Earthquakes continued to be felt by officials in the National Park. A gas-and-steam plume rose 1 km above the crater and drifted SW. Sulfur dioxide emissions were significant and deformation was detected. A thermal anomaly was also detected. Based on analysis of satellite imagery and reported seismic activity, the Washington VAAC issued a notice about a possible eruption from Nevado del Ruiz on 15 November. Cloud cover prevented observations of a possible ash plume but elevated seismicity was detected.

A few hours later seismicity decreased and a faint thermal anomaly was detected. About six hours after that seismic activity remained low and no anomaly was detected. Field measurements and analysis of satellite imagery continued to show a significant amount of sulfur dioxide in the atmosphere. A steam-and-gas plume rose m and drifted W. Seismic activity was low during September. Cloud cover mostly prevented observations of the volcano; a white gas plume rose m on 4 September and drifted W and SW.

During September field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere. Cameras located near the volcano showed gas plumes rising m above the crater and drifting NE on 23 August, and rising 1 km the next day. Gas plumes rose m and drifted W and N during August. Field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere during 24 and August.

Earthquakes indicative of fracturing rock were located SE at depths of 1. One event on 18 August was a M 2, located 1. Web cameras showed gas-and-ash plumes rising m and drifting W and NW during August.

A gas plume rose m and drifted NW on 19 August. During August field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere. Satellite images showed continuing sulfur dioxide emissions on 6 August. Web cameras near the volcano recorded a gas-and-steam plume rising m that drifted SW on 8 August.

Seismicity was low. Satellite imagery and ground-based observations on 22 July showed high levels of sulfur dioxide emissions. Satellite imagery and ground-based observations showed sulfur dioxide emissions. On 6 July a gas-and-ash plume rose m above the crater and drifted SW. On 8 July gas-and-ash plumes rose m above the crater and drifted NW and W.

On 10 July satellite imagery and ground-based observations showed sulfur dioxide emissions. Seismic signals indicated continuing gas and ash emissions. On 30 June an eruption produced an ash plume that rose 8 km above the crater and drifted SW.

The Alert Level was raised to I Red; "imminent eruption or in progress". According to news reports, communities around the volcano evacuated, and airports in Manizales, Pereira, and Armenia closed. By 2 July seismicity had decreased to low levels; the Alert Level was lowered to II Orange; "eruption likely within days or weeks".

Reports on 22 and 25 June stated that seismic signals had indicated continuing gas and ash emissions. Web camera images showed steam-and-gas plumes rising m and drifting NW on both days. On 26 June seismicity was low. Sulfur dioxide emissions continued to be significant.

Seismic signals on 15 and 18 June indicated continuing ash emissions. The plume was later detected in satellite imagery drifting more than 90 km NW. Ash plumes drifted almost 30 km SE on 8 June.

The next day seismic signals indicated that ash emissions continued. Field observers noted that sulfur dioxide emissions were significant. On 29 May activity significantly increased; at seismic signals indicated ash emissions that were confirmed by officials and residents near the volcano as well as with a web camera.

Sulfur dioxide plumes were detected by satellite and a sulfur dioxide odor was reported in multiple towns. Later that day ash emissions rose m above the crater. Seismicity continued to decrease. Cloud cover prevented observations on 27 April. Seismic signals indicated that an ash plume was emitted at on 30 April however cloud cover prevented observations. Seismicity fluctuated, but decreased.

Video data and observers in the city of Manizales 25 km NW indicated that gas-and-steam plumes rose m above the crater and drifted SE during the morning on 18 April. Satellite imagery detected a steam and sulfur dioxide plume the next day. During April high levels of sulfur dioxide were detected in satellite images.

Gas-and-steam plumes rose km above the crater. Earthquakes were located below or N of Arenas crater at depths of 1. Earthquake events at and 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. 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.

Observes in Manila reported a gas-and-steam plume that rose 1. Sulfur dioxide emissions also remained high; occasionally a sulfur odor was reported in Manizales, about 25 km NW. On 5 April a volcano-tectonic earthquake, M 2. During April gas-and-steam plumes drifted NW. Gas-and-steam plumes rose 3 km above the crater on 7 April and 1. Earthquakes indicating rock fracturing occurred W of Arenas crater, exhibiting a pattern observed prior to the eruptions in and , although less energetic.

Starting at on 29 March, and lasting for about 25 minutes, more than earthquakes were detected S of Arenas crater at a depth of 4 km. At on 31 March earthquake signals indicating rock fracturing and fluid movement markedly increased. Seismicity remained elevated during April; earthquakes were located below Arenas crater at depths between 0.

Gas plumes rose 2 km above Arenas crater and sulfur dioxide odors were reported by local people. Gas plumes rose 2 km above the crater and sulfur dioxide odors were reported by local people. On 8 March scientists aboard an overflight observed a gas plume that rose 1. They noted ash deposits on the glacier, near the crater rim and on the E flank, likely from an explosion on 22 February. Later that day a small explosion detected by the seismic network produced an ash emission that was observed with a camera installed in La Piranha NW.

Increased sulfur dioxide emissions were also detected. Earthquakes were located beneath the Arenas crater at depths of 0. The largest earthquake was M 1. A plume of white gas rose m above the caldera and a sulfur odor around the volcano was reported. The report also noted changes in deformation and geochemistry during the previous few months. Seismic levels fluctuated during October. A second VAAC report less than an hour later noted a brief seismic signal alert had been issued and that meteorological cloud cover prevented observations of the volcano.

Ash was not seen in satellite imagery later that day nor were there any additional reports of activity. On 9 June beginning at a swarm of volcano-tectonic earthquakes was recorded at Nevado del Ruiz. Next, hundreds of hybrid earthquakes were recorded, with more than 1, earthquakes occurring in 16 hours. This was the highest daily number of events recorded at Nevado del Ruiz since At the height of the activity the Alert Level was at Orange.

In addition to heightened seismicity felt by residents near the volcano, jet-like sounds were reportedly heard that corresponded with some of the hybrid earthquakes, and the strong scent of SO2 was reported near the summit. No ash emissions were reported. By 13 June seismicity had decreased. Click on the index link or scroll down to read the reports.

On 22 December, stronger earthquakes were detected, followed by a half hour of apparent harmonic tremor. During a visit to the crater in early January , increased fumarolic activity, evidence of phreatic explosions, and the wide deposition of sulfur salts over the adjacent snowcap were noted.

At times, a thin layer of ash had been ejected, which was analyzed by J. Tomblin and found to consist of alteration products and sulfur. There are no operating seismographs in the region. Abnormal fumarolic activity also continued. The one hot spring with frequent temperature monitoring, NW of the crater, had not shown any variation in temperature. Colombian officials have begun the necessary studies. Information Contacts: M. Increased thermal and seismic activity have continued.

They reported intense noise from the fumaroles as well as increased fuming which frequently made breathing difficult. Sulfur deposits were more extensive than those noted on their previous visit. On 22 February, there had been only a thin film of sulfur covering the surface near the fumaroles, but by July sand-sized material on the inner slopes of the crater was impregnated with sulfur deposits, creating a crust 10 cm thick.

A new crack was observed near the crater rim; it was 1. Ground temperatures were measured at various locations as the team descended into the crater. Isotopic studies are being conducted on waters collected at thermal vents on the flanks of the volcano. Much of the bottom of the crater was covered by a green lake that emitted hot steam from its surface. On 22 February, the lake had a pH of 0. The water in the lake had risen at least 1 m since February, covering a mud pool and its surrounding "mud volcano" seen on 22 February, and also nearby fumaroles.

Projecting into the SE side of the lake on 8 July was a peninsula that was thought to be the remnant of these features. An unusual thaw of the glacial ice that covers much of the summit area seemed to have contributed to the rise in the water level. Further evidence for thawing came from the presence of large blocks of ice scattered near the base of the crater.

A major increase in fumarolic activity producing an enormous yellow vapor cloud was reported on 23 July. Information Contacts: B. Salazar A. Calvache V. Ash emission began 11 September at about and lasted up to 7 hours, accompanied by a persistent roaring noise and electrical discharges.

A mudslide blocked a road on the E flank of the volcano. Before the eruption, tremors were recorded at about 1. Tremor was continuous during the ashfall, punctuated by discrete earthquakes. On 13 September, morphological changes in the glaciers on the W side of Ruiz were reported by ground and air observers. New fissures were also reported on the icecap N of and below the active crater near a small growing pond on the glacier surface. Nevado del Ruiz is a broad shield-shaped volcano covering more than km 2 , composed of an extensive sequence of hypersthene-augite andesite, hornblende andesite, basaltic andesite, and dacite.

Eruptions were reported in and , but no descriptions exist. Apparently these events produced no tephra. Ruiz is also reported to have been "smoking" in and Herd, Herd, D. Nacional, Manizales. After a series of magnitude-4 earthquakes on 22 December [] marked the onset of stronger seismicity, events were felt each month until an episode of strong phreatic activity on 11 September.

Seismologists have located about earthquakes, centered km NW of the active crater at average depths of km below sea level. Increased thermal and phreatic activity began in early and continued after the 11 September episode into early October. No juvenile material has been recognized in the tephra.

The vigorous 11 September phreatic activity began at from the summit crater Arenas and had ended by the next morning. Lithic blocks were deposited on snowfields as much as 2 km from the crater. Thunderous detonations and summit-area lightning accompanied the activity. As it traveled at an estimated km per hour, the lahar left the river channel at various locations, particularly along curves, and rose as high as m up canyon walls.

Valley residents were placed on alert but have not been evacuated. A hazard map has been prepared by the international team studying the volcano and distributed to Red Cross and Civil Defense officials. For 5 days preceding the 11 September activity, seismographs registered a very regular pattern consisting of 15 minutes of strong high-frequency tremor every hour. Although similar seismicity was recorded during three other periods of a day or less in August and September, none of these episodes was as intense or long-lasting.

Phreatic activity was continuous for the rest of September, emitting variable amounts of ash, typically darkening the snow to a few kilometers from the crater. Heavier emissions occurred on 23, 24, and 29 September, producing dense dark plumes that deposited trace ashfalls more than 10 km from the crater and lithic blocks on nearby snowfields. Activity had declined by early October. A steam plume km high was visible daily but contained no obvious ash.

Seismographs recorded microseismic events per day and an irregular harmonic tremor that may have been related to the steam plume. Geologists visited the crater at the beginning of October, noting a slight decline in fumarolic activity and little ash emission.

Glaciers seemed unchanged since September. Information Contacts: L. Rivera , Univ. Duque , Univ. Nacional, Manizales; A. Solano , Univ. An explosive eruption on 13 November melted ice and snow in the summit area, generating lahars that flowed tens of kilometers down flank river valleys, killing more than [25,] people. This is history's fourth largest single-eruption death toll, behind only Tambora in 92, , Krakatau in 36, and Mt.

The following briefly summarizes the very preliminary and inevitably conflicting information that had been received by press time. After the moderate explosive activity and km lahar of 11 September, there were several smaller ash ejection episodes through the end of September. Activity declined by early October to emission of a km steam plume that contained no obvious ash.

Low-level steaming with very little ash continued into late October, accompanied by occasional earthquakes, apparently tectonic, at about 12 km depth. Shallow km earthquakes, as many as 5 per day, were centered about 0. Three dry-tilt stations were established in late October, N, S, and W of the summit, and 11 days of data before 5 November suggested that some deflation was occurring. Seismicity began to increase by 7 November, characterized by a series of high-frequency seismic swarms, although fewer than were associated with the 11 September ash emission.

Continuous volcanic tremor began 10 November, but was weaker than the 11 September tremor. No additional changes in seismicity were apparent before the onset of eruptive activity at about on 13 November. Ashfall reportedly began about in heavy rain at Mariquita 59 km NE of the summit and lapilli with ash started falling about in Armero 46 km to the ENE figure 1.

At , Bernardo Salazar, tending seismic equipment 9 km from the summit, heard the start of strong explosions, much louder than those on 11 September, that shook the building and lit up the rain clouds "like a lamp.

