Differential GPS measurements at Santa Ana (Illamatepec) Volcanic Complex and associated deformation at the Coatepeque Caldera, September 2008 - September 2009

We describe a feasibility study of differential GPS measurements within a 13-station GPS network that spans the Santa Ana volcanic complex and Coatepeque Caldera. Caldera forming events occurred from 70-40 ka, and at Santa Ana/Izalco volcanoes eruptive activity occurred as recently as 2005. Twelve differential stations were surveyed for ~1-hour on a monthly basis from February 2009 through September 2009 and tied to a centrally located continuous-GPS station, which served as the network reference site. Repeatabilities of the horizontal (north and east) and components of the inter-station baselines range from ~ 3 - 4 mm, suggesting an approximate lower detection limit for the horizontal components of any short-term tectonic or volcanic deformation. Repeatabilities of the vertical baseline component range from 8 - 40 mm. Differential motions of the 12 sites relative to the continuous reference site reveal inflation from Feb. 2009 to July 2009 at several sites surrounding the caldera, with vertical displacements up to 139 mm, followed by deflation with measured vertical displacements greater than 40 mm on 1.8-7.4 km-long baselines. Uplift rates for the inflationary period reach 300 mm/yr with 1σ uncertainties of +/- 26 - 119 mm. Stations farther from the caldera do not exhibit similar uplift, suggesting a localized source. Elastic deformation modeling via a Mogi approach is currently underway in an effort to constrain possible deformation source parameters. The results suggest that differential GPS measurements are a useful monitoring tool for sub-tropical stratovolcanos and calderas.