Deformation of the Long Valley Caldera, eastern California from mid‐1983 to mid‐1988: Measurements using a two‐color geodimeter

Abstract

Deformation of Long Valley caldera in eastern California has been closely monitored since June 1983 with a two‐color geodimeter, which has a precision between 0.12 and 0.20 ppm of the baseline length. Initial trilateration measurements were concentrated in the south moat of the caldera, the source of the January 1983 earthquake swarm. Since then, the network has been expanded from 13 to 42 baselines. The additional baselines span the resurgent dome and the western half of the caldera. With a combination of measurements made several times per week on 7–11 baselines and monthly or yearly measurements on the remaining baselines, we have measured both the temporal and spatial deformation of the caldera. Although the M 6.4 Chalfant Valley earthquake was located 35 km from the nearest baseline in the network, we observed coseismic strain changes of up to 1.5 ppm on several baselines. Using fault planes selected to coincide with the hypo‐centers of the main shock and the M 5.9 foreshock, we have inferred a combined moment of 3.0×1025 dyn cm from the two‐color data. This cumulative moment of both earthquakes is equivalent to a magnitude 6.3 shock. Over the period from mid‐1983 to mid‐1988, the long‐term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension. During 1983, measurements show a maximum extension rate of 5.0 ppm/yr. After mid‐ to late‐1984, the extension rates decreased by a factor of 2–4 relative to those in the first year. With the exception of the coseismic step, the extension rates have been fairly constant during the interval between late 1984 and mid‐1988. The long‐term changes in rate can be modeled by using two Mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat. Modeling indicates that both the rates of slip and inflation have decreased since mid‐1983, but these two processes are still occurring at a measureable and constant rate.