Modeling gravity and trilateration data in Long Valley, California, 1983–1984

During the period July 1982 through July 1984 a series of repeated, high-precision gravity measurements have been undertaken in and around Long Valley, California. Data were collected at about 40 sites using three Lacoste-Romberg model G gravity meters. At the same time the U.S. Geological Survey was conducting a program of high-precision line length measurements throughout the same area. Significant changes were observed with both types of data during the intervals 1982–1983 and 1983–1984. Previous work by us has shown that deformation data during the interval 1975–1983 are consistent with a model for the inflation of Long Valley caldera which includes magma injection beneath the south central caldera at a depth of only 5 km. Simultaneous inversion of the trilateration and gravity data for the period 1983–1984 shows that most of the previous model is still reasonably consistent with the data and implies injected volumes of between 0.01 and 0.02 km3. Additionally, uplift during this period occurred along the Inyo Domes chain of vents and flows, indicating perhaps a new source of inflation in the northwest part of the caldera. This uplift amounts to a few centimeters and implies a volume increase of perhaps 0.004 km3 at reasonably shallow depth (3 km?). The total amount of uplift in the northwest is small in comparison to the total uplift centered on the central resurgent dome (total vertical displacement well in excess of half a meter). For purposes of comparison we examine several models, one of which involves injection at the base of the crust. This model implies much higher injected volumes, as much as 0.15–0.2 km3.