Options
Stavrakakis, G. N.
Loading...
Preferred name
Stavrakakis, G. N.
9 results
Now showing 1 - 9 of 9
- 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 - PublicationRestrictedA Reappraisal of the 1894 Atalanti Earthquake Surface Ruptures, Central Greece(2001-08)
; ; ; ; ; ; ;Pantosti, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;De Martini, P. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Papanastassiou, D.; National Observatory Athens, Institute of Geodynamics, P.O. Box 20048, Gr-11810, Greece ;Palyvos, N.; University of Athens, Department of Geology, Panepistimiopolis Zografou, 157 84 Athens, Greece ;Lemeille, F.; Institut de Protection et de Sûreté Nucléaire, F-92265 Fontenay-aux-Roses Cedex, France ;Stavrakakis, G.; National Observatory Athens, Institute of Geodynamics, P.O. Box 20048, Gr-11810, Greece; ; ;; ; This article presents the results of new field and aerial photo surveys of the Atalanti fault and of the mesoseismal area of the 20 and 27 April 1894 earthquakes. Coupled with a reanalysis of contemporary reports and previous investigations, these are used to gain a better understanding of the faults responsible for these events and their seismic behavior. The first shock was smaller and probably located inshore or offshore the Malessina peninsula. No resolving field evidence has been found to locate the seismogenic structure responsible for this shock. On the basis of the limited information available, we suggest the Malessina escarpment, a 12-km-long, ENE-trending, NW-dipping fault as a possible structure responsible for this event. On the other hand, the second and largest shock is definitely related to the Atalanti fault sensu stricto, a main WNW-trending, N-dipping active fault extending between the Platirema valley (a few km NW of the town of Atalanti) and Larymna. The total length of the rupture recognized in the field is about 32 km, but it can be extended further SE up to 40 km. No evidence for a longer rupture extending some other 20 km to the NW, between the Karagiozis river and Ag. Kostantinos, is found. The complex geometry of the fault with bends and step overs appears to be controlled by preexisting transverse structures. Minimum coseismic vertical throws, measured in the field after more than a century elapsed from the earthquake, are 30–80 cm, thus consistent with contemporary reports indicating 1-m average. Slip rates are not well constrained. The available estimates fall in the range 0.1–0.5 mm/yr confirming the smaller amount of crustal extension taking place in this area with respect to other nearby regions such as the Corinth gulf. No new data are available to define the average recurrence interval typical of the Atalanti fault. However, a reconsideration of the existing information induced us to rule out the possibility that the famous 426 B.C. earthquake occurred on the Atalanti fault. On the basis of the extent and size of the rupture recognized in the field, a M 6.8 is estimated for the second and largest shock.171 24 - PublicationOpen AccessCrustal structure of the Gulf of Corinth in Central Greece, determined from magnetotelluric soundings(1997-01)
; ; ; ; ; ;Chouliaras, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Pham, V. N.; Institut de Physique du Globe de Paris, Laboratoire de Geomagnetisme, Paris, France ;Boyer, D.; Institut de Physique du Globe de Paris, Laboratoire de Geomagnetisme, Paris, France ;Bernard, P.; Institut de Physique du Globe de Paris, Dèpartement de Seismologie, Paris, France ;Stavrakakis, G. N.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece; ; ; ; The magnetotelluric sounding method at 15 sites was employed to investigate the electrical properties of the crust and upper mantle near the epicentral region of the June 15 1995, Ms = 6.1, destructive earthquake in the Gulf of Corinth, Central Greece. The magnetotelluric results indicate the presence of a conductive zone in the mid-crust at a depth of 9 to 12 km near the seismogenetic region. The existence of this zone with a thickness of around 7 km can be explained by the presence of fluids in a zone of ductile shear. A second electrical discontinuity was also found at a depth of about 28 km and this may well correspond to the Moho below the Gulf of Corinth.137 137 - PublicationRestrictedShort Term and Short Range Seismicity Patterns in Different Seismic Areas of the World(1999)
