THE DENSITY AND DISPLACEMENT OF CONCEPCION VOLCANO, NICARAGUA

Two gravimetry surveys were conducted on and around Concepción volcano, Nicaragua, during the summer of 2007 and 2008 covering an area of 12.4 by 16.7 km. The first one was carried out using a LaCoste & Romberg instrument model G-58, while the second survey was done with a Burris instruments model B-38. A positive Bouguer anomaly (BA) in the range of 35 mGals delineates the structure of the volcano, indicating that the volcano is more dense than the surrounding sediments. These results are consistent with the model of Borgia and van Wyk de Vries (2003), and van Wyk de Vries and Matella (1998) for the gravitational spreading of this volcano, although the estimated density of the volcano is less than that used in previous volcano spreading models. In addition, steep gravity gradients delineate a geological feature running NW-SE cutting the west and south side of the volcano's base, about 6 km from the volcano's summit. This steep gradient in the BA correlates, to some extent, with near-surface faults on the WSW side of the volcano. We interpret the gravity anomaly to represent a deeper and more regional fault, which is not necessarily currently active. Geodetic GPS data collected on the volcano from 2001-2008 are used to interpret the stability of the edifice.