IMAGING THE MAGMATIC SYSTEM OF LAS SIERRAS-MASAYA VOLCANO, NICARAGUA, USING GEOPHYSICAL TECHNIQUES

The location and size of magma reservoirs or intrusive complexes at most volcanoes are unconstrained. Las Sierras-Masaya volcano, Nicaragua, is one example where to date geophysical and geochemical data have been unable to constrain the depth and size of any magmatic reservoirs. This information is of paramount importance as this basaltic caldera volcano is capable of basaltic plinian eruptions of greater than 10 km³ and more than 2 million people would be directly affected.

In order to provide some constraints on the location and size of the magmatic system, Bouguer gravity, magnetics and structural observations were used to image the intrusive complex beneath Las Sierras-Masaya volcano. Cracks and fissures, likely related to an intra-caldera annular structure, cross cut all lava flows in the northern section of the caldera where there are no surface eruptive features. Bouguer gravity data, analyzed using both inversion techniques and anomaly wavelength analysis, also detected a large (22 mGal) positive anomaly overlapping the northern edge of the caldera suggesting the presence of a large dense intrusive body deeper than 5 km. Finally, recent InSAR data suggests that the area in the centre of the annular structure is down dropping at ~ 1 cm/yr. Thus geophysical, geodetic and structural observations imply that this intra caldera annular structure likely represents a 2 km diameter caldera possibly created in the last plinian eruption (1.8 ka) and likely sourced from a shallower reservoir.