Methods used to extract the displacement and its error for each of the baselines are described in Langbein et al. (1993) and Langbein et al. (1995). The formal instrument variance of the two-color measurements is s 2 = a2 + b2 L 2, where a = 0.3 mm and b = 0.12 ppm of the baseline length L (Langbein et al., 1993). This does not account for any time-dependent noise due to local motion of the geodetic monuments (Langbein et al., 1995), which is of the order of 1 mm/yr1/2 (Langbein and Johnson, 1997). The two-color geodimeter measures distances to a precision of 0.5 to 1.0 mm for ranges between 3 and 8 km. The general trend of the data is 1) a general decrease in the rate of extension from 1983 through most of 1989, 2) an abrupt increase in the rate of extension on several baselines to 5 cm/yr. from late 1989 through early 1990, and 3) a decrease to 2 to 3 cm/yr. extension rate that has persisted since mid-1990.
For more details, contact: John Langbein (langbein@shasta.wr.usgs.gov)
References M. Battaglia, P. Segall, J. Murray, P. Cervelli, J. Langbein, 2002. .The mechanics of unrest at Long Valley caldera, California: 1. Modeling the geometry of the source using GPS, leveling and 2-color EDM data. J. Volcanol. Geotherm. Res. (in press).
Langbein, J., 1989. Deformation of the Long Valley caldera, eastern California, from mid-1983 to mid-1988: measurements using two-color geodimeter, J. Geophys. Res., 94, 3833-3849.
Langbein, J., Hill, D.P., Parker, T.N., Wilkinson, S.K., 1993. An episode of reinflation of the Long Valley Caldera, eastern California, 1989-1991. J. Geophys. Res , 98, 15,851-15,870.
Langbein, J. Dzurisin, D., Marshall, G., Stein, R., Rundle, J., 1995. Shallow and peripheral volcanic sources of inflation revealed by modeling two-color geodimeter and leveling data from Long Valley Caldera, California, 1988-1992. J Geophys. Res. 100, 12487-12,495.
Langbein, J., Johnson, H., 1997. Correlated errors in geodetic time series; implications for timedependent deformation. J. Geophys. Res. 102, 591-604.