N. S. Melis

Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations

[1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back‐arc and the near‐trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back‐arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE‐SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back‐arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench‐parallel flow is probably located beneath the shallow‐dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW‐SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench‐parallel flow within the mantle wedge.

Waveform Relocation and Focal Mechanism Analysis of an Earthquake Swarm in Trichonis Lake, Western Greece

Abstract In early April 2007, a series of moderate earthquakes (M L 4.1–4.8) oc-curred in the area of Trichonis Lake in western Greece. The earthquake activity waswell recorded by the Hellenic Broadband Seismic Network ( HL ) operated by the Na-tional Observatory of Athens. Initial locations for 156 events of the swarm showed adiffuse image of seismicity. Subsequently, 101 events are precisely relocated, calcu-lating source-specific station terms and differential travel times from waveform crosscorrelation. Uncertainties in relocations are estimated with a bootstrap approach byrandomly weighting the original picks and the differential times. Additionally, wave-forms of seven out of the eight largest earthquakes of the swarm were inverted inorder to derive regional moment tensor solutions. The results showed a tight north-northwest–south-southeast cluster located on an offshore extension of a similarlyoriented fault trace mapped onshore. Moment tensor solutions indicate normal fault-ing with a substantial component of left-lateral strike-slip motion. It is possible thatthis identified fault forms part of a link that connects the Gulf of Corinth rift system tothe west-northwest–east-southeast fault zone south of Trichonis basin.Online Material: Event catalog and focal mechanism for the eight largest events.Introduction