Supersoft sources in M31: Comparing the XMM‐Newton deep survey,ROSAT and Chandra catalogues

To investigate the transient nature of supersoft sources (SSSs) in M 31, we compared SSS candidates ofthe XMM‐Newton Deep Survey, ROSAT PSPC surveys and the Chandra catalogues in the same field. We found 40 SSSs in the XMM‐Newton observations. While 12 of the XMM‐Newton sources were brighter than the limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ∼10 yr earlier. Five correlate with recent optical novae which explains why they were not detected by ROSAT. The remaining 28 XMM‐Newton SSSs have fluxes below the ROSAT detection threshold. Nevertheless we found one correlation with a ROSAT source, which had significantly larger fluxes than during the XMM‐Newton observations. Ten of the XMM‐Newton SSSs were detected by Chandra with <1– ∼6yr between the observations. Five were also classified as SSSs by Chandra. Of the 30 ROSAT SSSs three were confirmed with XMM‐Newton, while for 11 sources other classifications are suggested. Of the remaining 16 sources one correlates with an optical nova. Of the 42 Chandra very‐soft sources five are classified as XMM‐Newton SSSs, while for 22 we suggest other classifications. Of the remaining 15 sources, nine are classified as transient by Chandra, one of them correlates with an optical nova. These findings underlined the high variability of the sources of this class and the connection between SSSs and optical novae. Only three sources, were detected by all three missions as SSSs. Thus they are visible for more than a decade, despite their variability (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Seismicity of western Macedonia, Greece

The seismicity of western Macedonia is examined in the present paper. On the basis of historical information as well as on instrumental data it is found that this area is characterized by low seismicity. The focal region of the Grevena-Kozani 1995 earthquake exhibits the highest seismicity in terms of probabilities for the generation of strong (M_s ≥ 6.0) earthquakes in a period of fifty years. Two other regions with relatively high seismicity were also distinguished (west of Edessa and around Prespes lakes). Accurate determination of focal parameters of all earthquakes occurred in the area during October 1975-April 1995, by the use of a 3-D crustal model shows that the seismic activity is related to the graben structures of the studied area. Finally, evidence is presented that the triggering of the 1995 earthquake may be related to the impoundment of the Polyfytos artificial lake.     

The seismic parameter b of the frequency-magnitude relation and its association with the geological zones in the area of Greece

The parameter b of the frequency-magnitude relation has been accurately calculated for each of the 21 seismic zones into which the Aegean and surrounding area (34°N–43°N, 18°E–30°E) has been divided on the basis of several seismotectonic criteria. These 21 seismic zones have been geographically separated into three (A, B, C) groups (regions), according to the values of the parameter b. The outer region (A) is characterized by a relatively high value (= 1.03) the middle region (B) by an intermediate value (= 0.84) and the innermost region (C) by a low value (= 0.60). The boundaries of these three groups of zones almost coincide with the boundaries of well-known geological zones. This observation is interpreted in terms of mechanical heterogeneity of the material, and of structural and stress conditions in the area.     

Attenuation of coda waves in northern Greece

The single scattering model has been applied for the estimation of codaQ values for local earthquakes that occurred in northern Greece during the period 1983–1989 and recorded by the telemetered network of the Geophysical Laboratory of the University of Thessaloniki. CodaQ estimations were made for four frequency bands centered at 1.5 Hz, 3.0 Hz, 6.0 Hz and 12.0 Hz and for the lapse time windows 10–20 sec, 15–30 sec, 20–45 sec, 30–60 sec and 50–100 sec. The codaQ values obtained show a clear frequency dependence of the formQ _c =Q ₀ f ⁿ , whileQ ₀ andn depend on the lapse time window.Q ₀ was found equal to 33 andn equal to 1.01 for the time window of 10 to 20 sec, while for the other windowsQ ₀ increased from 60 to 129, withn being stable, close to 0.75. This lapse time dependence is interpreted as due to a depth dependent attenuation. The high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity, in agreement with studies in other seismic regions.     

Determination of source,path and site parameters based on non-linear inversion of accelerometric data in Greece

Source, propagation path and site conditions are the factors affecting seismic ground motion. Consequently, recordings acquired at a seismic station are formed by the convolution of these three factors. In this work S-wave acceleration Fourier spectra of earthquakes recorded at local and regional scale, by the ITSAK accelerometric network for the period 2010–2016, are modeled as a product of source, propagation path (including geometric and anelastic attenuation) and site effects. The data set consists of 136 crustal earthquakes occurred in the broader Aegean area, with magnitudes 4.2?≤?M_w?≤?6.5 and epicentral distances 20 km?≤?R?≤?350 km, recorded at 112 broadband accelerometric stations installed at sites with various geologic conditions. Based on this data set, an iterative Gauss–Newton inversion method to solve the non-linear problem and retrieve the different terms of source, propagation path and site, is applied. This method uses an initial input model trying to find the best and at the same time a stable solution for the inverted parameters, which are, moment magnitude (M_w), corner frequency (f_c), attenuation quality factor (Q_s?=?Q_of^α), slope of the geometric attenuation (1/R^γ) and site transfer function (S(f)). The initial values of the starting model can be either known from other studies or inferred within a reasonable range. Depending on the level of knowledge on these input parameters, the associated standard deviation can be adjusted (large values for unknown parameters or small values for parameters which are well constrained). Results of the analyses exhibit satisfactory agreement of estimated source parameters with those proposed by seismological centers in Greece and propagation path properties similar to the ones determined in relevant previous studies for the same region. In addition, the site transfer functions obtained by the non-linear inversion are comparable with those calculated for the same sites using either standard spectral ratio or horizontal-to-vertical spectral ration (HVSR—receiver function) techniques. The aforementioned results are encouraging for reliable earthquake source parameters, propagation path properties and site effect assessment, in areas of intermediate to high seismicity.     

P and S velocity structures of the Santorini–Coloumbo volcanic system (Aegean Sea,Greece) obtained by non-linear inversion of travel times and its tectonic implications

One of the most prominent tectonic features of the Eastern Mediterranean region is the Hellenic volcanic arc in the Southern Aegean Sea, with the Santorini Island being one of the most active volcanic centers. Recent seismic studies show that the main seismic activity of the Santorini volcanic center is strongly associated with the volcanic processes, as well as with the seismo-tectonic regime of the broader Southern Aegean Sea area. The main cluster of local seismicity is located near the northeastern edge of the Santorini Island, beneath the Coloumbo Reef, which is a submarine volcanic seamount of the Santorini Island volcanic system.     

Engineering geological characterization of low strength anisotropic rocks in the Himalayan region for assessment of tunnel support

A detailed engineering geological assessment of low strength (6–35 MPa) anisotropic rocks at an ongoing Hydroelectric Project in the Himalayan Region has been carried out. The project (the Nathpa Jhakri Hydroelectric Project) will have one of the world's largest and longest water conducting systems, consisting of a 10.15 m diameter and 27.3 km long head race tunnel, a 942 000 m³ underground desilting complex and a 20 × 49 × 216 m powerhouse cavern in the area. Because these constructions are made in low strength metamorphosed anisotropic rocks like quartz mica schists, biotite schists and muscovite schists, it seemed necessary and worthwhile to undertake a comprehensive study of such rocks. The studies include petrographic and petrofabric analyses of the rocks, geo-mechanical properties, rock stress measurements, rock mass classification using the Q-System and data concerning joint geometry, joint roughness and joint strength. The use of mapped geological and geotechnical data along with the experimental and modelling studies have helped to better understand the behaviour of these rocks in underground openings.     

J-type carbon stars in the Large Magellanic Cloud

A sample of 1497 carbon stars in the Large Magellanic Cloud (LMC) has been observed in the red part of the spectrum with the 2dF facility on the Anglo-Australian Telescope. Of these, 156 have been identified as J-type (i.e. ¹³C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple two-dimensional classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c and j indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the   K − ( J − K )  colour–magnitude diagram, parallel to and 0.6 mag fainter than the N-star sequence. A subset of the J stars (about 13 per cent) are brighter than this J-star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature.     

Sediment non-linearity and attenuation of seismic waves: a study of accelerograms from Lefkas, western Greece

We investigate the dependence of the S-wave high-frequency spectral-decay parameter, κ (“kappa”) — a measure of wave attenuation — on ground-motion amplitude. 21 three-component accelerograms from two adjacent sediment sites in the town of Lefkas, western Greece, are used, representing 17 earthquakes with magnitudes M_w 4.7–7.0 and hypocentral distances 12–93 km. Recorded peak horizontal ground accelerations (PGA) and velocities (PGV) are 22–540 cm/s² and 1.3–54.5 cm/s.Fourier amplitude spectra are computed for S-wave windows, and the frequency range is visually determined where the high-frequency spectral decay can be approximated by a straight line on the linear-log plot; its slope (and hence κ) is computed by linear regression. κ is found to depend on hypocentral distance as κ=0.108+0.058R (r=0.518).As PGV increases from 1.3 to 54.5 cm/s, κ₀ (κ at 0 km, characterising inelastic attenuation in the site's subsurface geology) varies between 0.060 and 0.160 s. κ₀ is found to correlate very strongly with log MGA (r=0.645) (MGA — mean horizontal acceleration in the S-wave window) but also with log PGA (r=0.447) and log PGV (r=0.627). We attribute this behaviour to sediment non-linearity (shear-modulus degradation), resulting in the decrease of the site's dominant-resonance frequency (from about 3.5 to 2.4 Hz) and leading to the increase of κ₀. Our results imply that at sediment sites, an important contribution to κ comes from wave attenuation (damping) in the softest sediments and show that κ₀ is amplitude dependent, thus being a measure of sediment non-linearity.