The mechanics of unrest at Long Valley caldera, California: 1. Modeling the geometry of the source using GPS, leveling and two-color EDM data

We surveyed 44 existing leveling monuments in Long Valley caldera in July 1999, using dual frequency global positioning system (GPS) receivers. We have been able to tie GPS and leveling to a common reference frame in the Long Valley area and computed the vertical deformation by differencing GPS-based and leveled orthometric heights. The resurgent dome uplifted 74±7 cm from 1975 to 1999. To define the inflation source, we invert two-color EDM and uplift data from the 1985–1999 unrest period using spherical or ellipsoidal sources. We find that the ellipsoidal source satisfies both the vertical and horizontal deformation data, whereas the spherical point source cannot. According to our analysis of the 1985–1999 data, the main source of deformation is a prolate ellipsoid located beneath the resurgent dome at a depth of 5.9 km (95% bounds of 4.9–7.5 km). This body is vertically elongated, has an aspect ratio of 0.475 (95% bounds are 0.25–0.65) and a volume change of 0.086 km3 (95% bounds are 0.06–0.13 km3). Failure to account for the ellipsoidal nature of the source biases the estimated source depth by 2.1 km (35%), and the source volume by 0.038 km3 (44%).

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