Susanna Zerbini

Sea level in the Mediterranean: a first step towards separating crustal movements and absolute sea-level variations

Abstract The SELF (SEa Level Fluctuations: geophysical interpretation and environmental impact) project has been developed and realized in the framework of the Environment Programme designed by the Commission of the European Communities. The SELF project was aimed at providing a reliable base for the determination, in the Mediterranean area, of sea-level variations which could then be used as a possible indicator of climate changes and to study the interactions taking place among the ocean, the atmosphere: and the solid Earth. The project has made it possible to define a consistent network of well-established tide gauges encompassing the Mediterranean Basin as far as the Black Sea and to determine to centimeter accuracy the tide gauge benchmark heights in a global well-defined reference system such as the one provided by the SLR/VLBI space techniques. The SELF network constitutes, for the Mediterranean, the necessary prerequisite towards achieving the actual capability to separate vertical crustal movements from true sea-level variations. This has been accomplished through the use of space techniques namely SLR, VLBI and GPS in conjunction with Water Vapor Radiometer observations and absolute gravity measurements. The analysis of the available tide gauge records has shown a high spatial coherence of the annual to multidecadal sea-level variability. Sea-level fluctuations at periods longer than two months were found to be strongly correlated with air pressure. The seasonal cycle was found to be variable in time. Relative sea-level trends determined from records longer than 30 years are less than 1.5 mm/yr. Crustal movement rates as determined from the tide gauge records are in general of the order ± 1.0 mm/yr. The geological observations have shed light on the fact that a marked variability of crustal movements occurs on both the temporal and spatial scale, and it represents a major contribution to relative sea-level fluctuations. This fact has been verified for the selected sector which belongs to one of the more geodynamically active areas of the Central Mediterranean (Aeolian Archipelago). However, this work has shown that, at least at the tide gauges included in the present study, crustal movements are small compared to the decadal to multidecadal sea-level variability but of the same order as the long-term trend in sea level, thus necessitating a careful monitoring if crustal movement is to be separated from the oceanographic contribution to relative sea-level changes.

Height and gravity variations by continuous GPS, gravity and environmental parameter observations in the southern Po Plain, near Bologna, Italy

Abstract During 1996, a Global Positioning System (GPS) receiver and a superconducting gravimeter (SG) were installed at Medicina, Italy, in order to monitor ground deformation and signals related to global/regional processes and local environmental effects. First results provided by this combined observing strategy are presented, by focusing on common patterns as well as on the differences observed in the two time series. A marked annual signal of increasing amplitude, present in both series, is becoming quite recognizable towards the end of 1997. Seasonal loading effects induced by air pressure, the ocean and surficial water table were estimated and modeled for both data sets. For the gravity series, 12-h data of balloon radio sounding launches have been used to estimate the attraction effect of the seasonal vertical density distribution of the air pressure column above the station. Mass effects due to the surficial water table and the ocean were also estimated and accounted for. The comparison between the observed and modeled series, both in the case of GPS heights and gravity, shows a remarkable agreement and provides a convincing explanation for the observed seasonal fluctuations. In the July–September 1997 time frame, a sudden gravity increase occurred. This anomaly is in the order of 3 μGal and is present in the data series after accounting for the seasonal fluctuation. This step-like feature is responsible for most of the estimated positive linear trend present in the data series. The anomalous increase in gravity has been interpreted as due to mass/density variations, likely associated with the uprising of deep-seated waters in relation with local stress field changes. During the second half of 1997, concurrent seismic activity occurred both in the southern Po Valley and in central Italy (Umbria seismic crisis).

