The volcano has been relatively quiet since phreatic eruptions on 15-17 February sent plumes to 2 km. During April, May, and June the local seismic system registered 3, 27, and 17 events, respectively. The April seismic events were of unspecified type. The May seismic events included eight of high frequency; in addition, one hour of low-frequency tremor took place. Seismicity during June included some low-frequency events.

Geological Summary. Rincón de la Vieja is a volcanic complex in the Guanacaste Range of NW Costa Rica. Sometimes referred to as the Rincon de la Vieja-Santa María Volcanic Complex, it consists of a slightly arcuate 20-km-long ridge of 12 craters and pyroclastic cones constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. The Santa María cone, the highest peak of the complex, is located on the E side of the ridge and has a lake within the 400-m-diameter crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous reported eruptions possibly dating back to the 16th century, have been from the active crater, near the center of the complex, with an acidic 300-m-diameter lake.

Information Contacts: E. Fernandez, V. Barboza, E. Duarte, R. Sáenz, E. Malavassi, M. Martinez, and Rodolfo Van der Laat, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.