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The most reliable of the globally available relative data have been used to derive empirical for mulas which relate the subsurface fault length,

After approximately 60 years of seismic quiescence within Santorini caldera, in January 2011 the volcano reawakened with a significant seismic swarm and rapidly expanding radial deformation. The deformation is imaged by a dense network of 19 survey and 5 continuous GPS stations, showing that as of 21 January 2012, the volcano has extended laterally from a point inside the northern segment of the caldera by about 140 mm and is expanding at 180 mm/yr. A series of spherical source models show the source is not migrating significantly, but remains about 4 km depth and has expanded by 14 million m3since inflation began. A distributed sill model is also tested, which shows a possible N‐S elongation of the volumetric source. While observations of the current deformation sequence are unprecedented at Santorini, it is not certain that an eruption is imminent as other similar calderas have experienced comparable activity without eruption.

SUMMARY This study provides new constraints on the upper‐mantle structure from western Greece to central Anatolia using seismic data of permanent broad‐band networks recently installed in Greece and Turkey and from a two‐year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad‐band stations with a typical interstation distance of 60–100 km across the study area. The high‐resolution 3‐D shear wave velocity model of the mantle is obtained by inversion of fundamental‐mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damping. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3‐D model (0.02–0.1 km s−1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper‐mantle structure never reached at regional scale in the area. The overall structure is characterized by a low‐velocity zone (80–200 km depth) reflecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low‐velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high‐velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high‐velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low‐velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia.

At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short-period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper-crustal structure (<3-km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono-magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E-W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE-SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE-SW trending basin-like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE-SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation.

Compressional velocity structure of the crest and the upper mantle in southeastern Europe (broader Aegean area) is studied by inverting residuals of the first P arrivals from earthquakes in this region (16øE-3 IøE, 34øN43øN). The.dam used are from regional events recorded by the permanent network of stations during the period 1971-1987, enriched with data from experiments with portable seismogmphs in four regions of this broad area. This study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern Mediterranean lithosphere under the southern Aegean and gives further detailed information on the crustal anti upper mantle structure of the area. Important new information is the existence of a low-velocity crustal layer in western Greece and Albania and that the velocity anomaly in the mantle under the southern Aegean extends much farther and deeper to the northeast than the Benloft zone of the intermediate depth earthquakes indicates. Furthermore, evidence is presented about the possible existence of older subduct. ion in the northern Aegean and about the influence of the tectonic regime on the velocity field. Paper number 95JB00669. 0148-0227/95/95J-B.00669505.00 continuation in the northern Aegean together with ttie North Aegean Trough (Figure 1). Contours of average depth of 100 and 160 km intermediate depth earthquakes in the southern Aegean [Papazachos, 1990] are also shown in Figure 1. The velocity structure of the crust and upper mantle in this area has been studied using travel times from local earthquakes [Papazachos et al., !966; Panagiotopoulos. anal Papazachos, 1985] or explosions [Makris, 1972, 1978• Voulgaris, 1991 ], dispersion of surface waves [Papazachos et al., 1967; Calganile et al., 1982], and gravity data [Makris, 1973; Chadas et al., 1992]. A preliminary Moho map of the area based on all the previously published studies is presented in Figure 2 [Papazachos, 1993]. Strong variations of crustal thickness are observed. The ernst is moi'e than 40 km thick under the Dinarides-Hellinides mountain chain but is 1Qcally thinning to less than 25 km in the back are area (Aegean Sea). This thinning, which is also confirreed by gravity studies [Makris, 1976; Chailas et al., 1992; Papazachos, 1994], is mainly attributed to the back ate spreading of the Aegean Sea. The first results from seismic tomography were presented only recently and allowed a more detailed description of the lithosphere and the mantle. These previous studies [$pakman, 1986, 1988; $pakman et al., 1988, 1993; L•gdas et al., 1990; Ligdas and Main, 1991] tried to establish the principal features of the lithosphere-upper mantle system, to a depth of 800 km or more. Other studies concentrated on more local scales [Drakatos, 1989; Drakatos eta].]. The large-scale structure of the crust and uppermost mantle has not been studied in detail. In all the regional studies, the crust is treated as one layer [Drakatos and DrakopouIos, 1991; Ligdas et al., 1990] or as a part of the first layer [$pakman, ...

In the present article new predictive relations are proposed for the peak values of the horizontal components of ground acceleration, velocity, and displacement, using 619 strong motion recordings from shallow earthquakes in the broader Aegean area, which are processed using the same procedure in order to obtain a homogeneous strong motion database. The data set is derived from 225 earthquakes, mainly of normal and strike-slip focal mechanisms with magnitudes 4.5 Յ M Յ 7.0 and epicentral distances in the range 1 km Յ R Յ 160 km that have been relocated using an appropriate technique. About 1000 values of peak ground acceleration (PGA), velocity (PGV), and displacement (PGD) from horizontal components were used to derive the empirical predictive relations proposed in this study. A term accounting for the effect of faulting mechanisms in the predictive relations is introduced, and the UBC (1997) site classification is adopted for the quantification of the site effects. The new relations are compared to previous ones proposed for Greece or other regions with comparable seismotectonic environments. The regression analysis showed a noticeable (up to ϳ30%) variance reduction of the proposed relations for predicting PGA, PGV, and PGD values compared to previous ones for the Aegean area, suggesting a significant improvement of predictive relations due to the use of a homogeneous strong motion database and improved earthquake parameter information.