Seismic Hazard Assessment in the Area of Mystras-Sparta,South Peloponnesus,Greece, Based on Local Seismotectonic,Seismic, Geologic Information and on Different Models of Rupture Propagation

Strong seismic events once again confirm the view that great destructive earthquakes are produced by the reactivation of pre-existing faults although they have usually remained inactive for many, perhaps thousands of years. It is evident that such active seismogenic zones, with little or no seismicity, have presumably been ignored in the determination of the region's seismic hazard. At south Peloponnesus, Greece, is situated at Taygetos mountain. At its eastern front lies a large normal fault system, the southern segment being the Sparta fault. This area has been characterized by low seismicity for the last 25 centuries. However, during the 6th and 5th centuries B.C. several destructive earthquakes have been reported. That of 464 B.C., was the most destructive and devastated the city of Sparta. Detailed morphotectonic observations of this area, suggest that the earthquake of 464 B.C. could be related to the most recent reactivation of this fault. The ground accelerations that would be produced by a future activation of the Sparta fault, were calculated, by applying a method which takes into account information mainly from the seismotectonic parameters of the Sparta fault, the rupture pattern, the properties of the propagation medium and the local ground conditions. Moreover, these results were compared with those of other independent studies based mainly on the seismic data of the area. This method estimated greater expected values of ground acceleration than those computed by the conventional seismic hazard methods. The highest values correspond to the activation of the Sparta fault either in a unilateral rupture, which would start from the southernmost point of the fault, or in a circular one. Furthermore, an increase is observed of the order of 50% in the ground acceleration values in unconsolidated soft ground in relation to the corresponding values of hard ground. This revised version was published online in June 2006 with corrections to the Cover Date.     

40Ar-39Ar and Rb-Sr age determinations on Quaternary volcanic rocks

⁴⁰Ar-³⁹Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides⁴⁰K and⁸⁷Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the⁴⁰Ar and the³⁹Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×10⁵ years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the⁴⁰Ar/³⁶Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic⁴⁰Ar from trapped⁴⁰Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in⁸⁷Sr/⁸⁶Sr as large as 16 parts in 10⁴ and⁸⁷Rb/⁸⁶Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×10⁵to3.3±0.2×10⁵ years and are in good agreement with the⁴⁰Ar-³⁹Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial⁸⁷Sr/⁸⁶Sr. However, different tuffs show small differences in initial⁸⁷Sr/⁸⁶Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the⁴⁰Ar-³⁹Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.     

Geochemical and stratigraphic indicators of late Holocene coastal evolution in the Gythio area, southern Peloponnese, Greece

The study of past changes in sea level, and of historical and pre-historical coastal evolution, using coastal sediment stratigraphies is well-established over a range of geographic areas, in both seismic and aseismic settings. In the eastern Mediterranean, however, such studies are less common, and, notably, the use of sediment geochemistry, and its combination with lithostratigraphic studies to analyze palaeoenvironmental and palaeo-sea-level change, has not been explored to any significant extent, despite the fact that geochemical data have been successfully used elsewhere to aid in the identification of sea-level changes. Here, we use a combined geochemical, stratigraphic and microfossil approach to reconstruct late Holocene coastal evolution and sea-level change at two sites near Gythio in the southern Peloponnese, Greece. The sites show stratigraphic and geochemical evidence of the presence in Late Helladic times (ca. 1500 BC) of barrier-protected coastal lagoonal/wetland environments, which have gradually infilled over the last ca. 3500 yr. Archaeological remains and ceramic and charcoal-bearing horizons within the sediment sequences indicate Late Roman occupation of the area, although there is no sedimentary evidence of significant pre-Roman activity at the study sites. An apparent brackish wetland peat deposit at − 3.4 m (overlain by anoxic lagoonal clays) at Kamares (Kato Vathi) Bay shows a calibrated radiocarbon age of 1640–1440 BC, suggesting a relative sea-level rise of 0.8–1 mm/yr in this area over the past 3500 yr, in good agreement with previous archaeological and sea-level modelling studies. There is no evidence, based on the stratigraphic, microfossil or geochemical record, of sudden marine flooding events related to local or regional seismic activity, despite the presence of the area in a seismically active zone known to be subject to periodic earthquakes and tsunami. The data highlight the utility of combining geochemical and stratigraphic studies in the reconstruction of coastal evolution and the study of palaeo-sea-level changes, particularly in sequences (such as those described here) where microfossils are poorly preserved.     

