HR: 11:50h
AN: G42A-06    [Abstracts]
TI: Uplift, Subsidence, and Trapdoor Faulting at Sierra Negra Volcano, Galapagos Islands, from InSAR Observations and Mechanical Modeling
AU: * Yun, S
EM: shyun@stanford.edu
AF: Stanford University, Geophysics Department, Stanford, CA 94305 United States
AU: Zebker, H
EM: zebker@stanford.edu
AF: Stanford University, Geophysics Department, Stanford, CA 94305 United States
AU: Segall, P
EM: segall@pangea.stanford.edu
AF: Stanford University, Geophysics Department, Stanford, CA 94305 United States
AB: In the last 10 years, Sierra Negra volcano, on the island of Isabella in the Galapagos, has experienced rapid uplift, trapdoor faulting (Amelung and Jonsson et al., Nature 2000), renewed inflation, and subsidence (Geist et al., JVGR in press). Boundary element calculations based on the InSAR observations constrain the magma chamber geometry at Sierra Negra. The surface deformation during the periods of inflation was caused by pressurization of a sill-like intrusion increasing in thickness by a maximum of 0.5 meter (Yun et al., JVGR in press). However, for such a shallow intrusion only the top of the magma chamber can be resolved using surface deformation observations; the data are insensitive to the sides and bottom of the chamber. A simple thermal analysis shows that intrusion must be at least 40 meters thick to remain liquid during the period of observations, so that the magma chamber at Sierra Negra is likely a thick sill or a flat-topped diapir. We model the stress field in the volcano assuming magma chamber geometries and pressure changes found from analysis of the InSAR data. By simulating both the inflation and faulting events, we hope to gain insights into the stress state within the volcano, and the conditions that favor faulting on the intra caldera fault system versus dike propagation and eruption. The stress acting on the pre-existing intra-caldera fault is a combination of pre-inflation, gravitational, and magmatic contributions. We bound the change in excess magma pressure using InSAR observations prior to the trapdoor-faulting event, resulting in a lower bound on the shear stress that triggered the faulting. Our results will have important implications for the stress state within the volcano, the mechanics of induced faulting and dike propagation, and may lead to better forecasts of future behavior.
DE: 8439 Physics and chemistry of magma bodies
DE: 6924 Interferometry
DE: 6969 Remote sensing
DE: 3210 Modeling
DE: 3260 Inverse theory
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting