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On the origin of upper crustal shear wave anisotropy at Samos Island, Greece

Kaviris G., Spingos I., Kapetanidis V., Papadimitriou P., Voulgaris N.

Acta Geophysica

2021

Vol. 69, no. 3

1051--1064

Shear-wave splitting is associated to diferent sources in the upper crust. Preferentially oriented minerals, stress-aligned microcracks and tectonic structures have all been identifed as causes of seismic anisotropy in the upper crust. However, distinguishing between them and discovering the actual origin of the splitting efect has important implications; changes in the anisotropic properties of the medium related to the behavior of fuid-flled microcracks could have potential connections to the occurrence of an impending signifcant earthquake. The recent 2020 Samos Mw =6.9 event and its associated sequence was a great opportunity to study shear-wave splitting in the area. The spatial constrains in such studies, i.e., the requirement of events located very close to the receivers, did not permit exploring local anisotropy in the past, due to a severe lack of suitable data. To establish a background of splitting, we searched for any appropriate earthquake in a fve-year period preceding the mainshock. We performed an automatic analysis on over 200 event-station pairs and obtained 164 high-quality splitting observations between January 2015 and November 2020. Results indicated a strong connection to local structures; Sfast polarization axes seem to align with faults in the area. However, we also observed a period of increasing and decreasing time-delays, associated with an Mw =6.3 earthquake that occurred on June 2017 near Lesvos Island. The latter behavior implies the possibility of stress-induced anisotropy in the area. Thus, the Samos Island could be represented by two diferent sources of splitting; structures to the NW and microcracks to the SE.

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

Kaviris G., Fountoulakis I., Spingos I., Millas C., Papadimitriou P., Drakatos G.

Acta Geophysica

2018

Vol. 66, no. 6

1341--1357

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.