No major change in activity level has been observed during the 11 February-11 March report period but a correlation between measured data and crater activity was established. Most dry-tiltmeters began to show stronger variations (maximum amplitude 10 µrad) by the end of January. Electronic tiltmeter 'Inderena' (5.9 km NW of Arenas Crater) began to record inflation after 7 February that increased between 15 and 19 February to 0.5 µrad/day, the strongest anomaly since late September 1986.

After deformation flattened out on 20 February, several small ash emissions occurred 20-25 February. COSPEC values increased to a maximum of 4,900 t/d during this period but there was no recognizable correlation with the seismicity. The number of shallow, small-magnitude B-type events remained about the same since December 1986. Only harmonic tremor increased between 20 and 25 February. Its shallow source was evident from amplitude relations.

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: H. Meyer, INGEOMINAS, Manizales.