Deformation of Long Valley Caldera, Mono County, California, from 1975 to 1982

Abstract

Teleseismic and gravity data acquired in 1973 support the existence of a large body of partial melt (magma) beneath the resurgent dome of Long Valley caldera in eastern California. Whereas intermittent reoccupation of vertical and horizontal control networks in and near Long Valley caldera revealed only very small, probably tectonic movements between 1932 and 1979, a leveling survey in September 1980 showed a 250‐mm uplift of Highway 395 on the west side of the resurgent dome. This uplift was apparently related to expansion of the subjacent magma body beginning sometime between July 1979 and September 1980 and may have been coseismic with the Mammoth Lakes magnitude 6 earthquake sequence of May 25–27, 1980. These earthquakes ushered in a period of almost continuous low‐level seismicity, punctuated by relatively severe swarms in the southern moat. Vertical and horizontal displacements within Long Valley caldera between 1979 and 1982 define a striking radially symmetric pattern of uplift and horizontal distension that is consistent with expansion of the body of partial melt inferred from teleseismic and gravity data. Specifically, the observed deformation of the caldera floor approximates the theoretical deformation due to expansion of a spherical magma chamber at a depth of at least 7 km. The calculated maximum uplift is 430 mm and is located south of the center of the resurgent dome. Significant deviations from this simple model appear to be associated with either movement of mapped faults within and adjacent to the caldera or folding of weak surface deposits.