Displacement of Surface Monuments: Vertical Motion

Several measurements of vertical ground motion at Pition Flat Observatory, California, indicate the overall weakness and instability of the Earth's weathered surface with respect to the underlying rock. Cumulative long-period motions of order 0.5 mm per year dominate these records, though smaller elastic deformations caused by precipitation loading, atmospheric loading, and tidal strains are evident at higher frequencies; all of these help to characterize the near-surface material. The long-period records suggest hat near-surface weathering is the dominant influence on monument motion, at least at this site on crystalline rock in a semi-arid environment. Rainfall loading gives an average vertical modulus of 2.6 GPa for the material in the uppermost 26 m of the ground, compared with 88 GPa for granite under moderate confining stress; atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours. Earth tide records show Poisson's ratio to be 0.09 in contrast o the normative range of 0.2-0.25, establishing that horizontal strains couple only weakly into vertical ones, so that vertical strains near the surface are a poor measure of areal strain. The form of the ground-surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument-positioning error budget that increases with time. Because of ground instability, and the generally small rate of crustal tectonic motions, deeply emplaced monuments will be needed for observational programs designed to detect short-term changes in crustal deformation over baselengths of order 10 km and less.