Shear-wave splitting properties of the upper crust, during the 2013–2014 seismic crisis, in the CO2-rich field of Florina Basin, Greece (original) (raw)
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Physics of the Earth and Planetary Interiors
2020
The Florina basin (NW Greece) embodies sources of natural CO 2 emissions, which are aligned parallel to the NE-SW striking regional major normal faults. The study area is characterized by low to moderate seismicity, as historical information and instrumental data reveal. During February 2013 to January 2014 an outburst of seismic activity emerged, consisting of more than 2000 located earthquakes, with a mainshock of M w = 4.1 on 17 February 2013 (16:48 UTC). A local temporary network was installed in July 2013, whose recordings provided the necessary data for performing a Shear-wave Splitting (SwS) study of the upper crust. After the application of strict selection criteria, 532 SwS measurements were obtained for two stations, using a mixture of manual and fully automatic methods. Two polarization directions of the fast shear-wave were identified at one of the stations, with the main one striking NE-SW and the secondary in a NW-SE direction. The first direction is in agreement with the local stress field, as it is identified by focal mechanisms, and with the strike of faults along which soil gas CO 2 concentrations are distributed and can be explained by the Anisotropic Poro-Elasticity (APE) model. The second direction is consistent with the strike of mapped local faults, inherited since the Late Miocene extension, which are responsible for the formation of the Florina basin. The SwS results at the second station revealed an almost N-S dominant polarization direction, which is not consistent with the local stress field. This direction, obtained from events belonging to the southern group of the sequence, that was probably triggered by fluid intrusion, might be interpreted by spatiotemporal patterns of seismicity and CO 2 upwelling.
Geophysical Journal International, 2015
On 2010 January 18 and 22, two earthquakes of Mw 5.3 and 5.2, respectively, occurred near the town of Efpalio on the western Gulf of Corinth. We performed a shear wave splitting analysis using the cross-correlation method and calculated Vp/Vs ratios for events that occurred in the epicentral area of the Efpalio earthquakes, between 2009 January and 2010 December. The data analysis revealed the presence of shear wave splitting in the study area, as well as variations of the splitting parameters and Vp/Vs ratios. The average values of time-delay, fast polarization direction and Vp/Vs ratio for the time period before the Efpalio earthquakes, were calculated at 2.9 ± 0.4 ms/km, 92◦ ± 10◦ and 1.76 ± 0.04, respectively, while after the occurrence of the earthquakes, including the aftershock sequence, they were calculated at 5.5 ± 0.5 ms/km, 82◦ ± 9◦ and 1.88 ± 0.04. A few months after the occurrence of the Efpalio earthquakes, the mentioned splitting parameters were calculated at 3.6±0.4 ms/km and 83◦ ±9◦. Vp/Vs ratio exhibited a mean value of 1.87 ± 0.04. The mean fast polarization directions were in general consistent with the regional stress field, almost perpendicular to the direction of the extension of the Gulf of Corinth. The observed increase in the time-delays and Vp/Vs ratios after the Efpalio earthquakes indicates changes in the crustal properties, which possibly resulted from variations in the pre-existing microcrack system characteristics. We suggest that a migration of fluids in the form of overpressured liquids, which are likely originated from dehydration reactions within the crust, was triggered by the Efpalio earthquakes and caused the observed variations. The findings of this work are consistent with those of previous studies that have indicated the presence of fluids of crustal origin in the study area.
Geophysical Journal International, 2014
A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude Mw 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE–SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30? low-dip angle plane oriented NNE–SSW and a subvertical plane oriented NW–SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW–SE, which were identified by prior marine seismics investigations.
Annals Of Geophysics, 2013
On June 8, 2008, at 12:25 GMT, a Mw 6.4 earthquake, the Movri Mountain earthquake, occurred in the area of northwest Peloponnese, western Greece. The epicenter was located in the municipality of Movri, 35 km southwest of Patras. For this study, a crustal anisotropy analysis was performed in the epicentral area of the Movri Mountain earthquake. Specifically, the shear-wave splitting phenomenon and its temporal evolution in relation to the Movri Mountain earthquake was studied, using the cross correlation method. The data analysis revealed the presence of shear-wave splitting in the study area. Both before and after the Movri Mountain earthquake, the polarization directions of the fast component of the shear waves followed a general NNW-SSE direction. The observed mean fast polarization direction was not consistent with the regional stress field, which showed a general E-W direction of the maximum horizontal compressive stress. The differences between the estimated fast polarization directions and the properties of the regional stress field suggest the presence of a local stress field in the area around the fault. An increase in time delays was observed soon after the Movri Mountain earthquake. The average value of the delay times before the earthquake was ca. 18 ±2.6 ms, while after the earthquake this was ca. 40 ±4.6 ms. This increase in the time delay indicates changes in the crustal properties, which were possibly caused by variations in the preexisting micro-crack system characteristics related to the Movri Mountain earthquake, and the possible involvement of over- pressured fluids.