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Deformation and Stress-Change Modeling at Sierra Negra Volcano, Galapagos, from Envisat InSAR and GPS Observations

Sigurjon Jonsson(1), William Chadwick(2), Michael Poland(3) and Dennis Geist(4)

(1) ETH Zurich, Schafmattstr. 30, 8093 Zurich, Switzerland
(2) Oregon State University – NOAA, 2115 SE OSU Drive, Newport, OR 97365, United States
(3) USGS, P.O. Box 51, Hawaii National Park, HI 96718, United States
(4) University of Idaho, P.O. Box 443022, Moscow, ID 83844, United States

Abstract

Understanding the state of stress within volcanoes and how it changes with active geophysical processes is of vital importance to assess when and where magma may propagate to the surface and initiate an eruption. Observations of volcanic deformation help to constrain parameters of models that can be used to describe these processes and these models can then be used to calculate the resulting stress changes within the volcano. Here we use Envisat radar interferograms, along with continuous GPS observations, to constrain elastic dislocation models of magma accumulation and faulting at Sierra Negra volcano, Galapagos, in the years before an eruption took place in October 2005, which was the first eruption at Sierra Negra since 1979. Both InSAR and GPS data document a remarkable story of pre-eruption uplift and multiple trapdoor faulting events on an intra-caldera fault system. The total uplift from 1992 until the eruption began in 2005 was nearly 5 m, and it was accompanied by at least two trapdoor faulting events on an intra-caldera fault system, one in 1997-1998 and another on 16 April 2005. The pattern of uplift observed in the InSAR data from different time periods during 1992-2005 is consistent with filling and pressurization of a ~2 km deep sill under the caldera. Modeling results for the trapdoor faulting in 2005, on the other hand, indicate that slip occurred on a thrust fault that dips steeply (71 degrees) in towards the center of the caldera, and extends from the surface down to the sill at 2 km depth. Independently, the calculated stress changes caused by the inflating sill show that the Coulomb Failure Stress (CFS) change at the fault’s location is maximized on thrust faults with ~70 degree dip, agreeing exactly with the deformation modeling result, and implying that the inflation triggered the faulting. The calculations also show that the CFS change for E-W striking faults is significantly larger in the southern part of the caldera, where the faulting took place, than at other locations. The trapdoor faulting in turn relieved the mean stress (or pressure) within the sill by as much as 2-3 MPa, while it caused mean stress increase to the south of the faults. This suggests that while the faulting provides a mechanism for the sill to thicken and postpone eruptions, it also prevents the sill from growing to the south.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry