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Drakatos, G.
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Drakatos, G.
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- PublicationRestrictedOn the Mw 6.4 SW-Achaia (western Greece) earthquake sequence of 8 June 2008: Seismological, field, GPS observations and stress modeling(2008-12-15T21:56:02Z)
; ; ; ; ; ; ; ;Ganas, A.; Institute of Geodynamics, National Observatory of Athens ;Serpelloni, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Drakatos, G.; Institute of Geodynamics, National Observatory of Athens ;Kolligri, M.; Institute of Geodynamics, National Observatory of Athens ;Adamis, I.; Institute of Geodynamics, National Observatory of Athens ;Tsimi, Ch.; Institute of Geodynamics, National Observatory of Athen ;Batsi, E.; ; ; ; ; ; On 8 June 2008 an Mw(NOA)=6.4 earthquake occurred in NW Peloponnesus, western Greece. This event is the largest strike-slip earthquake to occur in western Greece during the past 25 years. The hypocentre was determined at 18 km depth beneath village Mihoi in SW Achaia. No surface rupture was observed. Many rock falls, slides and liquefaction features have been found as is typical for an earthquake of this size. Double-difference relocations of 370 aftershocks show a linear pattern of events and define a clear NE-SW striking mainshock fault plane. The aftershock region extends approximately 30 km in length, and the width of the surface projection of the aftershocks is as large as 10 km. The depth of the aftershocks rarely exceeds 22 km. Analysis of high-rate GPS data showed that station RLS (Riolos) which is located 12.8 km N5°W of the epicentre was displaced co-seismically 7 mm to the North in agreement with right-lateral kinematics of the rupture. Static (Coulomb) stress transfer analysis indicates loading of faults near the towns of Patras (north) and Amaliada (south), respectively. The earthquake put more emphasis on the role of strike-slip fault in the deformation of western Greece also indicating that seismic strain is partitioned between strike-slip and normal-slip events due to obliquity of the Nubia (Africa) subduction and the N-S extension of the overriding Aegean upper plate156 40 - PublicationOpen AccessP-wave crustal tomography of Greece with use of an accurate two-point ray tracer(1997-01)
; ; ; ;Drakatos, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Karantonis, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Stavrakakis, G. N.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece; ; The three-dimensional velocity structure of the crust in the Aegean sea and the surrounding regions (34.0º-42.OºN, 19.0ºE-29.0ºE) is investigated by inversion of about 10000 residuals of arrival times of P-wave from local events. The resulting velocity structure shows strong horizontal variations due to the complicated crustal structure and the variations of crustal thickness. The northern part of the region generally shows high velocities. In the inner part of the volcanic arc (Southern Aegean area), relatively low velocities are observed, suggesting a large-scale absorption of seismic energy as confirmed by the low seismicity of the region. A low velocity zone was observed along the subduction zone of the region, up to a depth of 4 km. The existence of such a zone could be due to granitic or other intrusions in the crust during the uplift of the region during Alpidic orogenesis.169 292 - PublicationOpen AccessThe seismic sequence of January–February 2014 at Cephalonia Island (Greece): constraints from SAR interferometry and GPS(2015-12-01)
; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ;; ; ;We analysed the ground deformation produced by the Mw = 6.1 2014 January 26 and Mw = 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.108 115 - PublicationRestrictedThe Mw 6.4 SW-Achaia (western Greece) earthquake sequence of 8 June 2008: Seismological, field, GPS observations and stress modeling(2009-10-20)
; ; ; ; ; ; ; ;Ganas, A.; Institute of Geodynamics, National Observatory of Athens, Athens, Greece ;Serpelloni, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Drakatos, G.; Institute of Geodynamics, National Observatory of Athens ;Kolligri, M.; Institute of Geodynamics, National Observatory of Athens ;Adamis, I.; Institute of Geodynamics, National Observatory of Athens ;Tsimi, Ch.; Institute of Geodynamics, National Observatory of Athen ;Batsi, E.; Institute of Geodynamics, National Observatory of Athen; ; ; ; ; ; On June 8, 2008 a Mw = 6.4 earthquake occurred in NW Peloponnese, western Greece. This event is the largest strike-slip earthquake to occur in western Greece during the past 25 years. No surface rupture was observed. Many rock falls, slides, and liquefaction features have been found as is typical for an earthquake of this size. Double-difference relocations of 370 aftershocks show a linear pattern of events and define a clear NE-SW striking mainshock fault plane. The hypocentrer was determined at 18 km depth beneath village Mihoi in SW Achaia. The 24-hr aftershock region extends approximately 30 km in length, and the width of the surface projection of the aftershocks ranges between 5–10 km. The depth of the aftershocks rarely exceeds 22 km. Analysis of high-rate GPS data showed that station RLS (Riolos) which is located 12.8 km N5 W of the epicenter was displaced co-seismically 7 mm to the North in agreement with right-lateral kinematics of the rupture. Static (Coulomb) stress transfer analysis indicates loading of faults near the towns of Patras (north) and Amaliada (south), respectively. The earthquake put more emphasis on the role of strike-slip in the deformation of western Greece also indicating that seismic strain is partitioned between strike-slip and normal-slip events due to obliquity of the Nubia (Africa) – Eurasia convergence.166 25 - PublicationOpen AccessA tomography image of the Aegean region (Greece) derived from inversion of macroseismic intensity data(1997-01)
; ; ; ; ;Stavrakakis, G. N.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Drakatos, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Karantonis, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Papanastassiou, D.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece; ; ; The three-dimensional attenuation structure beneath the Aegean sea and the surrounding regions was determined by inversion of seismic intensity data. A large number of seismic intensity data have been accumulated in a uniform scale in the Aegean region, where the seismic activity is much higher than that of the other Mediterranean regions. Nearly 11000 seismic intensity data from 47 earthquakes that have occurred in these regions were used to determine the seismic attenuation structure. The resulting structure reveals a remarkable contrast of attenuation. In the top layer (depth 0-20 km), low Q is dominant in the central Aegean sea, while high Q is dominant in the surrounding land areas, except for Southwestern Turkey. The low-Q regions correspond to areas of Neogene-Quaternary grabens where the high seismicity of shallow earthquakes appears. In the lower layer (20-40 km) low-Q areas are located in the southeastern part of the Hellenic arc. Some low-Q spots corresponding to the distribution of volcanoes exist along the volcanic arc. The low-Q spots might correspond to diapirs causing subduction volcanism.138 140 - PublicationRestrictedDisplacements recorded on continuous GPS stations following the 2014 M6 Cephalonia (Greece) earthquakes: Dynamic characteristics and kinematic implications(2015-01-03)
; ; ; ; ; ;Ganas, A.; Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece ;Cannavò, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Chousianitis, K.; Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece ;Kassaras, I.; Department of Geology, University of Athens ;Drakatos, G.; Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece; ; ; ; We report cm-size dynamic displacements of continuous GPS stations onshore the island of Cephalonia, Ionian Sea, Greece, following the passage of seismic waves from two (2) shallow earthquakes on Jan 26, 2014 and Feb 3, 2014, respectively. First, we estimated the displacements from the high-rate GPS data collected at NOA station VLSM, near to the epicenters, by using state-of-art data processing strategies. The time series of displacements were analyzed both in time and frequency domains. From the dynamic analysis of 1Hz data it is shown that the second event was recorded at station VLSM with higher amplitudes on both horizontal components, despite its smaller (22 %) moment magnitude, possibly due to its shallower depth. The static field of deformation is characterized by cm-size permanent motion in opposing directions between stations KIPO (western Cephalonia) and VLSM (eastern Cephalonia), in agreement with the right-lateral kinematics of both ruptures. The 7.4 cm northward motion of station KIPO implies that the western peninsula of Cephalonia island (Paliki) belongs to a separate crustal block with respect to the rest of the island. The northward motion of KIPO also implies that the Cephalonia Transform Fault (CTF) did not rupture during the 2014 events, because KIPO is located at the hanging wall of CTF. It is possible that the amount of accumulated strain along CTF since 1983 (M=6.8) can be released by a seismic event of M6.5-6.7, at any time.237 44 - PublicationOpen AccessTomographic image of the crust and uppermost mantle of the Ionian and Aegean regions(1997-01)
; ; ; ; ; ; ; ;Alessandrini, B.; Istituto Nazionale di Geofisica, Roma, Italy ;Beranzoli, L.; Istituto Nazionale di Geofisica, Roma, Italy ;Drakatos, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Falcone, C.; Istituto Nazionale di Geofisica, Roma, Italy ;Karantonis, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Mele, F. M.; Istituto Nazionale di Geofisica, Roma, Italy ;Stavrakakis, G. N.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece; ; ; ; ; ; We present a tomographic view of the crust and uppermost mantle beneath the Central Mediterranean area obtained from P-wave arrival times of regional earthquakes selected from the ISC bulletin. The P-wave velocity anomalies are obtained using Thurber's algorithm that jointly relocates earthquakes and computes velocity adjustments with respect to a starting model. A specific algorithm has been applied to achieve a distribution of epicentres as even as possible. A data set of 1009 events and 49072 Pg and Pn phases was selected. We find a low velocity belt in the crust, evident in the map view at 25 km of depth, beneath the Hellenic arc. A low velocity anomaly extends at 40 km of depth under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian sea close to the Calabrian and Aegean arcs. The tomographic images suggest a close relationship between P-wave velocity pattern and the subduction systems of the studied area.240 164