Report on Poas (Costa Rica) — March 2000
Bulletin of the Global Volcanism Network, vol. 25, no. 3 (March 2000)
Managing Editor: Richard Wunderman
Poas (Costa Rica) Conspicuous plumes and abundant low-frequency earthquakes in late 1999
Please cite this report as:
Global Volcanism Program, 2000. Report on Poas (Costa Rica). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 25:3. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN200003-345040.
10.2°N, 84.233°W; summit elev. 2708 m
All times are local (unless otherwise noted)
Comparative quiet continued at Poás; however, in addition to the fumarolic degassing often seen, seismicity was relatively high during the reporting period, June 1999-January 2000, when low-frequency earthquakes typically registered over 4,000 times per month (table 9). For comparison, a relative high in 1999 occurred in May when low-frequency events occurred ~1,400 times, and during a high in January 1998, when there were over 2,500 events. On 18 July 1999 an MR 3.1 earthquake occurred with 6 km focal depth and an epicenter 5 km NW of the active crater.
Table 9. A summary of seismic, temperature, and lake height data for Poás during July 1999-January 2000. Lake heights are with respect to the previous month and positive upwards (rising lake levels). The stated temperature for the pyroclastic cone's degassing refers to the value at an accessible point where the measurements are taken regularly. The seismic station POA2 lies 2.8 km SW of the active crater. "NR" indicates information absent and not reported. Courtesy of OVSICORI-UNA.[Skip text table]
Month Low-frequency Mid-frequency Total earthquakes earthquakes earthquakes Jun 1999 2,571 NR NR Jul 1999 2,996 64 3,061 Aug 1999 4,548 184 4,736 Sep 1999 5,264 274 5,542 Oct 1999 4,743 190 4,987 Nov 1999 4,142 221 4,393 Dec 1999 4,278 405 4,679 Jan 2000 NR NR low Month Crater lake Surface elevation change temperature of crater lake Jun 1999 NR NR Jul 1999 32°C -47 cm Aug 1999 37°C -32 cm Sep 1999 40°C NR Oct 1999 38°C NR Nov 1999 33°C 101 cm Dec 1999 35°C 156 cm Jan 2000 40°C ~140 cm
Tremor, which was seldom reported in 1999, took place for less than about 0.5 hours a day during October-November 1999. In contrast, tremor averaged only 0.1 hours a day during December 1999. In contrast, tremor durations of 20 to 70 hours were common in early 1998. Also appearing in the month of October 1999 were 5 unusual low-frequency events in conjunction with tremor; these low-frequency earthquakes had periods of 40-175 seconds.
During August -October, the pyroclastic cone's degassing led to unusually high plumes reaching 0.7 to 2 km above the crater floor. December plume heights ranged between 0.7 and 1 km. Some of the hottest temperatures were measured near the pyroclastic cone: up to 95°C during December-January and often over 92°C when reported during other months in late 1999.
Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.
Information Contacts: E. Fernandez, E. Duarte, V. Barboza, R. Sáenz, E. Malavassi, R. Van der Laat, T. Marino, J. Barquero, and E. Hernández, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.una.ac.cr/ovsi/).