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The mechanics of unrest at Long Valley caldera, California:
1. Modeling the geometry of the source using GPS,
leveling and two-color EDM data
J. Volcanol. Geotherm. Res., 127, 195-217, 2003.
[Printable article (1.3 Mb)]
M. Battaglia a, P. Segall a, J. Murray a, P. Cervelli a, J. Langbein b
a Department of Geophysics, Stanford University, Stanford, CA 94305-2215, USA
b U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, USA
Summary. This paper presents the results of an analysis using a combination of Global Positioning System (GPS), leveling, and two-color Electronic Distance Meter measurements to investigate the source of uplift in the Long Valley Caldera. The Long Valley Caldera, located east of the Sierra Nevada mountains in California, is the site of volcanic activity. It was formed during a huge eruption 760,000 years ago, and later additional smaller eruptions formed the resurgent dome, a small hill, in the center of the caldera. The most recent volcanic activity in the Long Valley area occurred north of the caldera about 600 years ago. Starting with a strong earthquake swarm in 1980, Long Valley caldera has experienced continuing bouts of seismicity and measurable uplift of the ground surface in recent years. A number of studies have concluded that a source of inflation exists about 7 km below the resurgent dome, causing the observed uplift. The inflation could be caused by magma or hydrothermal fluids (hot water with dissolved minerals).
One goal of the work presented in this paper was to improve estimates of the depth and the shape of the inflation source. The results support the existence of an ellipsoidal source at ~6 km depth below the resurgent dome. This source shape is more realistic than the idealized point sources of inflation often used in modeling volcanic deformation.
The source of the inflation (magma or hydrothermal fluids) can be investigated using gravity measurements which are sensitive to density variations of material underground. However, accurate measurements of ground uplift are crucial to the proper use of gravity data. Therefore, another goal of this paper was to use GPS and leveling data together to accurately measure uplift in the caldera. In a subsequent paper M. Battaglia showed that the gravity data from Long Valley are more consistent with the existence of magma, rather than hydrothermal fluids, beneath the caldera.
Fig. 1. Map of Long Valley caldera geodetic networks.
