Multi-parameter continuous observations to detect ground deformation and to study environmental variability impacts

Abstract

In the framework of the European Union SELF II project, a study was developed in order to assess the accuracy with which vertical crustal movements could be determined by means of continuous GPS and gravity observations in a relatively short timespan of a few years. The reliable knowledge of vertical rates at tide gauge stations is necessary to properly interpret sea level variations. For height determinations, continuous GPS and gravity measurements started in mid-1996 at Medicina, in the southern Po Plain. Additionally, continuous GPS observations have also been performed at Porto Corsini, on the Adriatic coast, where a tide gauge station is located which belongs to the Italian tide gauge network, and at a station in Bologna. Negative linear trends, of different magnitude, have been identified at the three stations. The time variability of gravity and GPS heights in relation to variations of several environmental parameters was investigated. A marked seasonal signal has been identified in both data series. It has been interpreted as the sum of different loading and Newtonian attraction effects modeled on the basis of the relevant environmental data series. At Medicina, the comparison between height and gravity series has shown that the seasonal variations are quite comparable both in amplitude and phase. The only remarkable difference between the two data sets is a sudden increase in gravity, in the order of 3 μgal, observed in mid-1997. This has been attributed to mass/density increase associated with uprising of deep-seated salty (connate) waters, likely triggered by local stress field changes. A simple model is proposed to check the feasibility of the suggested mechanism and the magnitude of the relevant gravity anomaly. This study demonstrated, among other aspects, the importance of collecting continuous, high accuracy, multi-parameter data series for an appropriate interpretation of signals related to environmental variability. For sea level fluctuation studies, the need for determining reliable long-term linear trends in station heights has been demonstrated for the Porto Corsini station. The sea level trend estimated by means of the tide gauge data and the GPS vertical crustal rate has been compared with the absolute sea level trend for the Northern Adriatic provided by the Topex–Poseidon satellite altimetry mission. Over the time frame of the satellite altimetry data set, the results provided by the two different measuring approaches agree within the errors of the estimated sea level trends.