ALBERT

Description: Northern Greece is an intracontinental region behind the Hellenic subduction zone, with widespread seismic activity (ranging from low to high), with strong destructive earthquakes of M [≥] 6.0 in historical to recent times. Geological and seismological data indicate that recent seismic activity is mainly localized along large, inherited, fault zones, which have transected Northern Greece since Oligocene-Miocene times. The main active fault zones in Thrace, and Eastern and Central Macedonia strike approximately east-west, with lengths of 40-120 km. Fault segments strike WNW-ESE to ENE-WSW and range from 10 to 30 km in length. In Western Macedonia the main active fault zones strike NE-SW to ENE-WSW with lengths of 40-60 km and consist of 10-30 km segments. The region's strong earthquakes are usually associated with reactivation of these fault segments and are estimated at M = 5.6-6.5. Focal mechanisms and fault-slip data from the fault zones indicate a change in the trend of extension axes from NNE-SSW in Eastern Macedonia-Thrace to NNW-SSE in Western Macedonia. Thus, neotectonic and seismological data suggest that variations in fault patterns, as determined from the large inherited fault zones transecting Northern Greece, are the major factor governing this change in the trend of maximum extension. This interpretation is consistent with the long-lived arcuate shape of the Hellenic subduction zone.

Description: We present a shear velocity model of the crust and uppermost mantle under the Aegean region by simultaneous inversion of Rayleigh and Love waves. The database consists of regional earthquakes recorded by portable broadband three-component digital stations that were installed for a period of 6 months in the broader Aegean region. For each epicentrestation ray path group velocity dispersion curves are measured using appropriate frequency time analysis (FTAN). The dispersion measurements for more than 600 Love wave paths have been used. We have also incorporated previous results for c. 700 Rayleigh wave paths for the study area. The single-path dispersion curves of both waves were inverted to regional group velocity maps for different values of period (632 s) via a tomographic method. The local dispersion curves of discrete grid points for both surface waves were inverted nonlinearly to construct 1D models of shear-wave velocity v. depth. In most cases the joint inversion of Rayleigh and Love waves resulted in a single model (from the multiple models compatible with the data) that could interpret both Rayleigh and Love wave data. Around 60 local dispersion curves for both Rayleigh and Love waves were finally jointly inverted. As expected, because of the complex tectonic environment of the Aegean region the results show strong lateral variations of the S-wave velocities for the crust and uppermost mantle. Our results confirm the presence of a thin crust typically less than 2830 km in the whole Aegean Sea, which in some parts of the southern and central Aegean Sea becomes significantly thinner (2022 km). In contrast, a large crustal thickness of about 4045 km exists in western Greece, and the remaining part of continental Greece is characterized by a mean crustal thickness of about 35 km. A significant sub-Moho upper mantle low-velocity zone (LVLmantle) with velocities as low as 3.7 km s1, is clearly identified in the southern and central Aegean Sea, correlated with the high heat flow in the mantle wedge above the subducted slab and the related active volcanism in the region. The results obtained results are compared with independent body-wave tomographic information on the velocity structure of the study area and exhibit a generally good agreement, although significant small-scale differences are also identified.

Description: Accelerating seismic strain caused by the generation of intermediate-magnitude preshocks in a broad (critical) region, accompanied by decelerating seismic strain caused by the generation of smaller preshocks in the seismogenic region are systematically observed before strong mainshocks. On the basis of this seismicity pattern a model has been developed that seems promising for intermediate-term earthquake prediction, called the Decelerating in-Accelerating out Seismic Strain Model'. Recent seismological data for the Mediterranean region are used here for backward and forward testing of this model. The selection of the broader Mediterranean region as a test area was motivated not only by the interest of time-dependent seismic hazard assessment in a high-seismicity and highly populated region but also by the fact that the Mediterranean is a natural geophysical and geological laboratory where both complex multi-plate and continuum tectonics are found in a more or less convergent zone. Within this complex geotectonic setting several geological phenomena such as subduction, collision, orogen collapse and back-arc extension take place, leading to the generation of a broad spectrum of mainshocks, reaching MW = 8.0 or greater for subduction-related thrust events and a variety of corresponding seismicity levels and neotectonic activity ranging from very low (e.g. large parts of Iberian peninsula) to very high (broader Aegean area). The backward procedure shows that all six strong (M [≥] 6.8) mainshocks that have occurred in the Mediterranean since 1980 had been preceded by preshock sequences that followed this seismicity pattern and satisfy all model constraints. Application of the model for future mainshocks has led to the identification of nine regions (in the Pyrenees, Calabria, NE Adriatic, Albania, Northern Greece, SE Aegean, NW Anatolia, western Anatolia, NE Anatolia) where current intermediate-magnitude seismicity satisfies the constraints of the model and corresponds to strong (M [≥] 6.2) mainshocks. The magnitudes, epicentres and origin times of these probably ensuing mainshocks, as well as their corresponding uncertainties, are estimated, so that it is possible to evaluate the model potential during the next decade (2006-2015). Furthermore, it is shown that geological observations of surface fault traces can contribute to the accurate location of the foci of future strong mainshocks in the Mediterranean and to an estimation of their sizes. For this purpose, globally valid relations between fault parameters based on geological observations (surface fault length, LS, and fault slip, uS) and measures of mainshock size (mainshock magnitude, subsurface fault length, L, and fault slip, u) are proposed.

