Seismic anisotropy around the Gulf of Corinth, Greece, deduced from three‐component seismograms of local earthquakes and its relationship with crustal strain

Abstract

Several thousand three-component seismograms from local earthquakes recorded during two field experiments in August 1991 and November !992 in the Gulf of Corinthhave been analyzed to detect shear wave splitting. After a first selection of the events located in the S window of the considered stations, a second very strict selection of the records is applied in order to avoid the effect of scattered or converted phases which can mimic the behavior of shear wave splitting. Two main directions of fast $ wave polarization have been detected: one oriented N105øE-N!20øE, the other N55øE-N75øE. The first one is perpendicular to the main direction of extension of the Gulf provided by focal mechanism, Global Positioning System measurements, and tectonic studies, and is thus consistent with the extensive-di!atancy anisotropy (EDA) model. The second direction is subparallel to the direction of the active normal fault closest to the sites. This suggests a local control of the anisotropy by these active faults, either by a local rotation of the total stress field, in which case the EDA model may still explain the anisotropy, or by the existence of a specific microstructure or macrostructure generated by the long-term fault activity (set of secondary fault planes parallel to the major one ), in which case the anisotropy direction would be significantly rotated from the stress direction (about 50ø). The anisotropic signature does not seem to be affected by the geology of the site (pre-Tertiary limestone and Pleistocene sediments), except for a station located on the thick Plio-Quaternary deposits of a delta, where the time delay is significantly larger. Introduction Numerous studies of shear wave polarization from local earthquakes have pointed out shear wave splitting in widely differing areas of the world [e.g., De Chabalier el el., 1992; Zollo and Bernard, 1989; Booth ei al., 1985; Iannaccone and Deschamps, 1989]. Crampin [1978] proposes that this effect results from the existence of anisotropy in the upper crust due to the presence of nearly vertical fluid-filled cracks, approximatively parallel to the maximum horizontal compressive stress direction. This is the hypothesis of extensive-dilatancy a•isotropy or EDA [ Crampin, 1985] which results in a seismic anisotropy of hexagonal symmetry with a symmetry axis normal to the direction of the maximum compression. A shear wave propagating through the anisotropic medium splits into two orthogonal components which travel at different velocities and thus sep-1Now at Paper number 95JB03464. 0148-0227/96/95JB_03464505.00 arate in time. The polarization direction of the fast split shear wave is nearly parallel to the direction of the cracks alignment and therefore parallel to the maximum horizontal stress direction. The time delay between the two split S waves is related to the crack density, to the path length through the anisotropic medium and to the direction of the path with respect to crack orientation. Nevertheless, there is now growing evidence that the EDA model cannot be an u...

Preliminary Shear-Wave Splitting Results in the Broader Aigion Area (Greece) During 2013

“…An important exception is the mean polarization direction in PSAR station by Bouin et al (1996), where its value was found equal to N70E and N60E, for 1991 and 1992, respectively, employing an automatic measurement procedure, while the current study obtained a measurement equal to N98E, in agreement with the N105E direction found by , who utilized the same visual inspection method as the current study. Results from station VVK are presented for the first time.…”

“…In comparison to older studies, our analysis provided varied differences. For instance, the mean time delay value measured in 2013 in the area of Psaromyta (PSAR station) is far higher, equal to 88 ms, than the ones observed by Bouin et al (1996), which were 38 ms for 1991 and 54 ms for 1992. The opposite is observed for LAKA station, where Kaviris et al ( , 2010 found time delay ranges (24 -104 ms and 10 -130 ms, respectively) similar to those derived by the current study.…”

Anisotropy Study of the February 4th 2008 Swarm in Nw Peloponnesus (Greece)

“…The shear-wave splitting phenomenon was observed in Milos island (Booth et al, 1989), Kalamata (Bouin, 1994), Almyros (Karnassopoulou, 1996), Gulf of Corinth (Bouin et al, 1996;Papadimitriou et al, 1999;Kaviris et al, 2008) and Attica (Papadimitriou et al, 2000), leading to conclusions for the local stress-field.…”

Seismic and electrical anisotropy in the Mornos Delta, Gulf of Corinth, Greece, and its relationship with GPS strain measurements

“…Crampin (1978) In the present study, we report and compare seismic and electrical anisotropy of the shallow crust at a site where we do not expect strong lateral heterogeneties, and where we could constrain the strain by GPS measurements. We selected the delta of the Mornos river, on the northern coast of the Gulf of Corinth (Greece), where a temporary experiment in 1991 has revealed a clear shear-wave splitting, interpreted as a crackinduced anisotropy in the sediments in relationship with the extensional strain of the area (Bouin et al, 1996).…”