Attenuation of shear-waves in the back-arc region of the Hellenic arc for frequencies from 0.6 to 16 Hz

Q_β for shear-waves is determined for the inner part of the Hellenic arc, the back-arc area, as a function of frequency in the range 0.6–16 Hz. We used 314 digital records from 32 earthquakes with magnitudes (M_w) ranging from 3.9 to 5.1. Epicentral distances ranged from 65 to 515 km. The data were obtained in 1997 during a 6-month operation of a digital portable network in Greece. The Q_β estimates were made for five frequency bands centred at 0.8, 1.5, 3.0, 6.0 and 12.0 Hz and the Q_β values obtained were 47, 79, 143, 271 and 553, respectively. The results show that Q_β for S-waves increases with frequency taking the form Q_β=55f ^0.91 (or Q_β⁻¹0.018f^−0.91). The high attenuation and the strong frequency dependence found, which is close to the frequency dependence of coda Q for Greece, are characteristic of an area with high seismicity, rapid extension, and in agreement with other similar studies in Greece.     

PART II: Comparison of Theoretical and Experimental Estimations of Site Effects

— To check the reliability and the quality of the theoretically estimated ground responses obtained from the 2-D simulation by the application of the hybrid method in PART-I, we compare some of them with those obtained at the same sites from observed data using the Standard Spectral Ratio (SSR). The comparison validates our synthetic modeling and shows that in cases of complex geometries, the use of at least 2-D numerical simulations is required in order to reliably evaluate site effects and thus facilitate the microzonation of the city of Thessaloniki.     

A plasticity constitutive model for unsaturated,anisotropic, nonexpansive soils

The paper describes and evaluates an incremental plasticity constitutive model for unsaturated, anisotropic, nonexpansive soils (CMUA). It is based on the modified Cam-Clay (MCC) model for saturated soils and enhances it by introducing anisotropy (via rotation of the MCC yield surface) and an unsaturated compressibility framework describing a double dependence of compressibility on suction and on the degree of saturation of macroporosity. As the anisotropic and unsaturated features can be activated independently, the model is downwards compatible with the MCC model. The CMUA model can simulate effectively: the dependence of compressibility on the level of developed anisotropy, uniqueness of critical state independent of the initial anisotropy, an evolving compressibility during constant suction compression, and a maximum of collapse. The model uses Bishop's average skeleton stress as its first constitutive variable, favouring its numerical implementation in commercial numerical analysis codes (eg, finite element codes) and a unified treatment of saturated and unsaturated material states.     

Travel time residuals in southeastern Europe

The Pn travel time relative residuals, in respect to a crustal model of the Aegean area, have been determined for 103 permanent seismological stations in southeastern Europe, western Turkey and the Middle East. The values of these residuals are considered to depend mainly on the crustal thickness beneath the seismological stations. Based on these values seven regions with different crustal thickness, varying between 31 Km and 42 Km, have been defined. The crust in these regions is continental. A region with very high negative residuals has been defined in the Middle East (Egypt, Israel, Lebanon). These negative residuals are attributed to different crustal structure of the eastern Mediterranean (oceanic crust with an extra thick sedimentary layer) and not the crustal thickness at the station sites.Independently from the interpretation, these Pn residuals can be used successfully to considerably improve (up to 2 Km) the determination of the earthquake foci locations.     

A microearthquake study of the Mygdonian graben (northern Greece)

