly the chemical and isotopic composition of its water is dominated by the mixing of seawater with meteoric water. But the simple mixing trend is modified by water-rock interaction processes, enhanced by the dissolution of endogenous CO2, leading to strong enrichments in Alkalinity, Calcium, Barium, Iron and Manganese

Static chamber methane flux measurements in volcanic/geothermal areas: preliminary data from Sousaki and Nisyros (Greece)

Methane plays an important role in the Earth’s atmospheric chemistry and radiative balance being the second most important greenhouse gas after carbon dioxide. Methane is released to the atmosphere by a wide number of sources, both natural and anthropogenic, with the latter being twice as large as the former (IPCC, 2007). It has recently been established that significant amounts of geological methane, produced within the Earth’s crust, are currently released naturally into the atmosphere (Etiope, 2004). Active or recent volcanic/geothermal areas represent one of these sources of geological methane. But due to the fact that methane flux measurements are laboratory intensive, very few data have been collected until now and the contribution of this source has been generally indirectly estimated (Etiope et al., 2007). The Greek territory is geodynamically very active and has many volcanic and geothermal areas. Here we report on methane flux measurements made at two volcanic/geothermal systems along the South Aegean volcanic arc: Sousaki and Nisyros. The former is an extinct volcanic area of Plio-Pleistocene age hosting nowadays a low enthalpy geothermal field. The latter is a currently quiescent active volcanic system with strong fumarolic activity due to the presence of a high enthalpy geothermal system. Both systems have gas manifestations that emit significant amounts of hydrothermal methane and display important diffuse carbon dioxide emissions from the soils. New data on methane isotopic composition and higher hydrocarbon contents point to an abiogenic origin of the hydrothermal methane in the studied systems. Measured methane flux values range from –48 to 29,000 (38 sites) and from –20 to 1100 mg/mˆ2/d (35 sites) at Sousaki and Nisyros respectively. At Sousaki measurement sites covered almost all the degassing area and the diffuse methane output can be estimated in about 20 t/a from a surface of about 10,000 mˆ2. At Nisyros measurements covered the Stephanos and Kaminakia areas, which represent only a part of the entire degassing area. The two areas show very different methane degassing pattern with latter showing much higher flux values. Methane output can be estimated in about 0.25 t/a from an area of about 30,000 mˆ2 at Stephanos and about 1 t/a from an area of about 20,000 mˆ2 at Kaminakia. The total output from the entire geothermal system of Nisyros probably should not exceed 2 t/a

Evidence of non-extensivity in the seismicity observed during the 2011–2012 unrest at the Santorini volcanic complex, Greece

During the period of October 2011–January 2012, an increase of earthquake activity has been observed in the volcanic complex of Santorini Island, Greece. Herein, the magnitude distribution of earthquakes as well as the temporal distribution of seismicity are studied. The statistics of both parameters exhibit complexity that is evident in the frequency-magnitude distribution and the inter-event time distribution, respectively. Because of this, we have used the analysis framework of non-extensive statistical physics (NESP), which seems suitable for studying complex systems. The observed inter-event time distribution for the swarm-like earthquake events, as well as the energy and the inter-event earthquake energy distributions for the observed seismicity can be successfully described with NESP, indicating the inherent complexity of the Santorini volcanic seismicity along with the applicability of the NESP concept to volcanic earthquake activity, where complex correlations exist