Observations on the Stress Related Variations of Soil Radon Concentration in the Gulf of Corinth, Greece

Our observations indicate a characteristic pattern in the long-term variation of soil radon concentrations, which seems to be consistent with the expected variation of regional stress in relation to seismicity. However, it seems that the major changes in radon level begin before the rock rapture, i.e. before the earthquake occurs. These conclusions have emerged after long-term observations with continuous and thorough real-time gamma-radiation monitoring in the seismically active area of the Gulf of Corinth, Greece. The recordings acquired close to a hot spring were of very high quality, implying that the deep hydraulic flow can possibly play a key role in the pre-earthquake variation of radon level. We were able to observe outstanding examples of radon level variations before significant seismic events in the Gulf of Corinth that cannot be attributed to other external factors such as atmospheric phenomena

Evidence for stability in coda Q associated with the Egion (central Greece) earthquake of 15 June 1995

International audienceOne hundred seventy-two seismograms, recorded at one seismological station located close to the source region of the Egion (Ms = 6.2) 15 June 1995 earthquake, were used to measure values of Q from the decay of the earthquake coda. The collected data were compared between events that occurred before and after the main event. The analysis showed no significant temporal variation in the value of coda Q for the region

Shallow attenuation in the west Corinth-Patras rift, Greece

International audienc

Regional ground‐motion prediction equations for amplitude‐, frequency response‐, and duration‐based parameters for Greece

Although all of the main properties of a ground motion cannot be captured through a single parameter, a number of different engineering parameters has been proposed that are able to reflect either one or more ground‐motion characteristics concurrently. For many of these parameters, especially regarding Greece, there are relatively few or no predictive models. In this context, we present a set of new regionally‐calibrated equations for the prediction of the geometric mean of the horizontal components of 10 amplitude‐, frequency response‐, and duration‐based parameters for shallow crustal earthquakes. These equations supersede previous empirical relationships for Greece since their applicability range for magnitude, and epicentral distance has been extended down to Mw 4 and up to 200 km, respectively, the incorporation of a term accounting for anelastic attenuation has been investigated, while their development was based on a ground‐motion dataset spanning from 1973 to 2014. For all ground‐motion parameters, we provide alternative optimal equations relative to the availability of information on the different explanatory variables. In all velocity‐based and contrary to the acceleration‐based parameters, the anelastic attenuation coefficient was found statistically insignificant when it was combined with the geometric decay and the coefficient accounting for saturation with distance. In the regressions where the geometric decay coefficient simultaneously incorporated the contribution of anelastic attenuation, its increase was found to be much less considerable in the velocity‐based than in the acceleration‐based parameters, implying a stronger effect of anelastic attenuation on the parameters that are defined via the acceleration time history

Strong ground acceleration seismic hazard in Greece and neighboring regions

In an early paper [Tectonophysics 117 (1985) 259] seismic hazard in Greece was analyzed using a relatively homogeneous earthquake catalogue spanning 1900–1978 and a strong motion attenuation relationship adapted to use in Greece. Improved seismic hazard analyses are obtained here using Gumbel's asymptotic extreme value distribution applied to peak horizontal ground acceleration occurrence, but now taking into account the increased length and quality of earthquake catalogue data spanning 1900–1999 and the burgeoning information on earthquake strong motion data and attenuation relationships appropriate for Europe and, explicitly, Greece. Seismic acceleration hazard results tabulated for six cities reveal (e.g. using arbitrarily the 50-year p.g.a. with 90% probability of not being exceeded) changes of about 10% in the new calculated values: two cities show an increase and four a decrease. These are relatively small and reassuring adjustments. Inspection of the available attenuation relationships leads to a preference for the models of Theodulidis and Papazachos, particularly with the model modification to produce a ‘stiff soil’ site relationship, as these relationships explicitly exploit the Greek strong motion database. Isoacceleration maps are produced for Greece as a whole from each attenuation relationship inspected. The final set of maps based on the Theodulidis and Papazachos models provide a foundation for comparison with the Seismic Hazard Zones adopted in the New Greek Seismic Code where scope can be found to modify zone shape and the level at which p.g.a.s are set. It should be noted that the generation of the present isoacceleration maps is based on a seismogenic zone-free methodology, independent of any Euclidean zoning assumptions

Temporal variation of shear-wave splitting parameters before and after the 2008 Movri Mountain earthquake in northwest Peloponnese (Greece)

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 pre-existing micro-crack system characteristics related to the Movri Mountain earthquake, and the possible involvement of over-pressured fluids

A catalogue of seismicity in Greece and the adjacent areas for the twentieth century

A magnitude homogeneous earthquake catalogue spanning the twentieth century has been assembled for Greece and adjacent areas. Data assemblage had to consider all of the common magnitude scales: MW, MS, mb, and ML (including ML(ATH) of the National Observatory of Athens (NOA)). Fourteen different correlation equations between magnitude scales were considered in order to approach magnitude homogeneity in the final catalogue. This catalogue contains an estimate of both MW and MS, for each earthquake, and the actual reported values of mb, MS, MW, and ML. The final catalogue includes 5198 earthquakes after having been truncated at 4 MW to eliminate large numbers of patchily reported microearthquakes. These 5198 earthquakes during 1900–1999 are within the area 33–43°N, 18–30.99°E, focal depths 1.0–215 km, and magnitude range 4.0≤MW≤7.7. The surface wave magnitude MS is always estimated for each earthquake to facilitate application of the catalogue to seismic hazard assessment, since seismic attenuation laws are more often expressed in terms of MS rather than MW. Alongside this new catalogue, the data of Papazachos and Papazachou for the period 550 BC–1899 then provide a full picture of the known seismicity of Greece

Local high-resolution passive seismic tomography and Kohonen neural networks — Application at the Rio-Antirio Strait, central Greece

International audienceA high-resolution passive seismic investigation was performed in a 150km2 area around the Rio-Antirio Strait in central Greece using natural microearthquakes recorded during three months by a dense, temporary seismic network consisting of 70 three-component surface stations. This work was part of the investigation for a planned underwater rail tunnel, and it gives us the opportunity to investigate the potential of this methodology. First, 150 well-located earthquake events were selected to compute a minimum (1D) velocity model for the region. Next, the 1D model served as the initial model for nonlinear inversion for a 3D P- and S- velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem using a least-squares method. The retrieved Vp and Vp/Vs images were used as an input to Kohonen self-organizing maps (SOMs) to identify, systematically and objectively, the prominent lithologies in the region. SOMs are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data. This analysis revealed the existence of five major clusters, one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes. The survey results provide, for the first time, a 3D model of the subsurface in and around the Rio-Antirio Strait. It is the first time that passive seismic tomography is used together with SOM methodologies at this scale, thus revealing the method’s potential

Temporal variation of shear-wave splitting parameters related to Movri Mountain earthquake in northwest Peloponnese (Greece)

<p>Geophysical Research Abstracts<br> Vol. 14, EGU2012-8549, 2012<br> EGU General Assembly 2012</p

Ambient noise tomography of the western Corinth Rift, Greece

International audienceThree years of continuous waveform data recorded at 22 stations from the Corinth Rift Laboratory and the Hellenic Unified Seismological Network are used to perform an ambient noise surface-wave tomography of the western Corinth Rift. All available vertical component time-series were cross-correlated to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves were measured for each station-pair by applying frequency-time analysis and then inverted to build 2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s, the spatial variation of the group velocities is correlated with the surface geology of the area. Lower velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within the underlying basement, our study highlights the presence of a distinct zone of lower velocities across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation of this low velocity includes two arguments, the present-day active tectonic regime and the possible involvement of fluids circulation processes at depth within a highly fractured upper crust in the vicinity of the major faults zones. In general, the results demonstrate good agreement with the major geological and tectonic features of the area, as well as with previous local earthquake tomography studies and support the assumption of fluid circulations at depth. This work intends to be the base for further investigations towards the study of the Corinth Rift structure using long-time series of ambient noise data