Iginio Marson

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.

Upper mantle properties of the Tuscan-Tyrrhenian area: a framework for its recent tectonic evolution

Abstract Within the Alpine-Mediterranean deformational system, several interesting features in the physical properties of the upper mantle have been recognized through the regional analysis of surface-wave dispersion. A large upper mantle anomaly (UMA), here called the Tuscan UMA, has been identified in the area northeast of Corsica, which includes part of the northern Tyrrhenian Sea and part of the North-Central Apennines. This anomalous upper mantle area together with the upper mantle characteristics of the Tyrrhenian Sea, located further to the south, provides a natural framework for comprehensive interpretation of the large data set of geological and geophysical observations, such as land and marine geology, volcanism, gravity, magnetic data, seismic data and seismology, deep temperatures, and surface heat flow.

Assessment of height variations by GPS at Mediterranean and Black Sea coast tide gauges from the SELF projects

Abstract In the framework of the European Union SEa Level Fluctuations (SELF) I and the SELF II Projects designed to study sea level variations around the Mediterranean and Black Seas, the Global Positioning System (GPS) technique was adopted to measure the ongoing crustal movements at tide gauge stations. Tide gauges measure sea-level variations with respect to a ground benchmark. In order to determine true sea-level variations of a few millimeters per year, it is necessary to estimate the ground vertical movement to a high degree of accuracy. Countries involved in the projects were Spain, France, Italy, Greece, Bulgaria and Russia. In the period from 1993 to 1998 repeated observations at more than 28 tide gauges and at a number of reference and intermediate stations were conducted. GPS measurements were complemented by Water Vapor Radiometers (WVR) at selected sites. Individual campaigns were evaluated and analyzed and showed height variations in the millimeter–centimeter range. All data were processed following common standards using the Bernese Software in a unified analysis to generate a combined solution. Based on the normal equations of all of the campaigns, a combined solution for the SELF Projects sites was generated. In the analysis of multi-technique anchor sites to the International Terrestrial Reference Frame ITRF97, it is shown that vertical rates of different techniques may be of different sign with respect to each other and also with respect to the ITRF97 combination. Vertical rates are not reliable in general and even the restriction to the use of long the long-time series sites KOSG, ONSA, WTZR, MADR, GRAZ and MATE did not ensure significant and unique vertical rates. Nevertheless, they had to be used for the reference frame definition. The results of repeated gravity observations and the continuous GPS observations at Porto Corsini and Medicina were compared to the epoch-wise GPS campaigns and revealed that the vertical rates are in conformance with each other if taken over the same period in time. The impact of the WVR observations on tide gauge position variation estimation is shown to improve the vertical component in the case of baseline-wise observations with two WVRs by up to 50%. It is shown, that the time span is too short for epoch-type observations to resolve significant height variations. The estimated rates of about 0–20 mm/year are most likely explained by systematic and random errors in the GPS observations. The absolute gravity observations at selected tide gauges are accurate to ±3 μGal and are in general agreement with the zero result from GPS. The main outcome of the SELF campaign observations therefore was the determination of a homogeneous zero-epoch data set and the creation of a database that includes the complete link from the GPS sites to the sea level in the five Mediterranean countries involved.

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.

Crustal stress crises and seismic activity in the Italian peninsula investigated by fractal analysis of acoustic emission, soil exhalation and seismic data

Abstract Crustal stress can be monitored by acoustic emissions (AE, ultrasound), which give an indication of whether a physical system is subject to stress, either of tectonic or endogenous origin. AE intensity critically depends on the damping of the signal; however, AE signals are clear indicators of the fatigue state of the crustal structures constituting the AE source. This aspect can be studied by fractal analysis of AE time series; these are, however, not suited for earthquake forecasting, as they only denote a changing state involving large lithospheric volumes. Several case histories from Italy show that an increased high-frequency AE activity (200 kHz) occurs approximately seven to eight months in advance of large earthquakes that affect areas of a few hundred kilometres radius, and an increased low-frequency AE activity (at 25 kHz) is observed several weeks in advance. Low-frequency AE also correlate with soil exhalation (water-well chemistry) and CH4, whereas fractal analysis of AE signals recorded close to a ‘future’ epicentral area gives a clear indication of the evolution of the system from about two months before the mainshock. This suggests that systematic monitoring of crustal stress variations may be used for assessing the time evolution of seismic activity.

Stromboli: a natural laboratory of environmental science

Abstract The science of environment is per se multi- and inter-disciplinary. It is not possible to separate the role of the physical, chemical, biological, and anthropic factors, respectively. Research must therefore rely on suitable natural laboratories, where all different effects can be simultaneously monitored and investigated. Stromboli is a volcanic island slightly North of Sicily, within a tectonic setting characterised by a Benioff zone, curved like a Greek theatre, opened towards the Tyrrhenian Sea, with deep earthquakes. Moreover, it is a unique volcano in the world in that since at least ∼3000 years ago, it has exploded very regularly, about every 15–20 min. Hence, it is possible to monitor statistically phenomena occurring prior, during, and after every explosion. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) has recently established a permanent Laboratory and an extensive interdisciplinary programme is being planned. A few main classes of items are to be considered including: (1) matter exchange (solid, liquid, gas, chemistry); (2) thermal and/or radiative coupling; (3) electromagnetic coupling; (4) deformation; (5) biospheric implications; and (6) anthropic relations since either the times of the Neolithic Revolution. Such an entire multidisciplinary perspective is discussed, being much beyond a mere volcanological concern. We present here the great heuristic potential of such a unique facility, much like a natural laboratory devoted to the investigation of the environment and climate.

Crustal stress and seismic activity in the Ionian archipelago as inferred by satellite- and ground-based observations, Kefallinia, Greece

Abstract Different observational techniques are compared in order to investigate possible correlations in seismic activity. The study site is the island of Kefallinà (Greece), where measurements available included (1) DInSAR, DGPS, and DEM data, (2) soil exhalation measured by monitoring Radon (Rn) well content, and (3) acoustic emissions (AE) at high and low frequency (point-like records with high temporal resolution). AE records provide: (1) relative time variation of the applied stress intensity and (2) the state of fatigue of stressed rock volumes, the AE source. Our results indicate that the large spatial scale (poor time resolution) may be considered quite satisfactory, whereas fractal analysis of the AE time series displayed some discrepancies when compared to analogous investigations in the Italian Peninsula. Therefore, some refinement is needed in order to reach more precise interpretations of the relevant information available with this kind of data. However, both sets of observations appear in agreement with each other, although more exhaustive investigations would require a suitable array of point-like AE and Rn (or other) measuring sites, as well as longer data series. The latter are particularly helpful for detailed interpretations of the different occurrences within tectonically complex settings where crustal stress crises are marked by various types of geological phenomena.

The role of gravity in the WEGENER project

Abstract Variations in the gravity field are introduced by mass or density redistribution in the vicinity of the measuring point as well as far field or global effects but also any crustal process which involves a height variation has a direct implication on the temporal variation of the gravity field. The measuring techniques involved in the WEGENER project include absolute and high precision relative gravity measurements and stationary measurements with superconducting gravity meters. The state of the art for both techniques is discussed and shown that systematic errors or the measurement of their changes can be detected by inter-comparison with other absolute gravimeters and frequently repeated measurements at a reference station monitored by a superconducting gravimeter. In the combination of the available gravity techniques it is possible to achieve a precision at the micro-Gal level for secular trends and a higher accuracy for period events which enable improvements in the modelling of environmental effects induced by ocean, atmospheric and ground water loading effects from the long term processes.