The principle of measuring unusual change of underground mass by optical astrometric instrument

:Optical observations from a ground-based astrometric instrument provide a unique set of data sensitive to variations of local plumb line, which represent the change of local gravitational field on the ground. The local gravitational change could be caused by density variation or movement of underground mass in addition to tidal action. Earthquake was found to be related with gravitational change. A seismic zone could have abnormal underground material density, causing the variation of plume line on the ground. In this frame, the astrometric telescope can measure the variation of local plumb line by observing the local astronomical longitudes and latitudes from stars in the Hipparcos catalog.In this study, we estimate the deflection angle of the plumb line on a ground site, and give a relation between the angle, abnormal mass and site distance (depth and horizontal distance). Then we derive the abnormality of underground material density using the plumb lines measured at different sites, and study the earthquake gestation, development and occurrence. Using the deflection angles of plumb lines observed at two sites, we give a method to calculate the mass and the center of gravity of underground materials. We also estimate the abnormal masses of latent seismic zones with different energy, using thermodynamic relations, and introduce a new optical astrometric instrument we had developed.