A detailed seismic anisotropy study during the 2011–2012 unrest period in the Santorini Volcanic Complex G. Kaviris ⇑ , P. Papadimitriou, Ph. Kravvariti, V. Kapetanidis, A. Karakonstantis, N. Voulgaris, K. Makropoulos Department of Geophysics–Geothermics, University of Athens, Panepistimiopolis, 157 84 Zografou, Greece article info Article history: Received 20 June 2014 Received in revised form 5 November 2014 Accepted 6 November 2014 Available online 15 November 2014 Keywords: Earthquake Santorini Volcanic Complex Seismic anisotropy APE model Temporal variation of normalized time delays Stress forecast abstract The Santorini Volcanic Complex (SVC) is an area in the Southern Aegean (Greece) which has been char- acterized by low seismicity rates for the last decades, especially in the Santorini Caldera where they have been very low until 2010. This pattern changed completely in February 2011, when intense microseismic activity was initiated within the Caldera. During the manual analysis of the events, the shear-wave split- ting phenomenon was observed, revealing the existence of an anisotropic upper crust in the SVC area. A detailed anisotropy study has been conducted using 231 events within the shear-wave window that ful- filled the selection criteria. The polarization direction of the fast shear-wave, the time-delay between the two split shear-waves and the source polarization direction were calculated after visual inspection, using both the polarigram and the hodogram representations. This procedure, applied for eight local stations, resulted in the determination of 340 splitting parameters. The obtained mean anisotropy directions are not homogeneous, revealing a complex regime in the activated area. Nevertheless, these results are explained by the APE model, related to the stress-sensitive behavior of fluid-saturated microcracked rocks. A detailed analysis of the temporal evolution of both the time-delay and anisotropy direction was carried out. The time-delays measured in the ‘‘band-1’’ window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity, as well as to the major M w = 5.1 and 5.2 events which took place about 40 km SW of Santorini on 26 and 27 January 2012, respectively. On the other hand, no significant temporal variations or 90° flips of the S fast polarization direction were observed. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The Santorini Volcanic Complex (SVC) is a part of the Greek Vol- canic Arc, situated in the back arc of the Greek subduction zone, and consists one of the most important volcanic centers of the Aegean (Papazachos and Comninakis, 1971; Angelier et al., 1982). The SVC consists of five different islands: Santorini, Therasia, Aspronisi, Palea Kameni and Nea Kameni. The spatial distribution of the first three islands forms a ring inside which Palea and Nea Kameni lie (Vougioukalakis, 2002). Concerning the tectonic setting of Santorini (Fig. 1), the conti- nental crust under the island complex is approximately 30 km thick, while the region at the NE of the island is characterized by alternating horsts and grabens, separated from each other by normal NE–SW trending faults (Perissoratis, 1995; Piper and Perissoratis, 2003). These faults are formed by an extensional stress field trending NNW–SSE (Mercier et al., 1989), with the local compressional field in a NE–SW direction, and are passing below the submarine volcano Colombo and the Santorini Caldera (Druitt et al., 2002). They are also characterized as a kind of ‘flower type’ structure which was developed above a strike-slip fault running below the Columbo tectonic line (Sakellariou et al., 2010) and the Columbo cape (Vespa et al., 2006). These faults are the pathways through which the generated underlying magma reaches the surface, forming the Columbo volcanic chain (Nomikou et al., 2014a). Another important formation is the Kameni line of faults (Pyle and Elliott, 2006), which lies under the islands of Nea and Palea Kameni and the submarine volcano Columbo, trending NE–SW. The volcanic activity of the island was initiated 0.6 million years ago and the formation of the present caldera took place after the Minoan explosion in 1610–1650 B.C. (Fouque, 1879; Bond and Sparks, 1976; Heiken and McCoy, 1984; Papadopoulos and Chalkis, http://dx.doi.org/10.1016/j.pepi.2014.11.002 0031-9201/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +30 210 7274841; fax: +30 210 7274787. E-mail addresses: gkaviris@geol.uoa.gr (G. Kaviris), ppapadim@geol.uoa.gr (P. Papadimitriou), fedrakra@geol.uoa.gr (Ph. Kravvariti), vkapetan@geol.uoa.gr (V. Kapetanidis), akarakon@geol.uoa.gr (A. Karakonstantis), voulgaris@geol.uoa. gr (N. Voulgaris), kmacrop@geol.uoa.gr (K. Makropoulos). Physics of the Earth and Planetary Interiors 238 (2015) 51–88 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi