A visit to the active crater on 6 October took place in bad weather, but scientists found strong fumarolic activity and a >10 m drop in lake level compared to September 1992. The lake level began changing after the 8 May 1991 phreatic eruption.

Low-frequency microseismic activity increased over the last 3 months, with three events in July, five events in August, and 93 events in September. Tremor was not reported in July, 165 minutes of tremor occurred in August, and no tremor was detected in September. Dry-tilt for the interval from November 1992 to July 1993 indicated an 11 µrad radial deflation; in contrast, the majority of intervals as far back as 1987 showed little or no change.

Geological Summary. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge 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. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide 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 historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillermo E. Alvarado, and Francisco Arias, ICE; Héctor Flores, Univ. de Costa Rica.