Daily seismic energy release showed a large increase in February, with the highest telemetric reading since monitoring began in 1985. The number of seismic events was similar to recent months. Long-period and high-frequency events dominated the seismicity. Tremor generally remained at lower levels, with brief pulses of high-amplitude, low-energy tremor.

Deformation changes (measured by dry-tilt and a short level-line) were generally only small to moderate. Electronic distance measurements were stable at the Refugio and Recio stations (~1.1 and 2.2 km from the summit, respectively). Nine SO₂ measurments were made by COSPEC during the month. One reading (on the 9th) showed an emission rate of 8,949 t/d. Mean rates in December and January were 1,220 and 1,800 t/d.

Geological Summary. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.