Results from new GPS and gravity monitoring networks at Fernandina and Sierra Negra Volcanoes, Galápagos, 2000–2002

GPS and gravity networks were established on Sierra Negra and Fernandina volcanoes in 2000 and remeasured in 2001 and 2002. After a decade in which the caldera inflated by over 2.5 m, Sierra Negra's caldera began to subside between 2000 and 2002, at a rate of about 9 cm/a. Neither inflation nor deflation was accompanied by eruption. Simple elastic modeling indicates that the deformation at Sierra Negra can be attributed to volumetric contraction of a 2.1 km-deep sill beneath the caldera, caused by either movement of magma out of the sill or loss of bubbles through degassing. Precise gravity measurements support but do not prove the former mechanism. Fernandina volcano is in a phase of slow refilling of the summit reservoir after the 1995 eruption, causing lateral expansion of the caldera rim by about 3 cm/a. Our estimate for the depth of the shallowest part of the Fernandina magma chamber is 1 to 2 km. Neither volcano shows evidence of flank spreading.