An estimate of hypocentre location accuracy in a large network: possible implications for tectonic studies in Italy

Data recorded by the Italian Telemetered Seismic Network (ITSN) of the Istituto Nazionale di Geofisica (ING) have been widely used in recent years to image slab structures and to find evidence for active processes along the Italian Peninsula. However, the use of seismic data for geostructural purposes may be affected by the well-known trade-off between earthquake location and seismic-velocity parameters. Furthermore, the confidence ellipse predicted by standard procedures may be inadequate for the representation of the probable error of a computed localization. This paper evaluates the probable errors on the hypocentre determinations of the seismic events recorded by the ITSN, using a Monte Carlo method.
We compute synthetic arrival times using a 1-D velocity model appropriate as an average for the Italian area. The hypocentres used are all those recorded by the ITSN during the period January 1992 to March 1994 (1972 events). Station locations are those of the current ITSN configuration. The synthetic arrival times are perturbed with a Gaussian distribution of errors and input to ING's standard hypocentral location procedure, but using crustal velocities differing by 10 per cent from those used to generate them. Each simulation is repeated at least 30 times. Average absolute shifts of hypocentres are assessed in grid cells of linear dimension 33 km covering the whole Italian region.
For regions within the ITSN, shifts are typically 5–10 km in location and up to 20 km in depth. However, for offshore and coastal regions, they are much greater: 50 km or more in both location and depth (far exceeding the equivalent uncertainties quoted by ING bulletins). Possible consequences of this are highlighted by producing a cross-section of subcrustal hypocentres from the Adriatic to the Tyrrhenian Sea, where the large uncertainty in depth precludes any confident interpretation of dipping tectonic features.     

Occurrence of the Indian genus Hemibos (Bovini, Bovidae, Mammalia) at the Early–Middle Pleistocene transition in Italy

The morphology of the horn-core structure and section shape of the Bos galerianus type specimen, as well as the general anatomy of the frontal and occipital areas of the skull, suggest that the skull is better attributed to the Indian genus Hemibos. This finding contributes to our understanding of faunal dispersal patterns into Europe at the Early–Middle Pleistocene transition.     

Isotopic ages and tectono-metamorphic history of the metamorphic basement of North-Eastern Sardinia

New Rb/Sr and K/Ar data on minerals and whole-rocks from the metamorphic basement of Northeastern Sardinia are presented. A formation of augen gneiss of volcano-sedimentary origin yields a Rb/Sr whole-rock age of 441±33 m.y.; a Rb/Sr isochron age of 306±10 m.y. is found for the minerals separated from one of these samples. K/Ar measurements on micas also yield ages of 319–284 m.y. A banded migmatite which originated through a process of metamorphic differentiation was analysed by the Rb/Sr method. Six bands, treated as whole-rock samples, fit an isochron of 344±7 m.y. Biotite and plagioclase from one of these bands yield an isochron age of about 300 m.y.The radiometric results reported in this paper and all those previously published are discussed in order to investigate the tectono-metamorphic history of this important segment of the ancient Mediterranean basement. It is argued that in this area there is only indirect evidence of a Caledonian orogenic event (late orogenetic acidic magmatites emplaced 458-441 m.y. ago) while the main features of the metamorphic basement must be related to the Hercynian orogeny the climax of which can be fixed at about 340 m.y. The concordance of the ages of the separated minerals (310-300 m.y.) suggests that the metamorphic succession stayed above the specific critical temperatures for about 40 m.y., after which it was suddenly uplifted contemporaneous with the emplacement of the, essentially post-kinematic, Hercynian granitoids.     

Crustal structure in the southwestern part of the Iberian Peninsula

Seismic investigations to determine the crustal structure in the southwestern part of the Iberian Peninsula have been initiated in 1970. First experiments were carried out during July 1970, when a series of ten shots was fired off Cabo de Sines (Portugal) in shallow water and recorded up to distances of 185 km along a SE-profile towards Huelva (Spain). The profile was reversed in December 1970, when a series of twelve shots was fired off the south coast near Fuzeta (east of Faro) and recorded up to distances of about 260 km along a NW-profile towards Cabo da Roca west of Lisboa. A considerable increase in the seismic efficiency of the explosions could be achieved by generating standing waves in the water.

