| A five-station electronic distance meter (EDM) network centered around the Novarupta dome in Katmai National Park, Alaska, was established in 1990 and resurveyed in 1993 and 1995. Both EDM and Global Positions System (GPS) measurements were made in 1993. The 1995 survey was restricted to the more accurate GPS surveying method. Analysis of EDM data in 1993 suggested an average increase of 22.5 millimeters in slope distances between stations from 1990 to 1993. Those changes were about 2-3 times the expected error for lines of this length (1.5 to 4.7 kilometers) and suggested that ground deformation was taking place in the Novarupta area. Loss of data from one of five GPS receivers in 1993 precluded complete comparison of GPS data with EDM results. In July 1995, the network was occupied with five P-code GPS receivers. Two 12-15 hour observations were made simultaneously at all five stations, which were located relative to a reference GPS station in Fairbanks. For final L1-only solutions and EDM line lengths between stations, one station was held fixed using these new coordinates. The 1995 GPS results, when compared with recomputed EDM line lengths for 1990 and 1993 and available 1993 GPS line lengths, indicate that the Novarupta site moved about 15-20 millimeters to the west and the Mainstreet station moved a similar distance to the northwest during the interval from 1990 to 1993. There is a suggestion that the Mainstreet stations also moved from 1993 to 1995. The movement at both stations is thought to be a result of a deformation source to the southeast outside the network or associated with the stability of the sites. The movement is not a result of volcano deformation centered at Novarupta. To further evaluate ground deformation in the Katmai area, extension of the network with GPS observations beyond the immediate vicinity of Novarupta is recommended. -- Kleinman, et.al., 1997 |
