Focal mechanisms in the southern Aegean from temporary seismic networks – implications for the regional stress field and ongoing deformation processes

Abstract

Abstract. The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located microearthquakes were analysed to produce 540 wellconstrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini-Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW-SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE-SSW. Intermediate-depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east-west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW-SE compression. Tensional principal stresses in the crust exhibit very good alignment with extensional strain rate principal axes derived from GPS velocities except in volcanic areas, where both appear to be unrelated, and in the forearc where compressional principal stresses are very well aligned with compressional principal strain rates. This finding indicates that, except for volcanic areas, microseismic activity in the southern Aegean is not controlled by small-scale local stresses but rather reflects the regional stress field. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the southeast, segmentation of the slab, arc volcanism and extension of the Aegean crust.

Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

“…Highest sea wave amplitudes ( Fig. 1) were observed on the south coast of Amorgos (20-30 m) and the north coast of Astypalea (10-20 m) by eye witnesses and are seen from marine flood deposits (Ambraseys, 1960;Galanopoulos, 1960;Dominey-Howes, 1996;Dominey-Howes et al, 2000;Okal et al, 2009), whereas on the coastlines of the two islands which faced away from the Santorini-Amorgos graben structure as well as on the northern coastline of Crete, on the Dodecanese islands and the Turkish west coast wave amplitudes of less than 4 m were observed. The only exception is the island Folegandros, where wave amplitudes of up to 14.6 m were observed locally on the western coast.…”

“…More than 3200 buildings were severely damaged. Most of these were on Santorini, where the second event apparently effected the final collapse of houses already damaged by the first event (Ambraseys, 1960;Galanopoulos, 1982).…”

“…While the two earthquakes have been unanimously located in the Santorini-Amorgos graben, focal parameters and focal depth are controversial (Table 1) or affected by large uncertainties (Galanopoulos, 1982;Comninakis and Papazachos, 1986;Makropoulos et al, 1989). For the first event, Shirokova (1972) proposed a fault plane solution of a ENE-WSW striking normal fault with a slight dip to the SSE, calculated from first-motion polarities of P phases (Fig.…”

“…Map of locations of the two earthquakes on 9 July 1956 at 03:11 and 03.24 UTC investigated in this study. Also shown are isoseismal lines and tsunami heights (in meters) taken from eye witnesses (Ambraseys, 1960;Galanopoulos, 1960;Okal et al, 2009) and studies of sedimentary deposits (Dominey-Howes et al, 2000).…”

“…The northern graben southeast of Amorgos only shows low seismic activity Brüstle, 2012). Stress tensor inversions of fault plane solutions Friederich et al, 2014) derived from available first P motion polarities of shallow highquality hypocenter locations indicate a SE-NW oriented tensional principal stress axis.…”