A calibration system for superconducting gravimeters

A new method for the calibration of a superconducting gravity meter is described, in which a 273 Kg annular mass is placed around the meter and is moved up and down. The geometry of the apparatus is easy to model and the accuracy in the computation of the gravity variation induced by the mass, 6.7µgal, is limited only by the accuracy in the knowledge of value of the gravitational constant. Measurements done in 91 and 92 for the calibration of the instrument GWR-T015 are described. The calibration factor has been determined with a precision of about 0.3%.     

From ERS-1/2 to Sentinel-1: two decades of subsidence monitored through A-DInSAR techniques in the Ravenna area (Italy)

Land subsidence due to underground resources exploitation is a well-known problem that affects many cities in the world, especially the ones located along the coastal areas where the combined effect of subsidence and sea level rise increases the flooding risk. In this study, 25 years of land subsidence affecting the Municipality of Ravenna (Italy) are monitored using Advanced Differential Interferometric Synthetic Aperture Radar (A-DInSAR) techniques. In particular, the exploitation of the new Sentinel-1A SAR data allowed us to extend the monitoring period till 2016, giving a better understanding of the temporal evolution of the phenomenon in the area. Two statistical approaches are applied to fully exploit the informative potential of the A-DInSAR results in a fast and systematic way. Thanks to the applied analyses, we described the behavior of the subsidence during the monitored period along with the relationship between the occurrence of the displacement and its main driving factors.     

Recent crustal deformations and tectonics of the Messina Strait area

Summary. The tectonics of the region at the convergence of the Tyrrhenian and Ionian basins is very complicated. The seismicity is particularly interesting in the area 38°–39° 30'N, 14° 30'–16° 30'E, where both deep events occur with compressive mechanism and crustal events with predominant normal faulting in excess of magnitude 7.0. The tectonic origin of this apparently contradictory behaviour can be traced to a remanent downgoing slab in the southern Tyrrhenian basin which is now inactive but is still undergoing considerable horizontal bending. Sedimentological studies suggest the bending to be accompanied by a marked uplift of Calabira reaching its highest rates in the area of the Strait of Messina. We performed a spirit levelling campaign in the Strait and found that a differential uplift of the inner lands with respect to coastlines took place in the period 1970–82. he question regarding the possibilities of either an absolute uplift of inner lands or a subsidence of coastlines can only be resolved after detailed tidal studies; the available tidal data seem to support the uplift hypothesis but they are substantially incomplete. The apparent pattern of vertical deformations seems to be a marked steady uplift with episodic subsidence periods in connection with large normal faulting earthquakes. Furthermore, our measurements and the results of a recently completed trilateration campaign tend to confirm the hypothesis of the existence of an angular graben in the Strait and a south–north right lateral slip of Sicily with respect to Calabria.     

Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea)

A combination of published and new radiometric dates on uplifted Holocene fossil beaches from northeastern Sicily and southern Calabria (southern Italy) is compared with the altitude of the inner margin of the Last Interglacial (LIg) (Late Pleistocene, 124 ka) and older marine terraces in order to gain a regional-scale outline of uplift rates and their temporal changes in a region which is one of the fastest uplifting sectors of the Central Mediterranean Sea. Late Holocene radiocarbon dates from Ioppolo (southern Calabria) and Ganzirri (northeast Sicily), two newly discovered sites are here presented for the first time. The Holocene uplift rates are highest at St. Alessio and Taormina in eastern Sicily (2.4 mm/y) and at Scilla in southwestern Calabria (2.1 mm/y), two sites located across the Messina Straits and which separate the island of Sicily from mainland Italy. Uplift rates decrease towards the south and north from this centre of uplift. Late Holocene uplift rates show an apparent increase of between 64 and 124% when compared with the longer-term uplift rates calculated from the LIg highstand terraces. Furthermore, we discovered that the locations of fastest Late Pleistocene and Late Holocene uplift rates spatially coincide. To what extent the Holocene increase in uplift rates results from incomplete elastic strain release along the major extensional faults which frame the seismotectonic of the area, or indicate a true change in regional tectonic processes, is not resolved. Nonetheless, the heterogeneity of uplift, with a well-defined centre that crosses the Messina Straits, and its persistence at different time-scales indicates a tight connection between wider regional processes and fault-related displacement in controlling crustal instability in this area.     

Origin and differentiation of clay minerals in pelagic sediments and sapropels of the Bannock Basin (Eastern Mediterranean)

Two cores from an anoxic basin of the southeastern Mediterranean Ridge were investigated to compare the clay mineralogy of pelagic sediments and of the interbedded sapropels. The sediments of Core BAN 84-02, raised from the basin floor, and those of Core BAN 84-08, from the eastern plateau of the Bannock Basin, provide evidence for different sedimentary environments. The anoxic conditions, which are still present near the bottom, produce an important decrease in smectite crystallinity (Core 02), whereas well-organized smectite persists in the normally oxygenated sediments (Core 08). Detrital clay minerals from various sources were deposited in the basin and no appreciable diagenesis was recognized downcore.

The clay mineralogy of the sapropels shows remarkable differences compared to the pelagic sediments. The changes observed are dependent on aggressive chemical reaction and on the sudden input of detrital crystalline sediments into the stagnant environment. A climatic curve registers the variable degree of clay mineral hydrolysis in continental areas and exhibits good correspondence with an already published oxygen isotope curve for the area.     

Calibrating a rainfall erosivity assessment model at regional scale in Mediterranean area

A simplified regression model is here calibrated on the basis of rainfall data records of Sicily (southern Italy), in order to show the model reliability in assessing the R-factor of the Universal Soil Loss Equation and its revised version (RUSLE) and to provide an estimate of long-term rainfall erosivity at medium-regional scale. The proposed model is a rearrangement of a former simplified model, formulated for the Italian environment, grouping three easily available rainfall variables on various time scales, which has been shown to be more successful than others in reproducing the rainfall erosive power over different locations of Italy. A geostatistical interpolation procedure is then applied for generating the regional long-term erosivity map with associated standard error. Areas with severe erosive rainfalls (from 2,000 up to more than 6,000 MJ mm ha⁻¹ h⁻¹) are pointed out which will correspond to areas suffering from severe soil erosion. Solving the problem of calculating the R-factor value in the RUSLE equation by means of such a simplified model here formulated will allow to predict the related soil loss. Moreover, given the availability of long time-series of concerned rainfall data, it will be possible to analyse the variability of rainfall erosivity within the last 50 years, and to investigate the application of RUSLE or similar soil erosion models with forecasting purposes of soil erosion risk.     

The RUSLE erosion index as a proxy indicator for debris flow susceptibility

Debris flows represent dangerous occurrences in many parts of the world. Several disasters are documented due to this type of fast-moving landslides; therefore, natural-hazard assessment of debris flows is crucial for safety of life and property. To this aim, much current work is being directed toward developing geotechnical-hydraulic models for the evaluation of debris flow susceptibility. A common base for such current models is parameterization of background predisposing and triggering factors such as inherent characteristics of geo-materials, topography, landscape and vegetation cover, rainfall regime, human activities, etc. which influence the occurrence of these processes on slopes. The same factors are also taken into account in soil erosion prediction models. Consequently, it seems worth investigating the effectiveness of the soil erosion index as debris flows susceptibility indicator. To this aim, a logistic regression analysis was carried out between the erosion index assessed by means of the Revised Universal Soil Loss Equation (RUSLE) model and the inventory of debris flows that have occurred in an area in Sicily (Southern Italy). Model assumptions were verified and validated by means of a series of statistical tools. Different possible scenarios were also evaluated by considering hypothetical changes in soil erosion rate under different rain erosivity conditions. Notwithstanding the rough approximations in model data collection, the outcomes appear encouraging.     

Geodetic surveys in Messina straits area

By using the fit of theoretical displacements on a fault according toMindlin andCheng (1950) dislocation theory, the vertical deformation field (maximum subsidence of 70 cm) associated to the Dec. 28 1908 MessinaM=7.0 earthquake is compared to the results of a spirit levelling survey that we performed in the Messina Straits area in 1981–1982. The differences in level from the last levelling campaign in the area, completed in 1970, show that the coastlines have undergone a moderate differential subsidence of about1 mm/year in the last decade. This is in agreement with the trend observed around 1908 and with the data of the Messina tide-gauge relative to the period 1897–1908 and 1910–1918. The lack of data in the period 1918–1970 does not allow definite conclusions about the vertical tectonic deformations in the area. Recent data on planimetric deformations have shown a South-North relative motion of Sicily. A comparison with the theoretical displacements indicates that a pure dip-slip of4 cm on the 1908 fault system does explain the observed vertical deformation, but not the horizontal.     

Evaluation of a seismic quiescence pattern in southeastern sicily

Southeastern Sicily experienced a very peculiar seismic activity in historic times, with a long series of ruinous earthquakes. A last large event, with magnitude probably in excess of 7.5, occurred on Jan., 11, 1693, totally destroying the city of Catania and killing 60,000 people. Only a few moderate events were reported since then, and a seismic gap issue has been proposed on this basis. A close scrutiny of the available data further shows that all significant seismic activity ceased after year 1850, suggesting one of the largest quiescence patterns ever encountered. This is examined together with the complex tectonic setting of the region, characterized by a wrenching mechanism with most significant seismicity located in its northern graben structure. An attempt to ascertain the imminence and the size of a future earthquake through commonly accepted empirical relations based on size and duration of the quiescence pattern did not provide any feasible result. A precision levelling survey which we recently completed yielded a relative subsidence of ~ 3 mm/yr, consistent with an aseismic slip on the northern graben structure at a rate of ~ 15 mm/yr. Comparing these results with sedimentological and tidal data suggests that the area is undergoing an accelerated deformation process; this issue is further supported by Rikitake's ultimate strain statistics. If the imminence of a damaging (M = 5.4) event is strongly favoured by Weibull statistics applied to the time series of occurrence of large events, the accumulated strain does not appear sufficient for a large earthquake (M 7.0). Within the limits of reliability of present semi-empirical approaches we conclude that the available evidence is consistent with the occurrence of a moderate-to-large (M 6.0) event in the near future. Several questions regarding the application of simple models to real (and complex) tectonic settings remain nevertheless unanswered.     

Ground displacement data around the city of Ravenna do not support uplifting Venice by water injection

The proposed injection of seawater in a brackish, sandy aquifer lying 600–800 m under the lagoon of Venice to lift Venice itself uniformly up to 30 cm in 10 years raises many problems. We discuss here whether a smooth uplift can be obtained as claimed by the proponents. We first underline the similarities between the Ravenna and Venice case. We then examine the subsidence around Ravenna because of fluid extraction and the measured surface displacements for the period when there is pressure recovery both in the depleted gas reservoir close to Ravenna and in the upper aquifer system. Uniform surface settlements are nowhere observed and strong influence of local heterogeneities cannot be ruled out. The whole picture suggests great caution and the need for much more investigation before undertaking such an injection.