The structure deduced exhibits some peculiar features. Below the Palaeozoic sediments a fairly high velocity of 6.4 km/sec is found for the dome-shaped basement in that area. The lower crust, which is separated from the upper crust by a distinct velocity inversion (with a minimum velocity of about 5.3–5.6 km/sec), is characterized by a velocity of 7.1 km/sec. From the geological evidence and the sequence of seismic velocities it must be concluded that the upper crustal block in the southwestern part of the Iberian Peninsula has been uplifted by about 2–5 km since Permo-Triassic time, thus emphasizing the significance of vertical movement in tectonic activity.

The top of the upper mantle (8.15 km/sec) was detected at a depth of 30 km close to the Atlantic coast in the west, while near the Algarve coast in the south the depth to the M-discontinuity is about 34–35 km. This result in conjunction with studies of earthquake focal mechanisms confirms the suggestion that the Iberian block is being underthrust under the African plate.     

Copepod grazing during spring blooms: Can Pseudocalanus newmani induce trophic cascades?

During late winter and spring of 2002 and 2003, 24 two- to three-day cruises were conducted to Dabob Bay, Washington State, USA, to examine the grazing, egg production, and hatching success rates of adult female Calanus pacificus and Pseudocalanus newmani. Here, we discuss the results of our grazing experiments for P. newmani. Each week, we conducted traditional microzooplankton dilution experiments and “copepod dilution” experiments, each from two different layers. Grazing was measured by changes in chlorophyll concentration and direct cell counts. Clearance rates on individual prey species, as calculated by cell counts, showed that Pseudocalanus are highly selective in their feeding, and may have much higher grazing rates on individual taxa than calculated from bulk chlorophyll disappearance. The grazing rates of the copepods, however, are typically an order of magnitude lower than the grazing rates of the microzooplankton community, or the growth rates of the phytoplankton. P. newmani ingested diatoms, but, at certain times fed preferentially on microzooplankton, such as ciliates, tintinnids, and larger dinoflagellates. Removal of the microzooplankton may have released the other phytoplankton species from grazing pressure, allowing those species’ abundance to increase, which was measured as an apparent “negative” grazing on those phytoplankton species. The net result of grazing on some phytoplankton species, while simultaneously releasing others from grazing pressure resulted in bulk chlorophyll-derived estimates of grazing which were essentially zero or slightly negative; thus bulk chlorophyll disappearance is a poor indicator of copepod grazing. Whether copepods can significantly release phytoplankton from the grazing pressure by microzooplankton in situ, thus causing a trophic cascade, remains to be verified, but is suggested by our study.     

Performance of the Italian strong motion network during the 2009, L��Aquila seismic sequence (central Italy)

On April 6, 2009, the town of L’Aquila in the Abruzzo region (central Italy) was struck by a seismic event at 01:32 (UTC), of magnitude M_W = 6.3. The mainshock was followed by a long period of intense seismic activity and within seven days after the mainshock there were seven events of magnitude M_W ≥ 5 that occurred from April 6 to April 13. This long seismic sequence was characterized by a complex rupture mechanism that involved two major normal faults of the central Apennines: the Paganica and the Gorzano faults. The strong-motions of the mainshock were recorded by 64 stations of the Italian Strong-motion Network (RAN) operated by the National Civil Protection Department (DPC). Six stations of a local strong-motion array were working in NW L’Aquila suburb area. One of them, located at about 6 km from the Paganica fault surface tip-line, set up in trigger mode, recorded continuously for more than 20 min the mainshock and the aftershocks. Besides the mainshock, the RAN stations recorded in total 78 foreshocks and aftershocks of M_L ≥ 3.5, during the period from January to December 2009. The corresponding waveforms provide the most extensive digital strong ground motion data set ever recorded in Italy. Moreover, the 48 three-component observations of events of magnitude M_W ≥ 5, recorded at a distance less than 15 km from each of the major involved faults, provide a significant increasing of near-field records available for the Italian territory. Six days after the mainshock, the strong-motion dataset, referred to preliminary locations of the events with M_L ≥ 4.0, was made available on the DPC web site () and at the same time it was delivered to the ITACA database (). This dataset has been used by many authors in scientific papers and by engineers, geophysicists and geologists for professional technical works. In this paper, the present-day available strong-motion signals from the L’Aquila sequence and the performance of the Italian strong-motion network in terms of the number and quality of recorded data, the geometry and data transmission system are described. In addition the role of the temporary network that represents an extension of the permanent Italian strong-motion network, supporting the emergency response by civil protection authorities and improving the network coverage has been evaluated.     

