On the reliability of long‐period response spectral ordinates from digital accelerograms

Using records from co‐located broadband and digital strong motion (SM) instruments, it is first shown that the displacement waveforms obtained by double integration of the accelerogram need not be free of unrealistic baseline drift to yield reliable spectral ordinates up to at least 10 s. Secondly, to provide objective criteria for selecting reliable digital SM records for ground motion predictions at long periods, a set of synthetic accelerograms contaminated by random long‐period noise has been used, and the difference between the original accelerograms and the spurious ones in terms of response spectra has been quantified, by introducing a noise index that can be easily calculated based on the velocity waveform of the record. The results of this study suggest that high‐pass filtering the digital acceleration record from a cutoff period selected to suppress baseline drifts on the displacement waveform appears to be in most cases too conservative and unduly depletes reliable information on long‐period spectral ordinates. Copyright © 2007 John Wiley & Sons, Ltd.     

Recent geophysical investigation at Somma-Vesuvio volcanic complex

Activity at Somma-Vesuvio volcanic area in southern Italy is monitored by seismic stations and periodic geodetic and gravity surveys. The seismic network, which consists at present of four vertical stations and one three-component station, recorded an increase in earthquake activity in 1978 and between November 1988 and March 1989. During the later activity, earthquakes were located in a cluster about 3 km beneath the summit of the volcano. Two tide gauges, two tiltmeters and a recording gravimeter are also operating at Somma-Vesuvio. Yearly levelling surveys are conducted along several closed routes that extend from as much as 6 km from the base of the volcano to the summit area. Survey results reveal no significant ground movement since 1959, except for a slight subsidence around the rim of the summit crater. Gravity changes have been larger than the expected 10 μGal uncertainty of the measurements. The lack of contemporary elevation changes implies that the observed gravity changes are the result of a slight change in density structure. The cone of Somma-Vesuvio has been very stable for the last few decades, showing no indications of a buildup to activity. The lack of surface movement should rule out a magma-supply rate to this volcano at the historic eruptive rate of 0.002 km³/yr.     

The ARIEL mission reference sample

The ARIEL (Atmospheric Remote-sensing Exoplanet Large-survey) mission concept is one of the three M4 mission candidates selected by the European Space Agency (ESA) for a Phase A study, competing for a launch in 2026. ARIEL has been designed to study the physical and chemical properties of a large and diverse sample of exoplanets and, through those, understand how planets form and evolve in our galaxy. Here we describe the assumptions made to estimate an optimal sample of exoplanets – including already known exoplanets and expected ones yet to be discovered – observable by ARIEL and define a realistic mission scenario. To achieve the mission objectives, the sample should include gaseous and rocky planets with a range of temperatures around stars of different spectral type and metallicity. The current ARIEL design enables the observation of ～1000 planets, covering a broad range of planetary and stellar parameters, during its four year mission lifetime. This nominal list of planets is expected to evolve over the years depending on the new exoplanet discoveries.     

Far-field Gravity and Tilt Signals by Large Earthquakes: Real or Instrumental Effects?

A wide set of dynamics phenomena (i.e., Geodynamics, Post Glacial Rebound, seismicity and volcanic activity) can produce time gravity changes, which spectrum varies from short to long (more than 1 year) periods. The amplitude of the gravity variations is generally in the order of consequently their detection requires instruments with high sensitivity and stability: then, high quality experimental data. Spring and superconducting gravimeters are intensively used with this target and they are frequently jointed with tiltmeters recording stations in order to measure the elasto-gravitational perturbation of the Earth. The far-field effects produced by large earthquakes on records collected by spring gravimeters and tiltmeters are investigated here. Gravity and tilt records were analyzed on time windows spanning the occurrence of large worldwide earthquakes; the gravity records have been collected on two stations approximately 600 km distant. The background noise level at the stations was characterized, in each season, in order to detect a possible seasonal dependence and the presence of spectral components which could hide or mask other geophysical signals, such as, for instance, the highest mode of the Seismic Free Oscillation (SFO) of the Earth. Some spectral components (6.5; 8; 9; 14, 20, 51) have been detected in gravity and tilt records on the occasion of large earthquakes and the effect of the SFO has been hypothesized. A quite different spectral content of the EW and NS tiltmeter components has been detected and interpreted as a consequence of the radiation pattern of the disturbances due to the earthquakes. Through the analysis of the instrumental sensitivity, instrumental effects have been detected for gravity meters at very low frequency.F.S.E. (Fondo Sociale Europeo -European Community -)     

The height datum problem and the role of satellite gravity models

Regional height systems do not refer to a common equipotential surface, such as the geoid. They are usually referred to the mean sea level at a reference tide gauge. As mean sea level varies (by ±1 to 2 m) from place to place and from continent to continent each tide gauge has an unknown bias with respect to a common reference surface, whose determination is what the height datum problem is concerned with. This paper deals with this problem, in connection to the availability of satellite gravity missions data. Since biased heights enter into the computation of terrestrial gravity anomalies, which in turn are used for geoid determination, the biases enter as secondary or indirect effect also in such a geoid model. In contrast to terrestrial gravity anomalies, gravity and geoid models derived from satellite gravity missions, and in particular GRACE and GOCE, do not suffer from those inconsistencies. Those models can be regarded as unbiased. After a review of the mathematical formulation of the problem, the paper examines two alternative approaches to its solution. The first one compares the gravity potential coefficients in the range of degrees from 100 to 200 of an unbiased gravity field from GOCE with those of the combined model EGM2008, that in this range is affected by the height biases. This first proposal yields a solution too inaccurate to be useful. The second approach compares height anomalies derived from GNSS ellipsoidal heights and biased normal heights, with anomalies derived from an anomalous potential which combines a satellite-only model up to degree 200 and a high-resolution global model above 200. The point is to show that in this last combination the indirect effects of the height biases are negligible. To this aim, an error budget analysis is performed. The biases of the high frequency part are proved to be irrelevant, so that an accuracy of 5 cm per individual GNSS station is found. This seems to be a promising practical method to solve the problem.     

Strong-motion observations from the OGS temporary seismic network during the 2012 Emilia sequence in northern Italy

Strong-motion data consisting of peak ground acceleration and velocity and 5 % damped response spectra are presented for 46 earthquakes of the Emilia seismic sequence which occurred in the Po Plain (northern Italy) in 2012. The data were recorded by the OGS temporary network installed close to the town of Ferrara following the main shock of May 20, 2012. Ground-motion peak parameters and spectral responses are compared with the ground-motion prediction equation (GMPE) of Bindi et al. (Bull Earthq Eng 9:1899–1920, 2011) for soft soils and reverse faults. Peak ground accelerations are in general in good agreement with those predicted by GMPE, while predicted peak ground velocities underestimate the observed data, especially for stronger events at more distant stations. The response spectra follow the trend in peak ground velocities, with observed values higher than predicted values at longer periods. This behavior has been interpreted as a site effect due to the deep soft alluvial cover of the Po Plain, which promotes ground motion characterized by a large low-frequency spectral content that is not yet well modeled by the Italian GMPE. A peculiar behavior was shown by the event occurring on June 6, 04:08:33 UTC, \(\hbox {M}=4.5\) , located at the eastern edge of the Po Plain, which produced peak ground accelerations exceeding three times the values estimated by attenuation laws. Such a great discrepancy could be related to post-critically reflected S-waves and multiples from the Moho (SmSM).     

Depositional facies and stratal geometry of an Upper Carboniferous prograding and aggrading high-relief carbonate platform (Cantabrian Mountains, N Spain)

Seismic‐scale continuous exposures of an Upper Carboniferous (Bashkirian–Moscovian) carbonate platform (N Spain) provide detailed information about the lithofacies and stratal geometries (quantified with differential global positioning system measurements) of microbial boundstone‐dominated, steep prograding and aggrading platform margins. Progradational and aggradational platform‐to‐slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable, high‐relief depositional systems. The Bashkirian is characterized by rapid progradation at rates of 415–970 m My^?1. Characteristic outer‐platform facies are high‐energy grainstones with coated intraclasts, ooids and pisoids, moderate‐energy algal‐skeletal grainstones to packstones and lower energy algal packstone and boundstone units. The Moscovian aggradational phase is characterized by aggradation rates of 108 m My^?1. Coated‐grain shoals are less common, whereas crinoidal bars nucleated in well‐circulated settings below wave‐base. Boundstones form a belt (30–300 m wide) at the platform break and interfinger inwards with massive algal‐skeletal wackestones (mud‐rich banks). The progradational phase has divergent outer‐platform strata with basinward dips of 12° to 2°. Steep clinoforms with dips of 20–28° are 650–750 m in relief and possibly sigmoidal to concave in the lower part. The basinward‐dipping outer‐platform strata might be depositional for less than 6°, consistent with lithofacies deepening seaward. The basinward dip is attributed to the downward shift of upper‐slope boundstone, forced by late highstand and relative sea‐level fall, and to compaction‐induced differential subsidence during progradation. The aggradational phase is characterized by horizontally layered platform strata. Clinoforms steepen to 30–45° reaching heights of 850 m and are planar to concave. The evolution from progradation to aggradation, at the Bashkirian–Moscovian boundary, is attributed to increased foreland‐basin subsidence and decreased boundstone accumulation rates. Progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top. Within the major phases, aggradational–progradational increments are produced by third‐ to fourth‐order relative sea‐level fluctuations.     

Modeling hydrothermal fluid circulation and gravity signals at the Phlegraean Fields caldera

The Phlegraean Fields caldera is an active volcanic system where episodes of ground deformation are accompanied by significant changes in geochemical and geophysical parameters monitored at the surface. These changes derive from a complex interaction between magmatic system and hydrothermal fluid circulation. We calculate the gravity changes associated with the variable density of hydrothermal fluids. We simulate the multi-phase and multi-component fluid circulation triggered by a pulsating magma degassing, periodically increasing the discharge of CO₂-enriched fluids into the shallow hydrothermal system. The simulated evolution of the hydrothermal system successfully reproduces the observed composition of gas discharged at the surface. At the same time, results indicate that changes in average fluid density generate a detectable gravity signal that is of the same order of magnitude of the observed changes. This contribution to gravity changes can explain the peculiar behavior of gravity data collected at Solfatara, where surface hydrothermal phenomena are present. Simultaneous fitting of two independent sets of monitoring data (gas composition and gravity changes) confirms the conceptual model proposed for the hydrothermal system at Solfatara, and it provides new insights for the interpretation of gravity data.     

Comparative high-resolution chemostratigraphy of the Bonarelli Level from the reference Bottaccione section (Umbria-Marche Apennines) and from an equivalent section in NW Sicily: Consistent and contrasting responses to the OAE2

The Bonarelli Level (BL) from the upper Cenomanian portion of the reference Bottaccione section (central Italy) is characterized by the presence of black shales containing high TOC concentrations (up to 17%) and amounts of CaCO₃ near to zero. In the absence of carbonate and, consequently, of relative carbon- and oxygen-isotopic data, the elemental geochemistry revealed to be a very useful tool to obtain information about the palaeoclimatic and palaeoceanographic evolution of the Tethys Ocean during the OAE2. Based on several geochemical proxies (Rb, V, Ni, Cr, Si, Ba), the BL is interpreted as a high-productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle. The enrichment factors of peculiar trace metals (Zn, Cd, Pb, Sb, Mo, U) provide further insight about the H₂S activity at the seafloor during the organic-rich sediment deposition and permitted us to evaluate the use of Ba as palaeoproductivity tracer in conditions of high rate of sulphate reduction.By comparing geochemical records from the reference Bottaccione section (central Italy) with those previously obtained for the coeval Calabianca section (northwestern Sicily), different degrees of oceanic anoxia were delineated and ascribed to different abundance and type (degradable or refractory) of organic matter, which are limiting factors in the bacterial sulphate reduction reactions and in subsequent euxinic conditions at seafloor in the Tethys realm. Based on a ciclostratigraphic approach, consistent fluctuations at 100 ky scale in the chemostratigraphic signals from the two sections are inferred to be expression of a strong orbital-climatic forcing driving changes in the oceanic environment during the BL deposition.