OVSICORI-UNA reported that the amplitude of seismic tremor at Rincón de la Vieja intensified on 17 April and then increased again on 25 April. The tremor signals were accompanied by long-period events occurring at a rate of sometimes hundreds per day. The intensities of the earthquakes were the highest recorded within the last five years. A total of 19 small eruptive events were recorded during 17-25 April and produced steam-and-gas plumes that rose as high as 2 km. Sulfur dioxide emissions began to increase on 21 April and increased more notably during 24-25 April, rising from around 77 tonnes per day (t/d) to around 493 t/d. Tremor amplitude fluctuated at high levels during 26-29 April, reaching a new peak at around 0200 on 28 April. Gas-and-steam emissions were continuous. The Alert Level remained at Level 3, Orange, the third highest level on a four-level scale.

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.

Source: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA)