ANISOTROPY STUDY OF THE FEBRUARY 4TH 2008 SWARM IN NW PELOPONNESUS (GREECE)

The Gulf of Corinth, located in central Greece, is characterized by normal faulting and by high seismicity since the antiquity. On 4 February 2008 a seismic swarm burst in NW Peloponnesus characterized by the occurrence of two moderate-size earthquakes of moment magnitudes 4.7 and 4.5, respectively. Analysis of the data revealed the existence of shear-wave splitting. The events that were chosen for the anisotropy study fit the selection criteria, having clear and impulsive S wave arrival phases on the horizontal components. In addition, the amplitude of the S wave phase on the vertical component was smaller than on the horizontal ones. The representations that were used to determine the polarization direction of the fast split shear wave, the time delay between the two split shear waves and the polarization direction of the source were the polarigram and the hodogram. The uniform fast shear wave polarizations, irrespective of the azimuth of each event, are consistent with the general NNE-SSW direction of extension in the Gulf and, therefore, in agreement with the extensive dilatancy anisotropy (EDA) model. Finally, a decrease of the time delay values was observed after the occurrence of the first moderate earthquake, implying changes of the medium’s properties

Evidence of shear-wave splitting in the eastern Corinthian Gulf (Greece)

The analysis of local earthquakes recorded by the Cornet network and located in the Gulf of Corinth (Greece) has revealed the existence of shear-wave splitting. The visual inspection technique is used to estimate the polarization direction of the fast shear wave and the time delay between the two split shear waves. The selected earthquakes are located close to one of the Cornet stations. Most of these earthquakes are recorded by only one station and their azimuth and angle of incidence are estimated using the covariance matrix decomposition method combined with the P-wave polarization direction. Polarigrams plotted for the horizontal components present a clear linear and almost constant polarization for each station, independent of the azimuth of the earthquake, except for one station where different S(fast) polarization directions are observed. The mean direction of the fast shear wave polarization at Paradeisi station is 146°N, at Sofiko station 104°N and at Villia station 142°N. At Desfina station, two different main S(fast) polarization directions were observed, one 143°N and the other 55°N. The calculated time delays between the two split shear waves are higher for the stations located in the eastern part of the Gulf. Comparing the mean S(fast) polarization direction with the direction of local faulting, we observe that they are approximately parallel at Sofiko station, while at Villia station they are almost perpendicular. In general, the obtained mean S(fast) polarization directions at the Cornet stations are perpendicular to the direction of the extension of the Gulf which is NNE-SSW and consistent with the extensive dilatancy anisotropy (EDA) model

An updated and extended earthquake catalogue for Greece and adjacent areas since 1900

A homogeneous earthquake catalogue for Greece and adjacent areas covering the period 1900–2009 is presented, to be used for reliable seismic hazard studies. The catalogues of Makropoulos and Burton (1981) and Makropoulos et al. (1989), covering the time span 1900–1985, were updated for the period 1986–2009 using instrumentally determined focal coordinates, except for the magnitude from the bulletin of the ISC. For <i>M</i><sub>s</sub>, which is the magnitude scale included in the previous versions, the same procedure applied since 1964 was adopted, using the ISC body wave magnitude (<i>m</i><sub>b</sub>) and a regression equation. In the present update, <i>M</i><sub>w</sub> is also calculated for the entire period 1900–2009 using a formula derived from all available moment magnitudes and directly determined by the moment tensor inversion method. Thus, a magnitude homogeneous catalogue concerning both <i>M</i><sub>s</sub> and <i>M</i><sub>w</sub> scales is presented. The extended catalogue contains 7352 events, 70% more than the 4310 events of the previous published (1989) version. The completeness test revealed that the catalogue is complete for magnitudes above 4 for the last 34 yr and that no earthquake with magnitude 6 or greater has been omitted in the whole instrumental era (1900–2009)

The MW = 6.3 2003 Lefkada earthquake (Greece) and induced stress transfer changes