Scientific objectives of current and future WEGENER activities

Abstract The WEGENER group has promoted the development of scientific space-geodetic activities in the Mediterranean and in the European area for the last fifteen years and has contributed to the establishment of geodetic networks designed particularly for earth science research. WEGENER currently has three scientific objectives which are related to plate-boundary processes, sea-level and height changes, and postglacial rebound. In a full exploitation of the space-geodetic techniques, namely SLR, VLBI and GPS, the individual scientific projects do not only pursue these objectives but also contribute to improving and developing the observation techniques as well as the modelling theories. In the past, particularly SLR observations within WEGENER-MEDLAS have provided a fundamental contribution to determine the regional kinematics of the tectonic plates in the Mediterranean with high precision. With GPS, spatially denser site distributions are feasible, and in several WEGENER projects detailed studies of tectonically active areas were possible on the basis of repeated episodic GPS observations. Current projects associated with WEGENER are successful in separating crustal movements and absolute sea-level variations as well as in monitoring postglacial rebound. These tasks require high-precision height determinations, a problem central to all of the present WEGENER activities. In these projects, continuously occupied GPS sites are of increasing importance. Time series of heights observed with continuous GPS can be determined with a few centimeters RMS error thus enabling the reliable estimates of vertical rates over relatively short time intervals. Regional networks of continuous GPS sites are already providing results relevant, for example, for the study of postglacial rebound. The Mediterranean area is an extraordinary natural laboratory for the study of seismotectonic processes, and the wealth of observations acquired in previous WEGENER projects together with new space-geodetic observations will allow the test of geophysical hypotheses linking three-dimensional deformations of the Earths surface to the dynamics of the Earths interior. In particular, it is anticipated that WEGENER projects will aim at a test of the slab-detachment hypothesis. The complex investigations on sea-level fluctuations presently carried out at basin scale from the Strait of Gibraltar to the Black Sea make it possible to study the present and recent past interactions of ocean, atmosphere and solid Earth, as well as to develop appropriate models to assess future aspects.

Geodetic measurements at the northern border of the Adria plate

Abstract This work describes measurements and results relevant to crustal movements at the northern border of the Adria plate. The measurements taken into consideration are subsurface observations made with extensometers and tiltmeters and episodic and continuous GPS (CGPS) observations. The subsurface measurements cover a remarkably long period of 34 years, while the episodic GPS campaigns have been performed over a time period of 7 years. The CGPS observations are available since July 1996, and 3.5 years of data have been considered in this work. The main characteristics of each different type of measurement are illustrated, and a first approach is made to compare the results. It is shown that environmental effects are present both in subsurface and surface measurements. Methods to interpret and remove these signals are also proposed. The correlation between crustal deformation and seismic events is investigated, and some examples are presented. The long-term observations are discussed with respect to a well-known model of plate motion. The subsurface crustal long-term observations reveal inversions in the direction of deformation with time scales of 10 or more years, providing important boundary conditions for future geodynamic models of the northern Adria plate. The results show that the comparison between subsurface deformation and CGPS measurements, if made over a sufficiently long period of time, in the order of 10 years, can give important contributions to the understanding of the geodynamical and physical mechanisms related to the earthquake occurrence in this area.

The goals, achievements, and tools of modern geodesy

Friedrich Robert Helmert (1843-1917) defined geodesy as the science “of measurements and mappings of the Earth’s surface”. Over time, this definition of geodesy has been extended, mainly as a consequence of technological developments allowing geodesy to observe the Earth on global scales with high accuracy. Today, geodesy is the science of determining the geometry, gravity field, and rotation of the Earth and their evolution in time. This understanding of modern geodesy has led to the definition of the “three pillars of geodesy”, namely (1) Geokinematics, (2) Earth Rotation and (3) the Gravity Field (see Figure 1.1 on page 4). These three pillars are intrinsically linked to each other, and they jointly change as a consequence of dynamical processes in the Earth system as a whole. The changes in Earth’s shape (including the surface of the water and ice bodies), i.e. the geokinematics, are the result of dynamic processes in the solid Earth and its fluid envelope, affecting mass distribution and angular momentum, and thus changing the gravity field and Earth rotation. Traditionally, geodesy has been a service science, providing an important utility to other sciences and many applications. This aspect has remained unchanged, and a principal tool and output of geodesy is a reference frame allowing the determination of the position of points relative to each other. But geodesy has developed into a science that can no longer satisfy this service aspect without encompassing and monitoring the whole Earth system, its kinematic and dynamics. As an additional benefit, geodesy is increasingly forced not only to “measure” the geokinematics, gravity field, and rotation, but also to “model” these quantities on the basis of mass transport and dynamics. The instruments (or measurement tools) are of crucial importance in geodesy. They in essence define the scope of the problems, which may be addressed by geodesy. Before the advent of the space age the geometrical aspects were studied mainly by measuring angles and time (time-tagging of the observations). In the best

Linking the Global Geodetic Observing System (GGOS) to the Integrated Global Observing Strategy Partnership (IGOS-P) through the Theme ‘Earth System Dynamics’

When setting up GGOS as a project, the IAG Executive Committee asked the GGOS Steering Committee to establish a relationship with IGOS-P. IGOS-P addresses a number of problems and components of Earth observing systems in the frame of specific Themes. The IGOS-P Theme process will also be an important mechanism for the development of the components of the Global Earth Observation System of Systems (GEOSS).

Height variations and secular changes in sea level

Abstract The problem of understanding, assessing and quantifying the causes of sea-level changes has received increased attention during the past several years, in particular in connection with predictions regarding a rise in the global mean sea level due to global warming. The impact aspect of sea-level rise is of particular concern for many coastal areas in densely populated regions of the world. The present availability of space geodetic techniques providing high-accuracy measurements of station positions and of their time variations makes it possible to separate vertical crustal movements and absolute sea-level fluctuations. Gravimetric methods play an important role in the assessment of sea-level variations by providing completely independent measurements of vertical crustal movements. Satellite altimetry now enables the measurement of the spatial variability of sea level. Geological observations can infer former sea levels which are important to understand the present trends. This paper addresses the study of height variations and secular changes in sea level by discussing the most interesting questions and issues, by presenting the current understanding of the phenomena involved, and finally by illustrating the measuring and modeling techniques to be adopted for a comprehensive approach to the problem.

Assessment of the possible contribution of space ties on-board GNSS satellites to the terrestrial reference frame

The realization of the international terrestrial reference frame (ITRF) is currently based on the data provided by four space geodetic techniques. The accuracy of the different technique-dependent materializations of the frame physical parameters (origin and scale) varies according to the nature of the relevant observables and to the impact of technique-specific errors. A reliable computation of the ITRF requires combining the different inputs, so that the strengths of each technique can compensate for the weaknesses of the others. This combination, however, can only be performed providing some additional information which allows tying together the independent technique networks. At present, the links used for that purpose are topometric surveys (local/terrestrial ties) available at ITRF sites hosting instruments of different techniques. In principle, a possible alternative could be offered by spacecrafts accommodating the positioning payloads of multiple geodetic techniques realizing their co-location in orbit (space ties). In this paper, the GNSS–SLR space ties on-board GPS and GLONASS satellites are thoroughly examined in the framework of global reference frame computations. The investigation focuses on the quality of the realized physical frame parameters. According to the achieved results, the space ties on-board GNSS satellites cannot, at present, substitute terrestrial ties in the computation of the ITRF. The study is completed by a series of synthetic simulations investigating the impact that substantial improvements in the volume and quality of SLR observations to GNSS satellites would have on the precision of the GNSS frame parameters.

SCF-Test of a Galileo-IOV retroreflector and thermal-optical simulation of a novel GNSS retroreflector array on a critical orbit

Thorough laboratory measurements performed at INFN-LNF (Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali di Frascati), in the framework of the ETRUSCO (Extra Terrestrial Ranging to Unified Constellations) experiment, proved fundamental to characterize retroreflectors for GNSS (Global Navigation Satellite System) satellites. The standard test developed, SCF-Test, was important to outline the weaknesses of past retroreflectors payloads (in use on GPS, GLONASS and GIOVE A/B satellites). For the upcoming deployment of the Galileo constellation ESA requested, in 2010, a full SCF-Test campaign to characterize a prototype retroreflector of the first IOV satellites. We report the results of a standard SCF-Test and the test of a simulated orbit, called GCO (Galileo Critical halfOrbit). The experience gathered with the ETRUSCO experiment was important for the subsequent project, ETRUSCO-2, whose aim is to develop and measure, in a newly built facility, a full size array of retroreflectors to be deployed on GNSS constellations. Here we report preliminary concurrent thermal and optical simulations of a simulated array. A simplified structure of the array was subject to a simulated space environment in a GCO; the resulting temperature distribution inside each retroreflector, was the input of the optical software to determine the variation of the intensity, throughout the orbit, coming back at a ranging station. The goal is to limit as much as possible signal fluctuations with respect to current deployed arrays.

Global and regional sea-level changes and the hydrological cycle

with contributions by the conference participants: P. Axe, A. Belperio, N. Bernier, R. Bingley, M. Bolgov, W. Bosch, A. Braun, C. Cabanes, A. Cazaneve, J. Chen, S. Cloetingh, G. Di Donato, J. Gregory, R. Hohmann, V. Klemes, P. Knudsen, M. Kuhn, R. Lane, G. Liebsch, J. Lowe, O. Mason, F. Mercier, M. Negusini, C. Paniconi, W. Peltier, H. Plag, F. Remy, B. Richter, A. Selivanov, C. Shum, F. Singer, N. Slotsvik, E. Stanev, N. Teferle, D. Thomson, J. Titus, K. Van Onselen, I. Vilibic, G. Woppelmann, J. Zwally