Decrease in deformation rate observed by two-color laser ranging in long valley caldera

After the January 1983 earthquake swarm, the last period of notable seismicity, the rapid rate of deformation of the south moat and resurgent dome of the Long Valley caldera diminished. Frequently repeated two-color laser ranging measurements made within a geodetic network in the caldera during the interval June 1983 to November 1984 reveal that, although the deformation accumulated smoothly in time, the rate of extension of many of the baselines decreased by factors of 2 to 3 from mid-1983 to mid-1984. Areal dilatation was the dominant signal during this period, with rates of extension of several baselines reaching as high as 5 parts per million per annum during the summer of 1983. Within the south moat, shear deformation also was apparent. The cumulative deformation can be modeled as the result of injection of material into two points located beneath the resurgent dome in addition to shallow right lateral slip on a vertical fault in the south moat.