D B Stone, J Kienle, and A M Goodliffe (Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-0800)
As part of an attempt to delineate the subsurface structure beneath the region around Novarupta, the main vent of the 1912 Katmai eruption, 145 surveyed points were occupied for gravity and magnetic measurements in 1989. A subset of 17 of these points were reoccupied in 1990 in an endeavor to detect subsurface changes. The 1990 field season also included additional aeromagnetic surveys flown at three elevations; 800m to help delineate the western boundary of the caldera; 1100m to fill in gaps in the 1989 survey and 1220m to extend a previous survey by Anma (1971).
The variations in gravity between 1989 and 1990 at the 17 reoccupied positions are up to 10 microgals, and are predominantly in the negative sense. Much of the variation is within the range of possible error, but could also be due to a regional depression in the water table. Ground movements cannot be ruled out as a contributory cause, as the results of the 1990 reoccupation of the geodetic net have not yet been received. Variations in the surface magnetics are again small and in the range of possible errors. Detailed magnetic profiles appear to show evidence of near surface intrusives around Novarupta. The 800m aeromagnetic shows a curvelinear region of high magnetic gradient believed to be the western caldera boundary. The position of this boundary is in agreement with previous theories. The other two aeromagnetic surveys better define the anomalies over the Turtle and Novarupta, as well as a possible anomalous body in the region of Falling Mountain.
Modelling based on the combination of ground magnetics, gravity, aeromagnetics and vertical magnetic gradients is helping to constrain the size and depth of the vent structures and also the dimensions of the caldera. Modelling at present favours two subsurface sources of anomalous fields located beneath the Turtle and Novarupta, respectively.
References:
Anma, K., An aeromagnetic survey in the Valley of Ten Thousand Smokes, Alaska, M. S. thesis, Univ. of Alaska, Fairbanks, 1971.