; ; ; ; ; ;Console, R.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Di Luccio, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Murru, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Imoto, M.; National Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba-shi, Ibaraki-ken, 305 Japan ;Stavrakakis, G.; National Observatory of Athens, Geodynamic Institute, PO Box 20048, GR-11810, Athens, Greece; ; ; ; The aim of this work is to quantitatively set up a simple hypothesis for occurrence of earthquakes conditioned by prior events, on the basis of a previously existing model and the use of recent instrumental observations. A simple procedure is presented in order to determine the conditional probability of pairs of events (foreshock-mainshock, mainshock-aftershock) with short time and space separation. The first event of a pair should not be an aftershock, i.e., itmust not be related to a stronger previous event. The Italian earthquake catalog of the Istituto Nazionale di Geofisica (ING) (1975–1995, M > 3:4), the earthquake catalog of the Japan Meteorological Agency (JMA) (1983– 1994, M > 3:0) and that of the National Observatory of Athens (NOA) (1982–1994, M > 3:8) were analyzed. The number of observed pairs depends on several parameters: the size of the space-time quiescence volume defining nonaftershocks, the inter event time, the minimum magnitude of the two events, and the spatial dimension of the alarm volume after the first event. The Akaike information criterion has been adopted to assess the optimum set of space-time parameters used in the definition of the pairs, assuming that the occurrence rate of subsequent events may be modeled by two Poisson processes with different rates: the higher rate refers to the space-time volume defined by the alarms and the lower one simulates earthquakes that occur in the nonalarm space-time volume. On the basis of the tests carried out on the seismic catalog of Italy, the occurrence rate of M > 3:8 earthquakes followed by a M > 3:8 mainshock within 10 km and 10 days (validity) is 0.459. We have observed, for all three catalogs, that the occurrence rate density for the second event of a couple (mainshock or aftershock) of magnitude M2 subsequent to a non aftershock of magnitude M1 in the time range T can be modeled by the following relationship: .T , M2/ D 10a 0Cb.M1M2/ with b varying from 0.74 (Japan) to 1.09 (Greece). The decrease of the occurrence rate in time for a mainshock after a foreshock or for large aftershocks after a mainshock, for all three databases, obeys the Omori’s law with p changing from 0.94 (Italy) to 2.0 (Greece).164 21 - PublicationOpen AccessThe October 9, 1996 earthquake in Cyprus: seismological, macroseismic and strong motion data(1999-02)
; ; ; ;Kalogeras, I.; National Observatory of Athens, Geodynamic Institute, Athens, Greece ;Stavrakakis, G.; National Observatory of Athens, Geodynamic Institute, Athens, Greece ;Solomi, K.; Ministry of Agriculture and Natural Resources, Geological Survey Department, Nicosia, Cyprus; ; On October 9, 1996, an earthquake of magnitude 6.8 occurred in the sea area SW of Cyprus, Eastern Mediterranean. This earthquake, which caused damage mostly in the area of Paphos and Limassol, triggered an accelerograph installed at Yermasoyia dam, north of Limassol. The Geodynamic Institute of the National Observatory of Athens in cooperation with the Geological Survey of Cyprus deployed five digital accelerographs in order to record large aftershocks. Although the aftershock activity lasted over four months and included a large number of earthquakes with magnitudes 4.5 and greater, only the largest aftershock of January 13, 1997, having a magnitude of 5.9, triggered two of these five accelerographs. Moreover another digital accelerograph, operated by the Water Development Department of Cyprus, was triggered and this record was also taken into account in this study. The first Cyprean strong motion records obtained to date, gave us the opportunity to compare the results from their analysis to the already proposed attenuation relationships from other areas of the world with a similar seismotectonic regime. Although a general fitting to the attenuation curves for subduction events and strike-slip/reverse fault events was found, the calculated peak ground accelerations were found to be lower than others. Unfortunately, due to the lack of data from previous Cyprean earthquakes, it was not possible to conclude to precise attenuation424 484 - PublicationRestrictedPaleoseismological Trenching across the Atalanti Fault (Central Greece): Evidence for the Ancestors of the 1894 Earthquake during the Middle Ages and Roman Times(2004-04)
