DYNAMIC CONTROLS ON FLUID FLOW AT MASAYA VOLCANO, NICARAGUA

Diffuse degassing is a direct result of the interaction between magma and groundwater. It can therefore be a useful volcano monitoring tool. However, the location and magnitude of the volcanic and hydrologic sources, and the geological structure of the rock through which the fumarole gases pass, strongly affect the surface emanations that we can measure. Therefore it is vital that a good conceptual model of the subsurface is available before diffuse degassing signals can be interpreted in light of changes in volcanic activity.

At Masaya volcano in Nicaragua temperature time series show that flank fumaroles 3-4 km from the active crater respond to changes in volcanic activity. These fumaroles are linked to the crater by a fracture zone with topographic offset. Magnetic profiles recorded in one fumarole zone over an area of 133 m x 125 m show a NE-SW trending anomaly of up to 6000 nT. The location corresponds to the topographic offset of the fracture zone. Modeling of the magnetic data suggests normal faults dipping SE at 60° which, together with hydrologic information from Transient Electromagnetic soundings (MacNeil et al. 2007), was used to construct a numerical model using the Tough2 code. Model output shows that dipping normal faults focus fluid flow toward the footwall of the faults. This is in excellent agreement with positive SP anomalies of 140 mV and elevated CO₂ fluxes of up to 2200 g/m²/d to the NW of the fracture zone at Masaya volcano. The models also suggest that fluid flow is inhibited over faults and that there is a very small amount of circulation on the hanging wall side. This corresponds to negative SP anomalies and an absence of CO₂ measured to the SE of the fracture.

Within the fracture zone between the active crater and Comalito cinder cone there are three distinct fumarole areas. Tough2 modeling shows that with heat injected at 1667 J/s uniformly along the base of the saturated zone, convection occurs that results in three zones of elevated surface fluid flux similar to those observed at Masaya. We are now working on using Tough2 models to constrain possible sources for rapid and cyclical variations in soil gas temperature measured at the fumarole zone near Comalito cinder cone during volcanic activity.