Metals in eggs and hatchlings of Dermochelys coriacea (testudines) in Espírito Santo,Brazil and their relation to hatching and emergence success

Certain concentration of metal may influence survival in embryonic stages, reducing the birthrate and hatching emergence. This study aimed to evaluate concentration of metal in eggs and hatchlings of Dermochelys coriacea and correlate metal concentrations in eggs with hatching and emergence success. The samples were collected at Biological Reserve Station, ES, Brazil. At the nests' opening, unhatched eggs and stillborn hatchlings were collected to survey the successful hatching and emergence of D. coriacea. The eggs shell, egg content, and dead hatchlings metal concentrations were compared to hatching and emergence success. A positive correlation was found between the concentration of copper (Cu) in the hatchlings and the success of the emergence (r² = .28, p < .05), and a moderate positive correlation between the concentrations of iron (Fe) and barium (Ba; r² = .44, p < .05) and success hatching. The concentrations of metals in eggs and hatchlings of D. coriacea in the Espírito Santo are below the levels that could be considered harmful to the species. In summary, there is a positive correlation between metals levels and hatching and emergence success.     

Magma mixing and convective compositional layering within the Vesuvius magma chamber

The pumice-fall deposits of the last two Plinian eruptions of Vesuvius-a.d. 79 Pompei and 3700 b.p. Avellino-show a marked vertical compositional variation from white phonolite at the base to grey tephritic phonolite at the top. In both Avellino and Pompei sequences a compositional gap separates white from grey pumice. Grey and white pumice have distinct Sr and Nd isotopic compositions (grey pumice: ⁸⁷Sr/³⁶Sr=0.70749-56, ¹⁴³Nd/¹⁴⁴Nd=0.512507 for Pompei; 0.70760-69, 0.512504 for Avellino; white pumice: 0.70757-78 for Pompei; 0.70729-42 for Avellino). K-feldspar separated from both grey and white pumice has, in all cases, a white ⁸⁷Sr/⁸⁶Sr ratio (0.70766-79 for Pompei, 0.70728-33 for Avellino). The observed variations are interpreted as reflecting a pre-eruptive zonation of the magma chamber. Although mineralogical evidence of interaction between magma and calcareous country rocks exists in both eruptions, crustal contamination has not significantly modified the isotopic signatures of the erupted products. Petrographic and isotopic evidence of syneruptive magma mingling occur in Pompei grey pumice as well as in Avellino white and grey pumice, but they do not fully explain all the observed geochemical and isotopic variations. These variations are related to the complex refilling history of the magmatic system and result by fractional crystallization and mixing processes acting within the magma chamber. Preliminary data from other Plinian and subplinian sruptions of the Somma-Vesuvius point out the repeticive behaviour of ⁸⁷Sr/⁸⁶Sr variation in the last 25 000 years, hence suggesting a single magma chamber and continuity of the feeding system.     

Tidal response to sea level rise and bathymetric changes in the German Wadden Sea

Ocean Dynamics - Tidally dominated coasts are directly affected not only by projected rise in mean sea level, but also by changes in tidal dynamics due to sea level rise and bathymetric changes. By...     

Time dependence of negative buoyancy and the subduction of continental lithosphere

The time evolution of negative buoyancy of a subducting slab is modelled from the beginning of subduction under various kinematic conditions (dip angle and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the negative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2–4) × 10¹³ N m⁻¹ range when the tip of the slab reaches a depth of 600–700 km. If continental material arrives at the trench and is subducted, the downward buoyancy decreases by an amount proportional to the volume of the subducted continental crust. Assuming that subduction stops when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subductable continental crust is estimated as a function of the dip angle, subduction velocity and geometry of the margin. In most cases, subduction of continental material down to depths of 100–250 km is possible, and continental subduction can continue for times up to 10–15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurrence of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite.