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Relative Seismic Quiescence Before Large Aftershocks (2000)

Drakatos, G.

Springer

Pure and applied geophysics 157 (2000), S. 1407-1421

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ISSN: 1420-9136

Keywords: Key Words: Aftershocks, prediction, quiescence, Greece.

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract —In terms of the modified Omori’s formula and Akaike’s Information Criterion, thirteen aftershock sequences have been analysed quantitatively, to check if any anomalous change of seismic activity took place before the occurrence of the largest aftershock. The analysis follows the method proposed by . The epicentre distribution of the main shocks covers the Aegean Sea (Greece) area and the surrounding regions. The earthquakes occurred from 1988 to 1998.¶Although difficulties existed for a quantitative treatment of seismic quiescence, the results of the temporal analysis of the aftershock activities seem to be promising. In eight of thirteen cases the observed temporal pattern is similar. The aftershock activity decreases significantly before the occurrence of the largest aftershock, with respect to the level expected according to the modified Omori’s formula. Thereafter, the activity recovers to its previous level just before or immediately after the occurrence of the main aftershock. Moreover, an estimation of the reliability of the method is made, based on the results of this study as well as of previous similar investigations. From a total of 41 aftershock sequences which have been analysed following Matsu’ura’s method, 29 of them (71%) show a remarkable relative quiescence before the occurrence of the main shock.¶Although it is difficult to construct a model which explains the precursory pattern, the real time monitoring and check for seismic quiescence could at least contribute to a ‘qualitative’ prediction of the largest aftershock which could at times be as large and disastrous as the main shock.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/PL00001126

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2

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3-D velocity structure beneath the crust and upper mantle of Aegean Sea region (1991)

Drakatos, G. ; Drakopoulos, J.

Springer

Pure and applied geophysics 135 (1991), S. 401-420

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ISSN: 1420-9136

Keywords: Seismic tomography ; Aegean ; velocity structure ; Greece ; Mediterranean

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract The region of the Aegean Sea and the surrounding areas in the Eastern Mediterranean lies on the boundary zone between the Eurasian and the African plates. It is a zone of widespread extensive deformation and, therefore, reveals a high level of seismicity. Three-dimensional velocity structure, beneath the crust and upper mantle of the region between 33.0°N–43.0°N and 18.0°E–30.6°E, is determined. The data used are arrival times ofP-waves from 166 earthquakes, recorded at 62 seismological stations. In total, 3973 residual data are inverted. The resultant structure reveals a remarkable contrast of velocity. In the top crustal layer, low velocities are dominant in Western Turkey and on the Greek mainland, while a high velocity zone is dominant in the Ionian Sea and in the southern Aegean Sea. In the upper mantle, high velocity zones dominate along the Hellenic arc, corresponding to the subducting African plate and in the northern part of the region, corresponding to the subducting African plate and in the northern part of the region, corresponding to the margin of Eurasian plate. A low velocity zone is dominant in the Aegean Sea region, where large-scale extension and volcanic activity are predominant, associated with the subduction of the African plate.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00879472

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3

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A quantitative study of some aftershock sequences in greece (1994)

Latoussakis, J. ; Drakatos, G.

Springer

Pure and applied geophysics 143 (1994), S. 603-616

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ISSN: 1420-9136

Keywords: Aftershock ; prediction ; Greece

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract Temporal features of the aftershock activities following twelve moderate shallow earthquakes in Greece have been studied quantitatively, by making use of the modified Omori's formula and Akaike's Information Criterion (AIC). Although the earthquake sequences occurred in different seismotectonic regions in Greece, similar seismic patterns before the occurrence of large aftershocks have been observed. Aftershock activity shows an appreciable decrease from the level expected from the modified Omori formula before the occurrence of the large aftershock. Also in some cases, the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the large aftershock, if the aftershock activity is monitored immediately after the occurrence of the main shock. In such case, the observed pattern may be useful in predicting a large aftershock.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00879500

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4

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Some features of aftershock patterns in Greece (1996)

Drakatos, G. ; Latoussakis, J.

