Temporal and areal gravity investigations at Long Valley Caldera, California

Abstract

Temporal gravity measurements over a broad network in the vicinity of Long Valley caldera, California, reveal significant gravity decreases over the interval June 1980 to July 1983 at five stations within or close to the caldera. The negative gravity changes reflect the late stages of an uplift of the caldera which began before the initial June 5–9, 1980, gravity survey (based on the gravity data) and after July 1979 according to the results of horizontal deformation measurements. Temporal gravity measurements at a more detailed network established within the caldera during June 1982 suggest that a low‐amplitude uplift occurred immediately after the January 1983 south moat earthquake swarm and that the uplift probably was accompanied by the addition of mass at depth. However, these data are complicated by gravity changes due to changes in depth to the water table. An areal gravity survey in the vicinity of the January 1983 swarm reveals two local gravity highs at 4–5 mGal amplitude and 2–3 km diameter. These local highs occur directly above the two areas that experienced the most intense earthquake swarm activity during the past few years and are believed to mark the locations of past intrusions into the south moat.