SHORT-TERM DYNAMIC BEHAVIOUR OF MASAYA VOLCANO FROM GEOPHYSICAL SURVEYS

The activity of Masaya volcano, Nicaragua, has been annually surveyed by micro-gravity in order to characterize the long term mass/density variations within the shallow magmatic system. These long term changes have been directly related to cyclical variations in gas flux. However, magmatic transfer can be a rapid process, requiring only several hours or days to generate a new intrusion. Furthermore, other short period cyclical phenomena may be present (e.g., changes in the hydrothermal systems) and responsible for noise in the measured signal during a typical dynamic gravity survey. In order to determine the origin and importance of this noise and fully characterize any short period variations, a continuous gravity survey was made in March 2006 and March 2007 in the summit crater of Masaya. During this period, significant short term residual gravity variations (~60 µGal) were measured with wavelengths of ~20 hours. In order to investigate these short term variations, we must characterize the largest source of noise, notably the hydrothermal and aquifer systems. Hydrothermal or meteoric fluids will generate self-potential (SP) signals thus high density SP mapping around the active cone, complemented by soil CO2 and temperature measurements, allowed us to delineate with precision the main hydrothermal system. When this data is processed by continuous wavelet transform (CWT), the depth and movement of these fluid cells can be localized. The SP signature of the main hydrothermal system on the active summit is characterized by an amplitude of ~230 mV and centered on the subsided Nindiri lava lake. CWT processing suggests the presence of 5 cells of hot hydrothermal fluids located at shallow depths (e.g., <200 m below the topographic surface). While this preliminary work can characterize the pressurized hydrothermal system, the origin of these gravity variations can not yet be unequivocally determined. The limited expanse of the hydrothermal system suggests it is only one of a number of sources for the observed gravity changes, including volcanic tide (i.e., magma movement due to the tidal forcing) and variable rates of magma vesiculation/ devesiculation. This combination of SP and continuous gravity can has the potential to give significant insight into short and medium term variations in magmatic activity.