A large earthquake of magnitude MW = 6.3 occurred on 14 August 2003 NW of the Lefkada Island, which is situated at the Ionian Sea (western Greece). The source parameters of this event are determined using body-wave modeling. The focal depth was found equal to 9 km, the constrained focal mechanism revealed dextral strike-slip motion (φ = 15°, Δ = 80° and λ = 170°), the duration of the source time function was 8 s and the seismic moment 2.9 × 1025 dyn cm. The earthquake occurred close to the northern end of the Kefallinia transform fault, where the 1994 moderate event and its aftershock sequence were also located. The epicentral distribution of the 2003 aftershock sequence revealed the existence of two clusters. The first one is located close to the epicentral area of the mainshock, while the second southern, close to the northwestern coast of the Kefallinia Island. A gap of seismicity is observed between the two clusters. The length of the activated zone is approximately 60 km. The analysis of data revealed that the northern cluster is directly related to the mainshock, while the southern one was triggered by stress transfer caused by the main event. © 2006 Elsevier B.V. All rights reserved

An updated and extended earthquake catalogue for Greece and adjacent areas since 1900

A homogeneous earthquake catalogue for Greece and adjacent areas covering the period 1900-2009 is presented, to be used for reliable seismic hazard studies. The catalogues of Makropoulos and Burton (1981) and Makropoulos et al. (1989), covering the time span 1900-1985, were updated for the period 1986-2009 using instrumentally determined focal coordinates, except for the magnitude from the bulletin of the ISC. For M s, which is the magnitude scale included in the previous versions, the same procedure applied since 1964 was adopted, using the ISC body wave magnitude (m b) and a regression equation. In the present update, M w is also calculated for the entire period 1900-2009 using a formula derived from all available moment magnitudes and directly determined by the moment tensor inversion method. Thus, a magnitude homogeneous catalogue concerning both M s and M w scales is presented. The extended catalogue contains 7352 events, 70% more than the 4310 events of the previous published (1989) version. The completeness test revealed that the catalogue is complete for magnitudes above 4 for the last 34 yr and that no earthquake with magnitude 6 or greater has been omitted in the whole instrumental era (1900-2009). © Author(s) 2012

Spatiotemporal properties of seismicity and variations of shear‐wave splitting parameters in the western gulf of corinth (Greece)

The Western Gulf of Corinth (WGoC) exhibits significant seismicity patterns, combining intense microseismic background activity with both seismic swarms and short‐lived aftershock sequences. Herein, we present a catalogue of ~9000 events, derived by manual analysis and dou-ble‐difference relocation, for the seismicity of the WGoC during 2013–2014. The high spatial resolution of the hypocentral distribution permitted the delineation of the activated structures and their relation to major mapped faults on the surface. The spatiotemporal analysis of seismicity revealed a 32‐km‐long earthquake migration pattern, related to pore‐pressure diffusion, triggering moderate mainshock‐aftershock sequences, as fluids propagated eastwards in the course of ~15 months. The anisotropic properties of the upper crust were examined through automatic shear‐wave splitting (SWS) analysis, with over 2000 SWS measurements at local stations. An average fast shear‐wave polarization direction of N98.8° E ± 2.8° was determined, consistent with the direction of the maximum horizontal regional stress. Temporal variations of normalized time‐delays between fast and slow shear‐waves imply alterations in the level of stress or microcrack fluid saturation during the long‐lasting pore‐pressure diffusion episode, particularly before major events. The present study provides novel insights regarding seismicity patterns, active fault structures, anisotropic properties of the upper crust and triggering mechanisms of seismicity in the WGoC. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Modeling landslide susceptibility in Greece: a weighted linear combination approach using analytic hierarchical process, validated with spatial and statistical analysis

A new model for Greece depicting, at a national scale, the spatial distribution of areas prone to landslide phenomena, taking into account preparatory and triggering criteria, is presented. This is a baseline study for prioritizing research in hazardous areas with more detail. Landslide occurrence is very high in certain areas of the Greek territory, such as Western Greece. Analysis is performed by applying two models, both based on slopes and geotechnical data, earthquake occurrence and precipitation. The first, which yields better results, also takes into account land use data. Since there was no access to landslide inventories, landslide susceptibility was performed on empirical estimates of the aforementioned criteria, using the analytic hierarchy process in order to derive the proper weights for each criterion. The final outcome is calculated based on the weighted linear combination of the above-mentioned criteria. The present study covers the area of Greece with an accuracy of 500 m × 500 m grid cell size. Results have been validated with observed events, landsliding urban areas and other studies, thus identifying landslide prone areas in a satisfying manner. © 2016, Springer Science+Business Media Dordrecht