Activity during 11 October-11 November. "A swarm of tectonic earthquakes on 28 October was the major recorded seismic activity for the report period. The swarm lasted 7.5 hours, and included >180 events with durations >= 5 seconds. Maximum magnitude was 2.9. The events were centered ~3 km W of Arenas Crater and 1 km NE of Olleta Crater, at ~5 km below summit level. In comparison, the much smaller swarm that initiated the present period of higher activity on 20 April occurred SE of Arenas Crater, at a shallower depth (~3 km).

"Long-period seismic events were less frequent than during September. Low-frequency seismic events continued at about the same rate and most still appeared to be centered near the SE station (Regio). Since about 6 November the seismic station nearest Arenas Crater (Refugio, 2.8 km to the W) recorded an increase in shallow seismic activity.

"Harmonic tremor was the lowest of any period since 4 May. Several times, the signal declined to background level for hours or days. As of early November, tremor typically lasted for a few hours, often beginning after low-frequency or shallow seismic events then slowly declining. At times, the increase or decrease in signal intensity was not simultaneous at all stations, suggesting that there was more than one tremor source.

"The highest SO₂ emission rate, 5,400 t/d, was measured on 31 October during a tremorless period. The average emission rate was ~3,000 t/d. Evidence of ash emission was less than during the previous month.

"After relatively strong fluctuating changes in September, deformation was weaker and less regular. The resultant tilt vectors pointed less consistently to the summit area. Some changes in deformation measurements may have been due to the effects of heavy rainfall."

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.