(Smithsonian / USGS Weekly Volcanic Activity Report) [less]

Earthquakes were located below or N of Arenas crater at depths of 1.5-2 km during 11-12 April. Earthquake events at 1146 and 1149 on 15 April were possibly associated with ash emissions which were not verified due to weather conditions. Earthquakes detected on 16 April occurred E of Arenas crater at depths of 1.5-4 km.
Gas-and-steam plumes were observed mainly in satellite imagery, by cameras located near the volcano, and from the city of Manizales (25 km NW). [less]

The reason is a steady increase in seismic activity and gas emissions from the summit crater. In recent days, more and more, usually small earthquakes have been occurring at shallow depths under the volcano, some of them felt. A steam nd gas plume is rising from the crater to 1.5 km and sulfur smell can be noticed in nearby villages and towns.
430 000 inhabitants from the valleys of the Río Chinchina, Manizales and four other rivers have been warned of mud flows and possible floods, which could be triggered especially during heavy rains, even when there is no eruption. [less]

- From March 27, there have been phases of volcanic tremor pulses related probably to deep magma movements
- Since the last week, there were seismic signals interpreted with rock fracturing, i.e. dike intrusions, located west of the active crater. Similar seismic activity was observed prior to the eruptions in November 1985 and September 1989, although this time it is less energetic. [less]

An overflight on 8 March revealed intense steaming from the main crater and the presence of ash on the summit glacier, the crater rim and on the volcano's eastern flanks, probably from the phreatic explosions reported earlier since 22 Februry. The steam plume reached an inpressive 1400 m (4,500 ft) height.
In addition, a thermal anomaly can be identified in the crater and levels of SO2 emission, as well as seismic activity with tremor signals have increased. This all adds to the likelyhood of a new eruption from the volcano, which, during its infamous 1985 eruption, killed about 30,000 people by a deadly lahar in the village of Armero. [less]

So far, the energy level of this increase is low compared with previous episodes of unrest, but small eruptions in the near future are possible.
The seismicity is located under the active Arenas crater at depths ranging between 0.5 and 3 km, and reached a maximum recorded magnitude of 1.4 on the Richter scale. [less]