9614687 Schwartz This proposal is to investigate the seismic and geodetic behavior of several andesitic stratovolcanoes in Costa Rica that are in different stages of their volcanic cycles in order to better understand the properties and dynamics of active magmatic systems and to improve volcanic hazard mitigation. In 1995, two permanent Global Positioning System (GPS) receivers and five stations equipped with broadband seismic sensors and electronic tiltmeters were established on Arenal Volcano, Costa Rica. These observations are among the first few broadband seismic recordings and continuous, high precision, geodetic measurements made on an active volcano. Preliminary interpretation of the GPS data suggest ongoing depletion of a relatively shallow magma reservoir and magmatic injection at shallower levels in the form of dikes or sills. Quantitative constraints on the geometry and behavior of this magmatic system will be obtained by expanding spatial resolution of the GPS measurements and analyzing the seismic recordings of the harmonic tremor and summit explosions at Arenal. Modest GPS networks and seismic observations on Irazu, Turrialba and Rincon de la Vieja (including 2 permanent GPS stations on Irazu) will allow the characterization of surface deformation associated with these volcanoes in between eruptive cycles. Baseline deformation data at presently inactive volcanoes is quite rare, but potentially very valuable for evaluating volcanic hazards and for understanding magmatic processes on the longer time scale of major eruptive cycles. ***
