Error analysis in hypocentral locations at Phlegraean Fields

The location reliability of the earthquakes occurred at Phlegraean Fields has been analyzed, and the theoretical errors, inferred from the diagonal elements of the covariance matrix, have been estimated. Using only first P-phase arrivals to the local network (22 stations) and assuming a reading error of 0.05 sec., the average error on the spatial coordinates is estimated to be of the order of 0.2 km.Shallow events (depth<1 km) are very poorly constrained in depth at the borders of the network. The use of both P and S arrival times, recorded by a smaller three component network (10 stations), improves the depth determination.Further analysis has been performed on a set of about 350 selected earthquakes, using two different velocity models.Differences in depth considerably greater than the theoretical errors, and showing highly different patterns have been found.Tests with artificial events, randomly distributed in space, indicate that the observed depth distribution is essentially due to the used velocity model.     

Chemostratigraphy of Lago Albano sediments (Central Italy):geochemical evidence of palaeoenvironmental changes in lateQuaternary

The results of the geochemical investigation on bulk sediment fromthree cores collected in Albano crater lake (Central Italy) are reported, andthe main markers of the palaeoenvironmental changes in the last 26ka are discussed. The sediment of Lago Albano consists of greysilt and mud, and is made of volcanogenic, calcareous, siliceous and organicmaterial. Some tephra layers provide a chronological framework for thesequence. The chemical features of the sediments are strongly impacted by theAlban Hills volcanism (Roman volcanic area), but there are also strong changesof organic/inorganic matter ratios, owing to variations in biologicalproductivity and terrigeous supply from the catchment. Six chemostratigraphiczones record the main steps of the transition from the cold and dry climate ofGlacial Maximum to the warmer and more humid Holocene climate. The most usefulgeochemical indices are: biogenic silica, CO₂, Br and organiccontent (OM) for biological productivity; Al, Y, CIA (Chemical Index ofAlteration), Al/Rb, Ti/Zr and Y/Al ratios for terrigenous clasticmaterial; OM/Al ratio for organic/terrigenous ratio; S/Fe andMo/Fe ratios and Authigenic U for redox diagenetic conditions of the lakesediment. The geochemical records agree rather well with those of thelithological and paleomagnetic studies, and despite the information obtainedare less detailed than those acquired by the study of biological remains, thepalaeoenvironmental significance inferred is consistent. A comparison of thechemostratigraphic results of Lago Albano with those obtained on coevalsediments from Central and Southern Italy lakes supports the role of thegeochemical investigation as important complement to more sophisticatedtechniques in the palaeoenvironmental reconstructions.     

Tephrostratigraphy of the last 170 ka in sedimentary successions from the Adriatic Sea

In this study are discussed new SEM-EDS analyses performed on glass shards from five cores collected in the Central Adriatic Sea and two cores recovered from the South Adriatic Sea. A total of 26 tephra layers have been characterized and compared with the geochemical features of terrestrial deposits and other tephra archives in the area (South Adriatic Sea and Lago Grande di Monticchio, Vulture volcano). The compositions are compatible with either a Campanian or a Roman provenance. The cores, located on the Central Adriatic inner and outer shelf, recorded tephra referred to explosive events described in the literature: AP3 (sub-Plinian activity of the Somma-Vesuvius, 2710 ± 60 ¹⁴C years BP); Avellino eruption (Somma–Vesuvius, 3548 ± 129 ¹⁴C years BP); Agnano Monte Spina (Phlegrean Fields, 4100 ± 400 years BP); Mercato eruption (Somma–Vesuvius, 8010 ± 35 ¹⁴C years BP; Agnano Pomici Principali eruption (Phlegrean Fields, 10,320 ± 50 ¹⁴C years BP); Neapolitan Yellow Tuff (Phlegrean Fields, 12,100 ± 170 ¹⁴C years BP). Some of these layers were also observed in the South Adriatic core IN68-9 in addition to younger (AP2, sub-Plinian eruption, Somma–Vesuvius, 3225 ± 140 ¹⁴C years BP), and older layers (Pomici di Base eruption, Somma–Vesuvius, 18,300 ± 150 ¹⁴C years BP). Significant is the tephra record of core RF95-7 that, for the first time in the Adriatic Sea, reports the occurrence of tephra layers older than 60 ka: the well known Mediterranean tephra layers X2 (ca. 70 ka), W1 (ca. 140 ka) and V2 (Roman origin, ca. 170 ka) as well as other tephra layers attributed, on the basis of geochemistry and biostratigraphy, to explosive eruptions occurred at Vico (138 ± 2 and 151 ± 3 ka BP) and Ischia (147–140 ka BP).     

Shear wave velocities and primary influencing factors of Campi Flegrei–Neapolitan deposits

The Campi Flegrei caldera (Italy) has been the site of intense seismic activity over the past decades. This area is densely populated and includes important towns such as Napoli with historical sites and supporting many industries. For the proper use and management of the region, the evaluation of the dynamic properties of near-surface rocks is necessary.

The volcanological pattern has been reconstructed from the lithostratigraphies of several drillings. The most interesting and widespread pyroclastic products are the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock). Both pozzolana and tuff products are covered by recent eluvial and coastal sandy deposits and younger volcanic products (<12 000 years). The characteristic ranges of the shear wave velocity (V_s) of the Campi Flegrei–Neapolitan soils and tuffs are defined and the primary influencing factors are evaluated. For the sandy deposits, the results show that eluvial and lacustral products have lower shear wave velocities than coastal products. For the volcanic products younger than 12 000 years b.p. the influence of vertical pressure is emphasized. As regards the pozzolana deposit (soil) and the Neapolitan Yellow Tuff (rock), a major influencing factor is shown by the textural characteristics and the different hardening degrees as a consequence of the diagenetic processes. The scattering of the V_s velocities for the same formation is so wide that only the variability ranges can be individuated. These results suggest a need to carry out detailed V_s measurements or, at least, to make a parametric study of the effect of the V_s ranges on seismic response analysis in order to give safe building codes.     

Ground deformations in collapsed caldera structures

A method is presented to analyze the effect of stress-strain discontinuities on the ground deformations generated by a pressure source. This is meant to simulate the effects due to caldera structures, likely to present fractured zones at the borders of the collapsed area. A method originally developed by Crouch (1976) to solve plane-strain problems has been used to simulate deformation curves for several source and discontinuity geometries. The main result is that the location of the discontinuities controls the extension of the deformed zone, and always reduces it with respect to a continuous medium. With respect to a homogeneous medium the presence of lateral discontinuities also acts towards lowering the overpressure required to produce a given amount of deformation. These results indicate that, when analyzing ground deformations in calderas, the use of classical methods involving continuous media should be avoided, or at least taken with caution. These methods, in fact, assume that the extension of the deformed zone is only linked to the source depth.Some examples of ground deformations in active calderas have been analyzed in the framework of the results obtained from theoretical modeling. Four calderas recently affected by ground deformations have been considered: Rabaul (New Guinea), Campi Flegrei (Italy), Long Valley and Yellowstone (U.S.A.). The effects of collapsed structures on the deformation field are possibly evidenced for all the four calderas. At Rabaul and Campi Flegrei, the fracture systems mainly affecting the ground deformations probably represent younger, innermost collapses and are well evidenced by seismicity studies. Ground deformations are here concentrated in an area much smaller than the one enclosed by geologically visible caldera rims. In particular, at Rabaul, the effect of the innermost collapse can explain the high concentration of the uplift in the period 1971–1985, previously modeled by a very shallow source (1–3 km) in terms of overpressure in the main magma chamber, probably located at 4–5 km of depth.     

The volcanic rocks from the Mount Agnello area (Fiemme Valley,Italy): A contribution to the knowledge of the mid-Triassic magmatism of the Southern Alps

Summary Middle Triassic volcanic rock outcrops in the Mount Agnello area (Fiemme Valley, Italy) are examined in this study. Chemical analyses based on main and trace elements (Rb, Sr, Zr, Y, Nb) allow this volcanism to be defined as shoshonitic. The amphiboles, which occur with thick opacitic rims, were determined as ferroan pargasites. They were often found in volcanites of calcalkaline and/or shoshonitic associations. Consequently, this volcanism can be associated with a newly orogenized area undergoing stabilization. These data support the more recent studies on the mid-Triassic volcanism in the Southern Alps and underline the particular position of this magmatism in the tectonic evolution of this area.

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Les roches volcaniques de l'aire de Mont Agnello (Vallèe de Fiemme, Italie): Une contribution à la connaissance du volcanisme du trias moyen des Alpes Meridionales

Résumé Sont examinées les volcanites du Trias moyen affleurantes dans l'aire de Mont Agnello (Vallèe de Fiemme, Italie). Les déterminations du chimisme, en particulier celles regardant les éléments en traces (Rb, Sr, Zr, Y, Nb), revèlent pour ce volcanisme une affinité shoshonitique qui est bien associable à une aire de recente orogenèse en voie de stabilisation. On a aussi analysé l'amphibole, present dans ces roches, caracterisé par d'epais bords de réaction et on l'à reconnu comme une pargasite riche en fer. Ces amphiboles ont été retrouvé fréquemment dans des volcanites d'association calcoalcaline et/ou shoshonitique.

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With 4 Figures