Seismicity, particularly shallow B-type activity, began to decline slightly on 20 March. In the following weeks, long-period events were more frequent, and high-frequency (A-type) events shifted N towards Arenas Crater and became more shallow. About 1-2 events were recorded daily. Tremor was weak but more continuous than during the previous report period.

Deformation, consisting mainly of NW-SE fluctuations, remained at slightly elevated levels, comparable to September/October 1986. Changes were recorded more strongly on lower-altitude stations. From 4-10 April an electronic tiltmeter (Inderena) 5.9 km NW of Arenas Crater recorded 2-3 µrad/day of eastward inflationary movement. Other tiltmeters recorded only minor amounts of inflation.

SO₂ emission, measured by COSPEC, declined to an average of 620 t/d (maximum 1,060 t/d on 27 March). There have been no reports of ash emission since late February. About 3 April, [hundreds to thousands of] dead [migratory] birds and small mammals were found 10-20 km from the crater, mostly to the NE (downwind). Preliminary analyses showed sulfur emission rates and rain acidity to be well below peak values, but biological analyses of animals revealed signs of carbon monoxide effects [see also 12:05].

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.