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 126 (1996), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: A quantitative study of 12 aftershock sequences, which occurred in Greece and the surrounding regions from 1971 to 1980, has been made. The aim of the study was to investigate the temporal pattern of each aftershock sequence and to examine the possibility of ‘predicting’ the largest aftershock. The magnitudes of the main shocks were ML 〉 5.0. The minimum and maximum numbers of aftershocks were 25 and 191, respectively.The temporal analysis was performed using the modified Omori formula and Akaike information criterion. First, we examine the Omori law to see if holds for the whole aftershock sequence. Second, we examine the same data set in order to find out if there is any possibility of ‘distinguishing’ two aftershock sequences as follows: (1) the first must include all the shocks since the occurrence of the main event until the occurrence of the largest aftershock and (2) the second must include all events following the occurrence of the largest aftershock until the end of the aftershock activity. In 10 cases out of 12, the model of two different aftershock sequences provides the best fit to the data.The aftershock activity prior to the large aftershocks was then examined. Although the main shocks occurred in different seismogenic regions, an almost identical behaviour of aftershock activity was observed. The activity shows an appreciable decrease from the level expected from the modified Omori formula. Also, in some cases, the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the main aftershock. These results seem to be independent of the magnitude of the ‘large’ aftershock.If the aftershock activity is monitored in real time, this observed temporal pattern may be useful in predicting a large aftershock.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb05272.x

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5

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Conventional and modern seismic investigations for rock quality determination at a dam site—a case history (1995)

Louis, J. ; Papadopoulos, T. ; Drakatos, G. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Geophysical prospecting 43 (1995), S. 0

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ISSN: 1365-2478

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences , Physics

Notes: Refraction seismics and modern tomographic surveys were employed at the Platanovrissi dam site on the Nestos river in northern Greece in an effort to investigate the dynamic elastic properties and rockmass condition of gneiss-schist outcropping in the area under investigation. Six seismic refraction traverses were run on the ground surface, inside the exploratory tunnels and between tunnels and the ground surface. The results provided information about the low-velocity surface layer parameters and the stress relief zones on the walls of all the tunnels in which seismic tomography tests were performed. Ultrasonic tests, conducted both in situ and in the laboratory, enabled the determination of the ratio Vp/Vs and the Poisson's ratio calculation. From the tomography data analysis and the geological interpretation of the results, three main velocity zones, ranging between 3500 and 4000, 3000 and 3500 and less than 3000m/s, were detected in each tunnel. High-velocity zones were found to be compatible with the hard rockmass of gneiss-schist material, while moderate velocities are closely related to jointed and fractured zones. Finally, low-velocity zones were well correlated with weathered gneiss-schist material.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-2478.1995.tb00280.x

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6

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Spectral characteristics of the 1986 September 13 Kalamata (southern Greece) earthquake (1989)

Stavrakakis, G. N. ; Drakopoulos, J. ; Latoussakis, J. ; [et al.]

Oxford, UK : Blackwell Publishing Ltd

Geophysical journal international 98 (1989), S. 0

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ISSN: 1365-246X

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: The seismic source parameters seismic moment M0, source dimension r, fault length l, average displacement (u), shear-stress drop Δs̀, radiated energy Es, and apparent stress n(s̀) are calculated for the Kalamata (southern Greece) earthquake of 1986 September 13. These source parameters are obtained by using teleseismic P and SH-wave spectra and making use of three independent spectral parameters, the long period spectral level Ω0, the corner frequency f0, and the parameter ε which controls the high-frequency decay of the displacement spectrum. The calculated far-field spectra are based on the direct phases of P-and S-waves and they have been corrected for the instrumental response and for an average radiation pattern assuming normal faulting mechanism. The striking feature of the obtained results is the low stress drop value of about 6–7 bar which is in deviation from the average value (30 bar) for interplate earthquakes. A seismic moment of 1.9 times 1025 and 2.2 times 1024 dyne·cm, a fault length of about 22 and 11 km, and an average displacement of 15 and 9 cm are obtained on the basis of P- and S-waves, respectively.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb05520.x

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The seismic sequence of January-February 2014 at Cephalonia Island (Greece): constraints from SAR interferometry and GPS (2015)

Briole, P., Elias, P., Parcharidis, I., Bignami, C., Benekos, G., Samsonov, S., Kyriakopoulos, C., Stramondo, S., Chamot-Rooke, N., Drakatou, M. L., Drakatos, G.

