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Arc volcanoes are underlain by complex systems of molten-rock reservoirs ranging from melt-poor mush zones to melt-rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short-lived or too small to be imaged, which has direct implications for hazard assessment and modelling of magma differentiation. Here we use a high-resolution technology based on inverting full seismic waveforms to image a small, high-melt-fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ~2 to at least 4 km below sea level (b.s.l.) at Kolumbo – a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high-temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive-flow models of magma differentiation.

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An alternative method is proposed for the robust estimation of a and b values of the Gutenberg-Richter relation. The main hypothesis is that b values depend on material properties and the seismotectonic setting and therefore should vary relatively smoothly in space. As far as the a values are concerned, more sharp variations are allowed because these values determine the seismicity level, once the b value is fixed. The study area is organized into a grid, and the a and b values are simultaneously determined for the whole grid by solving an appropriate linear system. Smooth b variations are imposed by introducing additional linear constraints, similar to the Occam's inversion used in tomography studies. The method is applied to Greece and the surrounding area, which is a high seismicity area. The results are in very good agreement with previous studies and further enhance our knowledge for the study area. Moreover, additional seismicity measures (return periods, probabilities, etc.) are estimated robustly because they depend on the a and b values obtained for this area.

The spatial distribution of the aftershocks that followed the September 1999 mainshock (Mw=5.9), which caused severe damage and loss of life in the nearby city of Athens, is examined in the present work. Ρ and S arrivals of seismic waves recorded by the permanent seismic network as well as by a number of digital seismographs and accelerographs, which had been deployed in the broader epicentral area shortly after the mainshock occurrence, were used for the determination of the focal parameters of the mainshock and its aftershocks. The spatial distribution of the aftershocks led to the recognition of the fault, which produced the September mainshock, while certain features of the rupture process may be deduced on the basis of their spatiotemporal variation