EDIFICE CONTRACTION FROM 1981 TO 2006 OF MOUNT BAKER, WASHINGTON: RESULTS FROM CAMPAIGN GPS RESURVEY OF EDM NETWORK

In the summer season of 2006, a campaign GPS study of Mount Baker, Washington, was conducted to determine if any net surface deformation has occurred on the stratovolcano during the past quarter century. Mount Baker is a particularly interesting candidate for geodetic study because of continued elevated degassing (CO₂ and H₂S) and heat flux from fumaroles in Sherman Crater since the onset of elevated activity in 1975. These elevated emissions suggest the possibility that a magmatic source has been degassing since at least this time. In 1981, a trilateration network of 14 benchmarks was installed by the USGS Cascades volcano observatory (CVO) and surveyed with an Electronic Distance Meter (EDM). We conducted a campaign-style GPS reoccupation of the network in the summer of 2006 to assess surface deformation of the volcano since the early 1980s. Ten of the fourteen stations were visited during the field season; nine were occupied with GPS for 4-32 hours, while one mark was not recovered and presumed destroyed. A pilot GPS survey in 2004 recovered one additional mark, and when combined with 2006 data, we have determined line length change since 1981 on ten of the eighteen trilateration lines. Five of the trilateration lines show shortening greater than error, which is estimated at about ± 3 cm. The data indicate contraction on the northern flank of the volcano, where four of the trilateration lines have shortened by 4.23 to 5.14 cm. An additional line connecting Sherman Crater to the southern flank shows contraction of 4.67 cm. These results suggest contractional strain is centered on the summit of Mount Baker. The average rate of shortening in any line is 2 mm/year or less.