Magma–Tectonic Interactions in Nicaragua: The 1999 Seismic Swarm and Eruption of Cerro Negro Volcano

A low-energy (Volcanic Explosivity Index [VEI] 1), small-volume (0.001 km ³ Dense Rock Equivalent [DRE]) eruption of highly crystalline basalt occurred at Cerro Negro volcano, Nicaragua, August 5–7, 1999. This eruption followed three earthquakes (each ∼ M _w 5.2) with strike-slip and oblique-slip focal mechanisms, the first of which occurred approximately 11 h before eruptive activity and within 1 km of Cerro Negro. Surface ruptures formed during these events extend up to 4 km from Cerro Negro, but concentrate ∼1 km south of Cerro Negro. Surface ruptures did not occur within 300 m of the cone, however, three new vents formed on the south flank and base of Cerro Negro and on trend with the Cerro La Mula–Cerro Negro volcanic alignment. Earthquake swarms were located northwest and southeast of Cerro Negro and seismicity was elevated for up to 11 days after the initial event. The temporal and spatial patterns of earthquake swarms, surface ruptures, and the eruption location can be explained using the Hill [J. Geophys. Res. 82 (1977) 1347] model for earthquake swarms in volcanic regions, where an eruption is triggered by tectonically induced changes in the regional stress field. In this model, tectonic strain, rather than magmatic overpressure causes dilation of the conduit for magma ascent. Numerical simulations for the 1999 eruption illustrate that the observed velocities (up to 75 m s ⁻¹ ) and fountain heights (50–300 m) can be achieved by eruption of magma with little excess magmatic pressure, in response to changes in Coulomb stress along the Cerro La Mula–Cerro Negro alignment. These observations and models show that 1999 Cerro Negro activity was a tectonically induced small-volume eruption in an arc setting, with the accommodation of extensional strain by dike injection.