GEOLOGICAL SURVEY Remeasurement of a Precision Gravity Network for

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade npmes is for descriptive purposes only and does not imply endorsement by the USGS. A precision gravity network consisting of approximately 75 stations (established to monitor the Lassen geothermal system) was remeasured to determine normal seasonal variations. Gravity measurements made in July and September, 1983 were compared to the original measurements made in 1981 and 1982. The average observed change in relative gravity was +.015 mGal with average uncertainties (computed standard errors) of.009 mGal. These small changes can be explained by water table fluctuations of less than a meter. We conclude that the network is sufficiently stable to be an accurate monitor of surface deformation, subsurface magma movement, or changes in the subsurface water mass