Seismic activity continued at high levels in December and January. During December, 423 high-frequency and 714 low-frequency earthquakes were recorded; a swarm of 101 high-frequency earthquakes occurred on the 11th. In January, low-frequency earthquakes increased to 1540 while high-frequency events decreased to 280. Shallow earthquakes were steady, with 62 recorded in December and 65 in January. The amount of seismic energy released increased slightly. Tremor signal was low and no ash emission was detected.

COSPEC measurements of SO₂ ranged from 450 to 3,000 t/d at the end of January, when a slight increase in SO₂ content was recorded in the plume. Deformation measurements that month showed minor to moderate changes. Most (70%) of dry-tilt changes were <8 µrad; Molinos Station (on the NW flank) continued to show instability with tilt variations reaching 15 µrad. The telemetric tilt data did not show any significant changes.

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, A. Nieto, and C. Carvajal, INGEOMINAS, Manizales.