R W Saltus (MS 964, USGS, Denver, CO 80225; 303-236-1375; saltus@musette.cr.usgs.gov)
D B Stone, J Kienle, and A M Goodliffe (Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775)
As part of on-going studies in support of proposed scientific drilling at Novarupta (the source of the 1912 eruption that formed the Valley of Ten Thousand Smokes, Eichelberger et al, 1991), we collected new gravity data in July 1991. These data, combined with previous measurements, define a gravity low associated with the active volcanic system in the vicinity of Katmai Pass. The anomaly spans approximately 400 square km, extends 24 km along the NE-SW strike of the magmatic arc and 16 km in the perpendicular direction, and reaches a minimum value of -27 mgal. Both Novarupta and the contemporaneously formed Katmai Caldera are on the flanks of the gravity low. Modeling the anomaly with a density contrast of -500 kg m-3 gives a source volume of 280 cubic km. Fitting spherical sources to gravity profiles across the anomaly yields centers at depth of 5 to 8 km. These models agree with previous results of Kienle (1969): the top of the anomalous body may be within 2 km of the surface.
The Katmai volcanic anomaly is nearly identical to the gravity signature of the Geysers volcanic field in northern California, and similar teleseismic delays are observed in both areas. Ward, Pitt and Endo (1991) have measured travel time delays of up to 0.9 seconds at a seismic station located near the gravity minimum at Katmai Pass; Oppenheimer and Herkenhoff (1981) found delays of up to 1 second near the gravity minimum at Mt Hannah in the Geysers/Clear Lake region. In both cases, the correlation of low-density and low-velocity sources in regions of high heat flow and recent volcanism strongly suggest that the anomalies are caused by magma bodies within the crust.