Ashfall at Armero increased about At a Caribbean Air Lines cargo plane flew through the eruption cloud at 8 km altitude. Pilot Manuel Cervera reported that "smoke" and the smell of sulfur filled the cabin, and his windows were etched. Lahars traveled down 11 flank valleys, the most destructive inundating the city of Armero, where an estimated [21,] of 25, residents died. The first mud, which reached Armero about [], was cold [reports that the mud became increasingly hot were incorrect].

Resident E. Nieto described "A frightening noise and then a blast of wind hit us and we saw fire falling from the sky. Geologists estimated that the lahars advanced at km per hour; Armero, at about m elevation, is 5 vertical kilometers below the summit of Ruiz. No pyroclastic flows were observed during the eruption, but geologists noted deposits with cross-stratification typical of those from surges.

The Refugio about 2 km NW of the summit was knocked down and its walls scattered radially downslope. Tephra fell on all but the S and SE flanks. Heavy weather clouds prevented observation of the eruption on NOAA's weather satellite images.

Lack of nearby wind data prevented precise altitude determination by correlation with cloud movement as indicated by Nimbus-7, but the available data suggested that it was upper tropospheric. As of 20 November, no additional strong explosions had occurred.

Geologists who flew over the volcano reported that the crater had enlarged to about m in diameter and m deep, and that new fumaroles had developed m from the crater, but they observed no lava flow. Newly installed telemetering seismometers recorded harmonic tremor of varying amplitude beginning 18 November at that preceded ash emission at An episode of stronger tremor that lasted from about to the next day was followed by a small explosion at Herd and R.

Matson and W. Barquero and R. Seismic swarms, latest with inflation; more on 13 November activity and products. Since the 13 November eruption, activity at Ruiz has been limited to emission of a vapor plume and a few seismic swarms, one accompanied by measureable inflation. Work by numerous geologists has yielded new information on the 13 November eruption, its products, and pre-eruption activity.

Pre November activity. The most vigorous seismic energy release at Ruiz occurred in the days preceding the 11 September ash emission.

The rate of energy release increased prior to the 13 November eruption, but more gradually than before the 11 September activity. Hypocenters were concentrated N and NE of the summit with best-located events concentrated at depths km below sea level figure 3. An increase in temperature of the thermal vent 'La Hedionda' on the NE flank may have been related to the increase in seismic activity. Details of the 13 November eruption sequence remain uncertain and field investigations were still in progress at press time.

An initial explosion at deposited a very thin, fine-grained layer of ash around the summit and NNE of the volcano. The main explosion started at or and continued for minutes. Five kilometers from the crater, tephra from the main explosion was 7 cm thick and included cm pumice fragments, but the deposit thinned rapidly and was only mm thick at Armero with similar amounts at Mariquita and Honda 75 km NE.

Preliminary estimates by Haraldur Sigurdsson and Steven Carey place the volume of tephra at roughly 39 x 10 6 m 3. Cloudy weather and lack of nearby wind data on 13 November impeded determination of the height of the Ruiz eruption column.

Based on the position of tephra diameter isopleths, Sigurdsson and Carey inferred that the top of the eruption cloud reached [31] km altitude, but emphasized that most of the tephra probably remained in the upper troposphere [Naranjo and others, ]. Mudflows that moved E down the valleys of the Lagunillas and Azufrado rivers and inundated Armero were overlain by airfall tephra within km of the volcano. The Armero mudflows emerged from both the Lagunillas and Azufrado valleys, which join upstream from the city.

The first wave of mud, probably from the Lagunillas, was apparently colder, lighter colored, more water-rich, and formed a more extensive deposit than the second wave, probably from the Azufrado, which was hotter, coarser, and darker-colored. The Lagunillas mudflow probably included water from a lake that had been trapped behind a debris dam in that valley's headwaters for at least several months. Preliminary chemical analyses of a few samples of the 13 November pumice suggest that it is a hypersthene andesite, very similar to an earlier pumice that was probably from Ruiz's last large eruption, in Little systematic variation was found in different-colored samples that had suggested mixed magma in hand specimen table 1.

