OVSICORI-UNA reported that increased eruptive activity continued at Rincón de la Vieja during 30 May-6 June. Seismic instruments continued to record low-magnitude volcano-tectonic earthquakes located S of the Pailas sector of Parque Nacional Rincón de la Vieja. Reports describing phreatic eruption events were issued almost daily. A phreatic event at 2215 on 30 May ejected incandescent material within the vicinity of the crater. A small phreatic eruption at 1753 on 31 May generated a small pyroclastic flow that traveled a short distance from the crater. Moderate phreatic eruptions were recorded during 1-2 June; the most energetic event occurred at 0902 and generated a plume of steam, gas, and ash that rose to 1.5 km above the crater and drifted N. OVSICORI-UNA raised the Alert Level from 2 to 3 (the third highest on a four-level scale) at 1650 on 2 June due to significant seismicity and significant emissions recorded during May.

Eruptive events were recorded during 3-4 June. The most energetic event occurred at 0624 on 3 June and generated a steam-and-gas plume that rose 1.5 km and drifted W. An eruptive event at 0526 on 4 June first generated emissions that rose to 500 m, followed at 0529 by a second plume of steam-and-gas rose to 3.5 km above the crater and drifted N. Steam-and-gas plumes with low ash content were generated from eruptive events during the nights of 4-5 June; the plumes rose 1-1.5 km above the crater and drifted W. At 0259 on 6 June a small phreatic eruption generated a plume that rose to 3 km and drifted NW.

During a press conference held by OVISOCORI-UNA, RSN, and CNE on 5 June, the public was reminded that although the recent volcanic activity is normal for Rincon de la Vieja, it is still necessary to remain cautious and that community emergency committees will remain activated. CNE maintained a Green Alert (first alert level on a four-color scale) for the districts of Dos Ríos (13 km N) and Aguas Claras (3 km NW) in the canton of Upala (22 km ENE), as well as the districts of Cañas Dulces (24 km ESE), Curubandé (18 km SW), and Mayorga (15 km W) in the canton of Liberia (21 km SW). The CNE reminded the public to stay away from the Azul, Pénjamo, and Azufrada rivers.

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.

Sources: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA), Comisión Nacional de Prevención de Riesgos y Atención de Emergencias (CNE), Red Sismologica Nacional (RSN: UCR-ICE), Universidad de Costa Rica and Instituto Costarricense de Electricidad