Description: A response-spectra database is compiled of hundreds of seismic records from intermediate-depth earthquakes (earthquakes whose foci are located between 45 to 300 km from the earth’s surface) with moment magnitudes of M  4.5–6.7 that occurred in the South Aegean subduction zone. The database consists of high-quality data from both acceleration-sensor and broadband velocity-sensor instruments. The database is much larger than previous databases used in the development of past empirical regressions enabling the determination of various parameters of ground-motion attenuation not previously examined. New variables accounting for the highly complex propagation of seismic waves in the Greek subduction zone are introduced based on the hypocentral depth and the location of the event, as these factors control the effects of the back-arc low-velocity/low- Q mantle wedge on the seismic-wave propagation. The derived results show a strong dependence of the recorded ground motions on both hypocentral depth and distance, which leads to the classification of the dataset into three depth-hypocentral distance categories. Ground motions from in-slab earthquakes, especially with hypocentral depths ( h )〉100 km, are amplified for along-arc stations, an expected effect of channeled waves through the high-velocity slab. The ground motions are also strongly attenuated in the back-arc region, due to the low- Q mantle wedge, which are almost independent of the recording hypocentral distance. In contrast, for shallower in-slab events (60 km〈 h 〈100 km), the corresponding differentiation of seismic motion for along-arc and back-arc stations is observed beyond a specific critical distance range. Moreover, for longer periods, both along-arc amplification and back-arc anelastic-attenuation factors strongly diminish, suggesting that the longer wavelengths of seismic waves are not affected by the complex geophysical structure, resulting in more similar ground motions for both back-arc and along-arc stations. Finally, results for interface events ( h 〈45 km) occurring along the outer Hellenic arc suggest their wave propagation is not affected by the presence of the low-velocity/low- Q S mantle wedge, but is mainly controlled by the differences of the anelastic attenuation between the Mediterranean and Aegean lithospheres.

Description: Robust relations correlating 10 different magnitudes of intermediate-depth and deep-focus earthquakes to moment magnitude are proposed, in order to be efficiently incorporated into the compilation process of homogeneous (with respect to magnitude) earthquake catalogs. By using global data available from International Seismological Centre (ISC), National Earthquake Information Center (NEIC), Comprehensive Nuclear-Test-Ban Treaty Organization’s International Data Centre (IDC), Institute of Physics of the Earth in Moscow, Russia, and China Earthquake Networks Center in Beijing, the performance of several widely used magnitude scales, such as body wave ( m b , m B ) and surface wave ( M s ), is examined with respect to the moment magnitude scale ( M w ). Similarly, appropriate M w -calibrated relations are also provided for regional magnitude scales such as the M JMA magnitude calculated by the Japan Meteorological Agency. The analysis also involves the integration of focal depth as an additional variable to some of the above magnitude-conversion relations. This depth effect proved to be important for ISC/NEIC’s body-wave ( m b IN) and surface-wave ( M s IN) magnitudes, leading to significant corrections for the estimated magnitudes. More specifically, a major change in the magnitude residual variation around the depth of 230 km was identified for the case of m b IN. Furthermore, the obtained results provide important observations on the behavior of certain magnitude scales. A typical case is the m b scale reported by IDC, which shows a systematic and large bias, with respect to the published M w data values.

Description: The 3D wave-propagation characteristics of the 4 July 1978 aftershock (M 5.1) of the 20 June 1978 strong mainshock (M 6.5) that struck the city of Thessaloniki are studied using a 3D finite-difference approach. Synthetics are estimated for a dense grid of receivers and compared with available accelerograms from soft-soil sites in the city of Thessaloniki, exhibiting a good agreement both in time and frequency domain for the frequency band studied (0.7–3 Hz). Moreover, the spatial distribution of various measures of ground motion (peak values, spectral values) is used for the quantitative study of site effects in the broader city area. Comparisons show that the coastal zone, including the city harbor and large areas of the eastern parts of the city, exhibit high values of ground motion (and significant site amplifications), in good qualitative correlation with the observed damage distribution of the mainshock of the 1978 seismic sequence. Finally, the 3D synthetics are compared with available 2D simulations, as well as amplifications derived from macroseismic information for three typical cross sections spanning the urban area of the city. The comparisons confirm the strong spatial variability of ground motion throughout the Thessaloniki area, as well as the superiority of 3D modeling of actual recordings against previous modeling attempts. These results verify the practical usefulness of 3D wave-propagation tools for hazard mitigation, especially of specific target events, in complex geometry sedimentary basins such as the Thessaloniki area.