Taylor B.,University of Hawaii at Manoa |
Weiss J.R.,University of Hawaii at Manoa |
Goodliffe A.M.,University of Alabama |
Sachpazi M.,Geodynamic Institute |
And 2 more authors.
Geophysical Journal International | Year: 2011
A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (∼35° in the centre and 45-48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15-20° in the centre and 19-30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E-W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090-120° in the basement and early rift to 090-100° in the latest rift, reflecting a ∼10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6-11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4-14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15-20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation. © 2011 The Authors Geophysical Journal International © 2011 RAS.
Savvaidis A.,Institute of Engineering Seismology and Earthquake Engineering EPPO |
Margaris B.,Institute of Engineering Seismology and Earthquake Engineering EPPO |
Theodoulidis N.,Institute of Engineering Seismology and Earthquake Engineering EPPO |
Lekidis V.,Institute of Engineering Seismology and Earthquake Engineering EPPO |
And 19 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013
For the seismic action estimation according to Eurocode (EC8) one has to characterize site conditions and suitably estimate soil amplification and corresponding peak ground motion for the site. For this reason, as specified, one has to define a design spectrum through the ground-type/soil-category (S), and the peak ground acceleration (PGA) of the reference return period (TNCR) for the corresponding seismic zone and for structural technical requirements chosen by the designer. Ground type is defined through geophysical/geotechnical parameters, i.e. (a) the average shear wave velocity up to 30 meters depth, (b) the Standard Penetration Test blow-count, and (c) the undrained shear strength of soil. Through the GEO-CHARACTERIZATION THALIS-PROJECT we combine different geophysical and geotechnical methods in order to more accurately define the ground conditions in selected sites of the Hellenic Accelerometric Network (HAN) in the area of Crete Island. More specifically in the present efforts, geological information shear wave velocity and attenuation model calculated from seismic surface geophysical measurements is used. Additionally we utilize the ground acceleration recorded through HAN from intermediate depth earthquakes in the broader area of South Aegean Sea. Using the recorded ground motion data and the procedure defined in EC8, the corresponding elastic response spectrum is calculated for selected sites. The resulting information are compared with the values defined for the corresponding EC8 spectrum for the seismic zone comprising the island of Crete. As a final outcome of this work we intend to propose regional normalized elastic spectra for seismic design of structures and urban development planning and compare them with Eurocode. © 2013 SPIE.
Margaris B.,ITSAK Institute of Engineering Seismology and Earthquake Engineering |
Skarlatoudis A.,Aristotle University of Thessaloniki |
Savvaidis A.,ITSAK Institute of Engineering Seismology and Earthquake Engineering |
Theodoulidis N.,ITSAK Institute of Engineering Seismology and Earthquake Engineering |
And 2 more authors.
Geotechnical, Geological and Earthquake Engineering | Year: 2011
The destructive earthquakes that occurred over the last years in the broader Greek region urged the need for acquiring high quality strong ground motion recordings. This necessity led to the enhancement of accelerographic networks by deploying a significant number of new sensors all around Greece, improving their spatial coverage. Within the framework of this work, a new, more efficient, strong motion data processing technique is presented and the properties of the updated Greek strong motion database are presented. The resulted high quality dataset will be used in various applications of engineering seismology, soil dynamics and earthquake engineering with the main goal being the derivation of new ground motion prediction equations (GMPEs) for the broader Aegean area. Updated seismological and geotechnical information such as site classification of ITSAK accelerographic stations based on VS30, is adopted for the derivation of new GMPEs. © Springer Science+Business Media B.V. 2011.
Pearce D.,Massachusetts Institute of Technology |
Rondenay S.,University of Bergen |
Sachpazi M.,Geodynamic Institute |
Charalampakis M.,Geodynamic Institute |
Royden L.H.,Massachusetts Institute of Technology
Journal of Geophysical Research: Solid Earth | Year: 2012
 The western Hellenic subduction zone (WHSZ) exhibits well-documented along-strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab density are believed to drive deformation rates along the WHSZ; however, this hypothesis has been difficult to test given the limited seismic constraints on the structure of the WHSZ, particularly beneath northern Greece. Here, we present high-resolution seismic images across northern and southern Greece to constrain the slab composition and mantle wedge geometry along the WHSZ. Data from two temporary arrays deployed across Greece in a northern line (NL) and southern line (SL) are processed using a 2D teleseismic migration algorithm based on the Generalized Radon Transform. Images of P- and S-wave velocity perturbations reveal N60E dipping low-velocity layers beneath both NL and SL. The ∼8 km thick layer beneath SL is interpreted as subducted oceanic crust while the ∼20 km thick layer beneath NL is interpreted as subducted continental crust. The thickness of subducted continental crust inferred within the upper mantle suggests that ∼10 km of continental crust has accreted to the overriding plate. The relative position of the two subducted crusts implies ∼70-85 km of additional slab retreat in the south relative to the north. Overall, our seismic images are consistent with the hypothesis that faster sinking of the denser, oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the WHSZ. © 2012. American Geophysical Union.
Pavlou K.,National and Kapodistrian University of Athens |
Kaviris G.,National and Kapodistrian University of Athens |
Chousianitis K.,Geodynamic Institute |
Drakatos G.,Geodynamic Institute |
And 3 more authors.
Natural Hazards and Earth System Sciences | Year: 2013
Seismic hazard assessment and seismicity changes are investigated in the Kozani-Grevena area, at the western margin of internal Hellenides in NW Greece. The region is of great interest, since it was characterized by very low seismic activity until 1995, when the "unexpected" Kozani-Grevena earthquake (Ms = 6.5) occurred. This event is of significant importance for Greece, since it, along with the 1999 Athens earthquake, initiated the modification of the Greek Building Code. In order to detect any seismicity changes, the seismicity of the region was divided into three time windows: the first up to 1973, the second from 1900 to 1994 and the third covering the entire instrumental period. For the above mentioned time windows, seismic hazard assessment was performed using the extreme values method. The results indicate an increase of the peak ground acceleration (PGA) values after the impoundment, with the exception of the area in the vicinity of the NE edge of the Dam. Before the occurrence of the 1995 event, the epicentral region also exhibited higher PGA values than before the impoundment. The most significant increase in PGA values is observed SE of the Polyphyto artificial lake, where the largest values are observed for the second and the third period. The coincident increase in the number of earthquakes and in the PGA values may be attributed to the impoundment of the Polyphyto Dam. The maximum expected magnitude is calculated by the extreme values method and Gumbel's third asymptotic distribution. The results reveal similar values of maximum expected magnitudes (Mmax = 6.5), independent of the seismicity rate, indicating that the 13 May 1995 earthquake was not an "unexpected" event, since the magnitude of an oncoming earthquake depends mainly on the tectonics of the region and the characteristics of the active faults. © Author(s) 2013.