During the night of 4-5 August 1999, strong seismic activity occurred near Cerro Negro and the earthquakes with magnitudes up to 4.8 were felt throughout NW Nicaragua, especially in the big cities of León (20 km away, where many people could not sleep because of the seismic events) and Chinandega (40 km away). The strongest event was even felt 70 km away in Managua. The Nicaraguan seismic network recorded hundreds of earthquakes and strong seismic tremor at the seismic station at the volcano and at the MIRAMAR station (7 km away).

Three notices were received from the GOES alarm network concerning Cerro Negro. Distinct hot spots, indicating small plumes over the volcano, were detected on infrared satellite imagery at 0055, 0155, and 0235 on 5 August.

Explosive eruptions began at about 1000 on 5 August 1999. Ash clouds at heights of about 7,000 m were reported by aircraft. Ashfall was reported from some places SW of the volcano. The activity issued from four new vents outside the main crater, very near to the parasitic crater Cristo Rey, on the S flank of Cerro Negro. The vents formed cones ~40 m high during the day.

Wilfried Strauch visited the volcano that afternoon and observed explosions every few seconds, sometimes generating lava fountains ~300 m high. The activity alternated among the different new cones. No significant amounts of volcanic ash were emitted at this time. Local residents ~1 km from the volcano reported that seismicity was extremely strong during the night. Fissures appeared in the soil near their houses, releasing vapor.

INETER informed Civil Defense and other institutions on the night of 4 August about the seismic activity. Civil Defense officers visited the volcano early in the morning of 5 August, but could not yet detect signs of volcanic activity. When they got the information about the beginning of the eruption they proceeded with the evacuation of nearby villages, involving several hundreds of people.

Volcanic ash advisory statements on 6 August indicated that well-defined hot spots were still occasionally visible on GOES-8 multi-spectral imagery through 1615. No ash was visible in the imagery at that time, and thick clouds moved over the area later in the day. Imagery obtained under clear skies at 1015 on 7 August revealed no ash or hot spot.

Geological Summary. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: Wilfried Strauch, Instituto Nicaraguense de Estudios Territoriales (INETER), Division of Geophysics, Apartado 2110, Managua, Nicaragua; Benjamin van Wyk de Vries, Magmas et volcans Observatoire du Physique du Globe, Departement des Sciences de la Terre, Université Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France (URL: http://modis.higp.hawaii.edu/); Washington Volcanic Ash Advisory Center, NOAA Satellite Services Division, NESDIS E/SP23, NOAA Science Center, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).