The isotopic composition of titanium in the Allende and Leoville meteorites

We describe the analytical techniques developed for the precise measurement of the titanium isotope abundances using a TiO⁺ ion beam. Terrestrial, lunar, and bulk meteorite samples yield identical results. Using a normalization to ${}^{46}\text{Ti}{}^{48}\text{Ti}$ for mass dependent isotope fractionation, we obtain the normal Ti composition: ${}^{46}\text{Ti}{}^{48}\text{Ti}\text{= 0.108548}$; ${}^{47}\text{Ti}{}^{48}\text{Ti}\text{= 0.099315 ± 0.000005}$; ${}^{49}\text{Ti}{}^{48}\text{Ti}\text{= 0.074463 ± 0.000004}$; ${}^{50}\text{Ti}{}^{48}\text{Ti}\text{= 0.072418 ± 0.000004}$ (2σ grand mean), taking ${}^{18}\text{O}{}^{16}\text{O}\text{= 0.002045}$ and ${}^{17}\text{O}{}^{16}\text{O}\text{= 0.00037}$. Measurements on thirteen coarse-grained and fine-grained Ca-Al-Ti-rich inclusions from the Allende and Leoville meteorites show the presence of widespread, significant, nonlinear isotope anomalies in the Ti isotopes which were not used for normalization. The data require the addition of at least three exotic components. The distinct correlation of non-linear effects for the most neutron-rich isotopes of Ca and Ti and the absence of substantial effects at ⁴⁶Ca in the FUN samples EK-1-4-1 and C-1 indicate that the effects reflect neutron-rich equilibrium or quasi-equilibrium nucleosynthetic processes in the outer layers of a supernova core. The results on Ca and Ti in conjunction with the isotopic effects on other elements (Mg, Sr, Ba, Nd, Sm) show that the samples represent mixtures of different nucleosynthetic components from distinctive processes (‘e’, ‘r’, ‘p’) which do not appear to be related to processes in the same stellar sites.     

Mg and Ca isotopic study of individual microscopic crystals from the Allende meteorite by the direct loading technique

We have developed a direct loading technique for determining the isotopic composition of Mg in chemically unseparated samples. This technique has a sensitivity and precision comparable with those of the conventional technique of analyzing pure Mg salt and eliminates contamination introduced during the chemical separation of Mg. This results in a significant reduction in sample size required for an analysis. This technique was combined with other characterization techniques of microscopic samples (e.g. optical microscopy, SEM, EPMA), and was applied to four single crystals of pure phases from an Allende inclusion ranging in size from 25 to 150 μm. Using a total sample of 4 μ, we found an anorthite crystal showing an enrichment in ²⁶Mg of 10% and were able to construct an ²⁶Al-²⁶Mg isochron which confirms our previous results obtained on macroscopic samples of the same inclusion. The isotopic composition of Ca was also measured along with Mg, on a directly loaded anorthite crystal from this inclusion, during the same mass spectrometer run and was shown to be essentially normal. Thus, the direct loading technique is applicable to Ca and will be useful in a correlative study of isotopic effects of different elements on individual microscopic samples. The extension of this technique to other elements appears feasible but will require extensive testing to control possible interferences.     

In search of the Earth‐forming reservoir: Mineralogical,chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites

High‐precision isotope data of meteorites show that the long‐standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth‐forming reservoir, we performed a comprehensive petrographic, elemental, and multi‐isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent‐body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium‐tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent‐body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.     

The May 13, 1995, Kozani-Grevena (NW Greece) earthquake: Source study and its tectonic implications

At 08:47 GMT, on May 13, 1995, a strong earthquake of M_s = 6.6 occurred in the NW part of Greece (Western Macedonia) and caused serious damage in the Kozani and Grevena prefectures, but fortunately no fatalities. The maximum observed macroseismic intensity was IX + of the Modified Mercalli scale. The main shock was preceded by several foreshocks and followed by intense aftershock activity lasting several months.The Institute of Geodynamics of the National Observatory of Athens, in order to monitor and study the aftershock activity, installed a seismic network of nine (9) stations operated for a period of 50 days. Thousands of aftershocks were recorded. Based on the analysis of recorded data, a NE-SW trending zone dipping NW is defined.In the field a surface rupture of normal slip was observed, following a NE-SW direction for a length of 8 km with a 4 cm down throw of the NW area. This break was located along a pre-existing minor normal fault, while a main fault system exists 10 km to the SE.The focal mechanism of the main shock shows normal faulting, which is in agreement with the field observations. Moreover focal mechanisms of several well defined aftershocks were computed, showing various types of faulting.     

3-D Crustal Velocity Structure from Inversion of Local Earthquake Data in Attiki (Central Greece) Region

Three-dimensional velocity structure of the upper crust was determined by inversionof P-wave travel times in the region of Attiki Prefecture (Greece), located at the eastern part of the Greek mainland, which is traditionally considered as an area of low seismicity. An earthquake of Ms = 5.9 occurred there, on September 7, 1999, causing extensive damage in the metropolitan area of Athens. A two-step tomographic procedure was applied to investigate the 3-D crustal velocity structure. The data set consisted of travel time residuals of 466 very well located earthquakes. In order to improve the initial velocity model, before the inversion of the data, a `minimum 1-D' initial velocity model was obtained and, therefore, for the first time a reliable velocity model for the region was derived. Theresults show that the velocity distribution is influenced rather from the geology than the seismotectonic regime of the region. At shallow depths (less than 4 km) the velocity increases gradually with respect to the depth without any sharp variation. On the contrary, the lateral variation of velocity seems to be affected from the geological regime of the region. The low velocities at this depth seem to be typical for sedimentary basins, likethose of the investigated region. At deeper layers (larger than 5 km) a different picture is obtained where both the lateral and vertical velocity variations are sharply pronounced. At the depth of 8.5 km, the aftershock area is covered by relatively low velocities, while a region of higher velocity covers the central part of Attiki almost coinciding with the transition zone between the Pelagonian and Attico-Cycladic massifs. Finally, the influence of the geological regime is well expressed by a high velocity anomaly, which is predominant beneath the Palaeozoic and Mesozoic metamorphic basement of the investigated region.