OPTIMIZATION OF KINEMATIC GPS DATA ANALYSIS FOR LARGE SURFACE DEFORMATION FROM THE JULY 2003 DOME COLLAPSE AT SOUFRIèRE HILLS VOLCANO, MONTSERRAT

The continuous GPS network is an increasingly effective technique in monitoring volcanic processes. It is important then to resolve the atmospheric changes from the position variances to obtain precise positioning. In a region like the Caribbean, where Montserrat lies, this area is subjected to rainy and even hurricane seasons. During the 12-13 July 2003 eruption and dome collapse of Soufrière Hills Volcano (SHV), the established cGPS receivers collected data at 30 sec intervals covering the massive dome collapse and volcanic unrest. Data were then originally processed with GOA-II (v. 5) using high-rate (30 s) final, precise orbit, clock, and earth orientation parameter products, while treating the antennae as a kinematic buoy. When data was being run through GOA, the parameters for the random walk of the wet zenith delay, elevation cutoff, the horizontal gradient and random walk of position were kept at the default values encoded in the program. After reviewing the position data, one GPS station, HERM recorded a maximum vertical displacement of -1.98 m from its mean, with negligible horizontal movement, rebounding within an hour. This estimate of vertical site displacement was an order of magnitude larger than those estimated at other sites on SHV. This study investigated the standard parameters and reprocessed the data using various parameters to increase the precision of GPS position estimates during the eruption. BGGY, a station located 48 km northeast on Antigua was used as a control to attain the best constraints for modeling the atmospheric variations more accurately for this type of environment since this station is subjected to the same weather patterns but is unaffected by volcanic activity. Once the best constraints were decided with statistical analyses for verification, HERM was reprocessed. The vertical movement has a maximum of 2.5 cm with a variation of 30 cm in the wet zenith troposphere. Original default parameters were too constraining on the atmospheric variation for this tropical environment but now ground deformation and volcanic processes can be better understood.