During 1993 Rincón de la Vieja continued fumarolic venting in Rincón crater. Gas columns rose to 500 m high and contained sufficient gas concentrations to irritate the eyes and sting the skin of observers on the crater margin. The crater lake was clear gray in color, with clouds of suspended sulfur and several areas of discontinuous bubbling. It had a temperature of 35°C.

When seismically active, as in January and September 1993 (figure 8), both high- and low-frequency signals were common. A swarm of 25 high-frequency events took place on 26-29 January. On 26 March, 10 small-amplitude high-frequency events registered, their S - P (S minus P) times were

Figure 8. Seismic events at Rincón de la Vieja received at station RIN3, 5 km SW of the active crater, January 1993-January 1994 (October-December 1993 data presumed unrecorded). Courtesy of OVSICORI.

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.