In May, minor ash emissions were associated with pulses of high-amplitude tremor that lasted a few minutes at the beginning of the month, and had longer durations after 21 May. Seismicity decreased in May but remained at high levels. The number of high-frequency events declined from 2,683 in April to 1624 in May, and low-frequency shocks dropped from 2,303 to 1,902. Hypocenters were concentrated S and SW of Arenas Crater at depths of 1-5 km. The maximum daily energy release (1.2 x 10⁸ ergs) occurred 31 May when 42 high-frequency and 306 low-frequency earthquakes were registered. Relatively good weather and consistent winds (1-5 m/s velocity) allowed SO₂ measurements by COSPEC on 13 days. The maximum measured rate of SO₂ emission was 5,190 t/d on 13 May, with a monthly average of 2,435 t/d. Most of the highest SO₂ values corresponded with ash emissions and increases in the tremor signal. No significant changes were registered by electronic or dry-tilt.

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: M. Calvache, F. Gil, and C. Carvajal, INGEOMINAS, Manizales.