; ; ; ; ; ; ;Pantosti, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;De Martini, P. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Papanastassiou, D.; National Observatory Athens, Institute of Geodynamics, P.O. Box 20048, Gr-11810, Greece ;Lemeille, F.; Institut de Protection et de Sûreté Nucléaire, F-92265 Fontenay-aux-Roses Cedex, France ;Palyvos, N.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Stavrakakis, G.; National Observatory Athens, Institute of Geodynamics, P.O. Box 20048, Gr-11810, Greece; ; ;; ; The Atalanti fault bounds to the southwest the Evoikos Gulf, one of the major extensional basins of central Greece. This fault ruptured during the 1894 earthquakes, producing at the surface a complex, ca. 30-km-long rupture. Paleoseismological trenching performed at three sites along this fault provided the first insights on its seismogenic behavior. Unfavorable trench stratigraphy and scarcity of datable material made the identification and characterization of individual paleoearthquakes quite difficult. However, by integrating paleoseismological, geological, historical, and archaeoseismological data, we defined three surface-faulting earthquakes. The most recent event is the 1894 earthquake; the penultimate occurred during the Middle Ages between A.D. 770 and 1160, whereas the third event back occurred in Roman times between 50 B.C. and A.D. 230 and is interpreted to be the Opus earthquake of A.D. 105. These results suggest that 1894-type earthquakes repeat each 660–1120 yr. The average minimum slip per event and vertical slip rates are of the order of 45 cm and 0.4–1.6 mm/yr, respectively. These values are in agreement with other geological estimates and with geodetic measurements. Because of the short time elapsed since the 1894 earthquake, the Atalanti fault does not appear to contain an important seismogenic potential. On the other hand, these results may shed light on the potential of other seismogenic sources threatening the area.193 27 - 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 - PublicationRestrictedA physically based strong ground-motion prediction methodology; application to PSHA and the 1999 Mw = 6.0 Athens earthquake(2007-02)
; ; ; ; ; ; ; ; ;Hutchings, L.; Lawrence Livermore National Laboratory, Hazards Mitigation Center, PO Box 808, L-201, Livermore, CA 94551-0808, USA. ;Ioannidou, E.; Department of Geophysics-Geothermics, University of Athens, Athens 15783, Greece ;Foxall, W.; Lawrence Livermore National Laboratory, Hazards Mitigation Center, PO Box 808, L-201, Livermore, CA 94551-0808, USA. ;Voulgaris, N.; Department of Geophysics-Geothermics, University of Athens, Athens 15783, Greece ;Savy, J.; Lawrence Livermore National Laboratory, Hazards Mitigation Center, PO Box 808, L-201, Livermore, CA 94551-0808, USA. ;Kalogeras, I.; Institute of Geodynamics, National Observatory of Athens, Athens, Greece ;Scognamiglio, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Stavrakakis, G.; Institute of Geodynamics, National Observatory of Athens, Athens, Greece; ; ; ; ; ; ; We present a physically based methodology to predict the range of ground-motion hazard for earthquakes along specific faults or within specific source volumes, and we demonstrate how to incorporate this methodology into probabilistic seismic hazard analyses (PSHA). By ‘physically based,’ we refer to ground-motion syntheses derived from physics and an understanding of the earthquake process. This approach replaces the aleatory uncertainty that current PSHA studies estimate by regression of empirical parameters with epistemic uncertainty that is expressed by the variability in the physical parameters of the earthquake rupture. Epistemic uncertainty can be reduced by further research.We modelled wave propagation with empirical Green’s functions. We applied our methodology to the 1999 September 7 Mw = 6.0 Athens earthquake for frequencies between 1 and 20 Hz.We developed constraints on rupture parameters based on prior knowledge of the earthquake rupture process and on sources within the region, and computed a sufficient number of scenario earthquakes to span the full variability of ground motion possible for a magnitude Mw = 6.0 earthquake with our approach. We found that: (1) our distribution of synthesized ground motions spans what actually occurred and that the distribution is realistically narrow; (2) one of our source models generates records that match observed time histories well; (3) certain combinations of rupture parameters produced ‘extreme,’ but not unrealistic ground motions at some stations; (4) the best-fitting rupture models occur in the vicinity of 38.05!N, 23.60!Wwith a centre of rupture near a 12-km depth and have nearly unilateral rupture toward the areas of high damage, which is consistent with independent investigations.We synthesized ground motion in the areas of high damage where strong motion records were not recorded from this earthquake. We also developed a demonstration PSHA for a single magnitude earthquake and for a single source region near Athens. We assumed an average return period of 1000 yr for this magnitude earthquake and synthesized 500 earthquakes distributed throughout the source zone, thereby having simulated a sample catalogue of ground motion for a period of 500 000 yr. We then used the synthesized ground motions rather than traditional attenuation relations for the PSHA.201 28 - 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