Oxford University Press

In: Geophysical Journal International

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Publication Date: 2015-10-22

Description: We analysed the ground deformation produced by the M w = 6.1 2014 January 26 and M w = 6.0 2014 February 3 Cephalonia earthquakes, western Greece. Campaign GPS measurements and RADARSAT-2 synthetic aperture radar (SAR) interferometry provide constraints on the overall deformation produced by the sequence. TerraSAR-X and COSMO-SkyMed SAR interferometry provide constraints on the second earthquake separately. Two permanent GPS stations captured the two coseismic offsets and show no pre- or post-seismic transients. Most of the deformation is concentrated in the Paliki peninsula which is consistent with the location of the seismicity and the damages. Both GPS and SAR interferometry indicate areas with large deformation gradients probably due to shallow effects. Given the limitations on the data and on the knowledge of the structure and rheology of the crust, we used a simple elastic model to fit the ground displacements. Although such model cannot fit all the detail of the deformation, it is expected to provide a robust estimate of the overall geometry and slip of the fault. The good data coverage in azimuth and distance contributes to the robustness of the model. The entire sequence is modelled with a strike slip fault dipping 70° east and cutting most of the brittle crust beneath Paliki, with an upper edge located at 2.5 km depth and a deeper edge at 8.5 km. This fault is oriented N14° which corresponds to the azimuth of the Cephalonia Transform Fault (CTF). The fit to the data is significantly improved by adding a secondary shallow strike-slip fault with low dip angle (30°) with a component of reverse faulting on that shallow fault. The modelling of the February 3 event indicates that the faulting is shallow in the north of Paliki, with a centroid depth of ~3.2 km. The fit is improved when a single planar fault is replaced by a bent fault dipping ~30° in the uppermost 2 km and ~70° below. The fault of the January 26 earthquake, inferred from the difference between the two above models, is located south and beneath the February 3 fault, with a centroid depth of ~6.4 km. We interpret the 2014 fault zone as an east segment of the CTF located ~7 km east of the main axis of the CTF, which location is constrained by the elastic modelling of the interseismic GPS velocities. The aftershock sequence is mostly located between the January 26 fault and the axis of the CTF. According to our analysis, the Paliki peninsula is partly dragged north with the Apulian platform with ~7 mm yr –1 of shear accommodated offshore to the west. During the last 30 yr three main sequences occurred along the CTF, in 1983, 2003 and 2014 breaking a large part of the fault, with a gap of 20–40 km left between Cephalonia and Lefkada.

Keywords: Seismology

Print ISSN: 0956-540X

Electronic ISSN: 1365-246X

Topics: Geosciences

Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).

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Assessment of Earthquake-Induced Landslide Hazard in Greece: From Arias Intensity to Spatial Distribution of Slope Resistance Demand (2016)

Chousianitis, K., Del Gaudio, V., Sabatakakis, N., Kavoura, K., Drakatos, G., Bathrellos, G. D., Skilodimou, H. D.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2016-01-27

Description: The earthquake-induced landslide hazard in Greece is evaluated by means of a parametric time probabilistic approach. First, by taking into account the characteristics of seismicity affecting Greece, we calculated occurrence probabilities for different levels of seismic shaking expressed in terms of Arias intensity for a time interval of 50 yrs. Then, by utilizing the linkage of Newmark displacement with Arias intensity and critical acceleration, we mapped the spatial distribution of the slope strength demand that imply a 10% probability that displacement will exceed critical thresholds for potential seismic triggering of incoherent and coherent landslides. The obtained results suggest that the areas of western Greece and the broader region of the Gulf of Corinth are the most exposed to conditions of seismic triggering of both kinds of landslides. The derived slope strength demand maps could provide the basis for the assessment of whether particular slopes have a significant failure probability by comparing the estimated strength demand with the actual critical acceleration values calculated from slope material properties and slope angle. We exemplified this approach at a coastal area of the western Gulf of Corinth, where, by considering situations expected in dry and wet periods, we also demonstrated that earthquake effects on slope stability depend on the season of earthquake occurrence.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics