The gravity low associated with the Geysers hydrothermal system is the southwest lobe of a regional 24-mgal gravity low centered near Mount Hannah in northern California. By subtracting the effect of a deep low-density source beneath Mount Hannah, a large 3- to 5-mgal gravity low is defined. This gravity low is interpreted to coincide with the present and past locations of the Geysers hydrothermal system. The producing steam field is now to the northeast of the anomaly and overlaps its northeastern boundary. The northeastern boundary of the anomaly consists of dense ultramafic and mafic rocks, between 0.5 and 1.0 km in thickness, which mask whatever density anomaly is associated with the steam reservoir below them. A three-dimensional density model constructed to fit this residual gravity field employs a low-density volume of about 100 km3 that has a density contrast of −0.06 g/cm3. If a deep low-density source is included in the model, the shallow density cannot be less than −0.04 g/cm3. This shallow low-density anomaly could be caused by a combination of high temperatures, unsaturated conditions, dissolution, and fracturing, and it may be flanked by density gradients due to chemical precipitation at the boundaries of the hydrothermal system.
The possibility of a deep low-density source to the gravity low indicates a possible extension of the deep low-density source that produces the 24-mgal low centered near Mount Hannah. An overlapping low-velocity anomaly coincides with the low-density region below Mount Hannah, and the low-velocity anomaly also extends beneath The Geysers. These low-density and low-velocity sources may be representative of the heat source that generated the Geysers hydrothermal system.