X - 2 Melini and Piersanti : Global Seismicity and Sealevel

We analyze the effect of seismic activity on sealevel variations, by computing the time-dependent vertical crustal movement and geoid change due to coseismic deformations and postseismic relaxation effects. Seismic activity can affect both the absolute sealevel, by changing the Earth gravity field and hence the geoid height, and the relative sealevel, i.e. the radial distance between seafloor and geoid level. By using comprehensive seismic catalogues we assess the net effect of seismicity on tidal relative sealevel measurements as well as on the global oceanic surfaces, and we obtain an estimate of absolute sealevel variations of seismic origin. We improved the computational methods adopted in previous analyses considering the issue of water volume conservation through the application of the sealevel equation and enabling us to evaluate the effect of an extremely large number of earthquakes on large grids covering the whole oceanic surfaces. These new potentialities allow us to perform more detailed investigations discovering a quantitative explanation for the overall tendency of earthquakes to produce a positive global relative sealevel variation. Our results confirm the finding of a previous analysis that, on a global scale, most of the signal is associated with few giant thrust events, and that RSL estimates obtained using tide-gauge data can be sensibly affected by the seismic driven sealevel signal. The recent measures of sealevel obtained by satellite altimetry show a wide regional variation of sealevel trends over the oceanic surfaces, with the largest deviations from the mean trend occurring in tectonically active regions. While our estimates of average absolute sealevel variations turn out to be orders of magnitude smaller than the satellite measured variations, we can still argue that mass redistribution associated with aseismic tectonic processes may contribute to the observed regional variability of sealevel variations. A detailed study of these tectonic contributions is important to acquire a complete understanding of the global sealevel variations and will be subject of future investigations.