A total of 399 microearthquakes were recorded in July (figure 4) at a seismic station (RIN3) 6 km SW of the crater. Six hours of low- and medium-frequency tremor (1.3-3.2 Hz), were recorded in episodes 12 minutes to 3 hours long. Low-frequency earthquakes were also recorded, with durations that reached 175 seconds.

Figure 4. Daily number of earthquakes at Rincón de la Vieja, July 1991. 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: J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.