Continuous gravity and tilt observations in an active geodynamic area of southern Italy: the Calabrian Arc system

Abstract

Calabria (southern Italy) is a site of considerable seismic activity related to the ongoing evolution of the Calabrian Arc system, where a complex lithospheric structure is present. For over a century the Calabrian region has been going through a period of relative seismic quietness, yet its seismic hazard is at the highest levels in the Mediterranean basin due to several catastrophic earthquakes present in the historical records. In order to strengthen the geophysical monitoring of this region, a gravity and tilt recording station was set up in the premises of the University of Calabria. The recorded signals should allow to estimate a tidal anomaly, possibly correlated with the difference between some local feature of the lithosphere or geodynamic activity and the corresponding characteristics of the model used to calculate the reference gravity tide. We report here on the results obtained by the analysis of more than two years of continuous gravity and tilt observations at a site in the northern part of the region. The tidal analysis of the gravity records, covering the time interval May 2011 - September 2013, has provided amplitudes, amplification factors and phases of the main lunar and solar gravity tidal waves. A reliable model of the gravity tide is necessary for accurate processing of discrete absolute and relative gravimetric measurements and to detect in the gravity signals possible components correlated to major seismic activity. The Ocean Tide Load (OTL) effect was accounted for in the determination of the tidal field spectral parameters. The most widespread DDW99/ NH Earth’s model, adopted here as reference, fits the obtained results well enough. The tidal residual vectors do not highlight any significant anomaly ascribable to the complex structure of the lithosphere beneath the region. The analysis of the tilt records points out a manifest influence of the fluctuations of the air temperature on ground slope, at annual and diurnal periods. The spectral analysis highlights the presence, on both E-W and N-S components, of a significant S1 solar wave. Values of the thermal admittance, at diurnal period, have been estimated for both tilt components. A weak energy at the frequency of 1.932 cycles/day, at the limit of statistical significance, is also observed, identifiable as the main semidiurnal lunar wave M2 of the crustal tide. The amplitude and the amplification factors of such a wave are consistent with the values expected by the DDW99/NH model. Beyond everything, the obtained results of the analyses of gravity and tilt records have provided models for the gravity tide and tidal field in the Calabrian region. Moreover, it turns out that the response of the complex lithospheric structure in the Calabrian Arc system to the tidal stress field does not produce any significant anomaly related to the adopted model. Moreover, the absolute measurements of the gravity acceleration, carried out in 1994 and 2013, yielded coinciding values, implying that during the time interval of 20 years not any significant vertical movement and/or mass redistribution in the underground occurred in the area, despite its intense geodynamic activity.