Stress interaction between magma accumulation and trapdoor faulting on Sierra Negra volcano, Galápagos

Abstract

Understanding the state of stress within volcanoes and how it changes with active geophysical processes is important 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 to calculate the resulting stress changes within volcanoes. Here I use geodetic observations to constrain elastic dislocation models of magma accumulation and faulting at Sierra Negra volcano, Galápagos, in the years before the October 2005 eruption, which was the first eruption at Sierra Negra since 1979. Both InSAR and GPS data document a remarkable story of ~ 5 m pre-eruption uplift during 1992–2005, which was accompanied by at least three trapdoor faulting events on an intra-caldera fault system, in January 1998, April 2005, and just before the eruption in October 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 2.2 km deep sill under the caldera. Modeling results for the trapdoor faulting in April 2005, on the other hand, indicate that slip occurred on a thrust fault that dips steeply (71°) in towards the center of the caldera, and extends from the surface down to the sill at 2.2 km depth. Independently, the calculated stress changes caused by the inflating sill show that the Coulomb Failure Stress change (ΔCFS) at the fault's location is maximized on a 72° dipping thrust fault, agreeing almost exactly with the deformation modeling result, and implying that the inflation triggered the faulting. The calculations also show that the ΔCFS for optimally oriented faults (any strike and dip) is significantly larger in the southern part of the caldera, where the faulting took place, than at any other location. The trapdoor faulting in turn relieved the pressure within the sill, but caused compression 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.