During March and April 1984, a temporary network of 29 portable stations was operated in the region of the Mygdonian graben near Thessaloniki (northern Greece), where a destructive earthquake (M_s = 6.5) had occurred in the Summer of 1978. During a period of six weeks we recorded 540 earthquakes with magnitudes ranging from −0.2 to 3.0. From this set of data, 254 events are selected which according to us have a precision in epicenter and depth better than 1.5 km. A total of 54 single-event focal mechanisms have been determined.The seismicity and focal mechanisms show a rather complex pattern. There are no clear individual faults, but the E-W and NW-SE striking zones show N-S extension. Zones striking NNE-SSW show dextral strike-slip motion but NW-SE zones with sinistral strike-slip are also observed.In the center of the graben where the 1978 earthquake was located, we observe several thrust mechanisms distributed in two groups showing either NW-SE or E-W compression; these earthquakes seem to be located 2 km above the earthquakes showing normal mechanisms.The mean direction of the T-axes, found from the focal mechanisms, trends N15° and dips sub-horizontal.We propose a model for the formation and evolution of a complex graben system comprising several stages. In the initial stage the deformation occurs along pre-existing NW-SE or NNE-SSW faults, with normal or strike-slip movements. In the second stage, a new, E-W trending group of normal faults is formed over the ancient fault network. These new faults have a direction perpendicular to the mean T-axis and accommodate better the actual state of stress. At this stage the initial faults adjust to the deformation produced by the E-W trending new faults, and may constitute geometric barriers to the evolution of the new normal faults.     

Cenozoic erosion and the preglacial uplift of the Svalbard–Barents Sea region

The creation of the huge fans observed in the western Barents Sea margin can only be explained by assuming extremely high glacial erosion rates in the Barents Sea area. Glacial processes capable of producing such high erosion rates have been proposed, but require the largest part of the preglacial Barents Sea to be subaerial. To investigate the validity of these proposals we have attempted to reconstruct the western preglacial Barents Sea. Our approach was to combine erosion maps based on prepublished data into a single mean valued erosion map covering the whole western Barents Sea and consequently use it together with a simple Airy isostatic model to obtain a first rough estimate of the preglacial topography and bathymetry of the western Barents Sea margin. The mean valued erosion map presented herein is in good volumetric agreement with the sediments deposited in the western Barents Sea margin areas, and as a direct consequence of the averaging procedures employed in its construction we can safely assume that it is the most reliable erosion map based on the available information. By comparing the preglacial sequences with the glacial sequences in the fans we have concluded that 1/2 to 2/3 of the total Cenozoic erosion was glacial in origin and therefore a rough reconstruction of the preglacial relief of the western Barents Sea could be obtained. The results show a subaerial preglacial Barents Sea. Thus, during interglacials and interstadials the area may have been partly glaciated and intensively eroded up to 1 mm/y, while during relatively brief periods of peak glaciation with grounded ice extending to the shelf edge, sediments have been evacuated and deposited at the margins at high rates. The interplay between erosion and uplift represents a typical chicken and egg problem; initial uplift is followed by intensive glacial erosion, compensated by isostatic uplift, which in turn leads to the maintenance of an elevated, and glaciated, terrain. The information we have on the initial tectonic uplift suggests that the most likely mechanism to cause an uplift of the dimensions and magnitude of the one observed in the Barents Sea is a thermal mechanism.     

Recent discoveries of supersoft X‐ray sources in M 31

Classical novae (CNe) have recently been reported to represent the major class of supersoft X‐ray sources (SSSs) in the central area of our neighbouring galaxy M 31. This paper presents a review of results from recent X‐ray observations of M 31 with XMM‐Newton and Chandra. We carried out a dedicated optical and X‐ray monitoring program of CNe and SSSs in the central area ofM 31. We discovered the first SSSs in M 31 globular clusters (GCs) and their connection to the very first discovered CN in a M 31 GC. This result may have an impact on the CN rate in GCs. Furthermore, in our optical and X‐ray monitoring data we discovered the CN M3 1N 2007‐11a, which shows a very short SSS phase of 29–52 days. Short SSS states (durations ≤ 100 days) of CNe indicate massive white dwarfs (WDs) that are candidate progenitors of supernovae type Ia. In the case of M31N 2007‐11a, the optical and X‐ray light curves suggest a binary containing a WD with M_WD > 1.0 M_⊙. Finally, we present the discovery of the SSS counterpart of the CN M31N 2006‐04a. The X‐ray light curve of M31N 2006‐04a shows short‐time variability, which might indicate an orbital period of about 2 hours (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)