Table 1. Preliminary analyses of bulk compositions of Ruiz pumice and glass septa Numbers 1, 2, 5, and 6 are from electron microprobe analyses by William Melson and Deborah Reid Jerez; 3 and 4 are X-ray fluorescence analyses by Joseph Taggart. Post November activity. No significant eruptive activity occurred in the succeeding weeks.

The vapor column varied in height from m to Possible new fissures have been observed near the summit along with possible development of a depression SW of the summit. However, the fissures may have been pre-existing features exposed by clearer weather and seasonal snowmelt.

Slight advances of some of the summit glaciers have been noted, but no large-scale ice movements were apparent and there was no evidence of significant melting from below. Six telemetering seismometers have been installed, ringing the summit at elevations of 4,, m, supplementing the four-station seismic net that was in place before 13 November. Telemetering tiltmeters were emplaced at 4, m elevation on the NW flank, 4, m elevation on the NNW flank, and on the NE flank, and EDM lines have been established, in addition to the dry-tilt network installed on the N flank in October.

Seismic energy release was at relatively low levels shortly after the 13 November eruption, but the slope of the energy release curve steepened in the succeeding weeks. Earthquake swarms that were small but of increasing energy occurred and 27 November, and December. Maximum magnitudes were 2. Locations were available for only a few events, which were centered along a generally N-S trend, usually somewhat N of the crater.

The swarms were not accompanied by measurable tilt episodes or obvious changes to the plume. The rate of seismic energy release doubled during the first day of a stronger swarm December and Civil Defense personnel were put on alert. However, seismicity declined 13 December, and the seismic energy release curve was nearly flat December. Information Contacts: P. Van der Laat , Univ. Nacional, Heredia; E.

Sigurdsson and S. Carey , Univ. Williams and D. Lowe , Louisiana State Univ. Stoiber and B. Gemmell , Dartmouth College; D.

Hearn , D. Klick , D. Herd , and R. Taggart, Jr. Melson and D. Jerez , SI; P. Explosive activity on 4 January ejected a small amount of ash and was accompanied by vigorous seismicity.

The 4 January activity did not generate mudflows or cause any apparent changes in river flow, but residents of low-lying areas were temporarily evacuated as a precautionary measure. A series of earthquake swarms followed the 13 November eruption, including a strong swarm December that was accompanied by deformation figure 5. Epicenters were generally S of the active crater, extending E and W under the flanks.

Before the 4 January eruption, focal depths decreased from km to km below a datum at 3. Equipment problems prevented remeasurement of EDM lines immediately before the 4 January eruption, so the amount of pre-eruption inflation is uncertain. The net change in the lengths of several radial lines of 5 km average length measured 3 days after the eruption was about 10 cm, but this figure probably included substantial post-eruption deflation.

By December, small but distinct changes in rate or direction of tilt had begun to appear on all four electronic tilt stations at 4, m elevation on the NE flank, 4, m on the W flank, about 3, m on the NW flank, and 4, m on the SE flank.

Movement of cracks in summit glaciers continued through December and early January at roughly constant rates. Extensional changes of cm per day were measured near the head of the Azufrado valley, and both extensional and compressional motion of a few mm to 5 cm per day elsewhere. Little baseline data exist on typical rates of glacier advance on Ruiz.

Strong seismicity began 3 January at about , and was saturating seismographs within less than an hour. The seismicity was initially characterized by superimposed high- and low-frequency tremor, but tremor amplitude declined somewhat around and low-frequency B-type earthquakes and explosion events accompanied the tremor. Darkness initially prevented direct observations of the summit, but ash began falling about The eruption cloud was small, generally m high, occasionally rising to 1 km above the summit.

Ashfalls were minor, concentrated around the summit and in a narrow zone to the WNW. Several hundred meters from the vent, new ash was only about 7 mm thick; 3 km downwind the deposit was only 2 mm deep; and only traces of ash were found more than 10 km away. Vigorous seismicity continued until about noon, then declined slowly until the eruption ended in mid-afternoon. Most residents returned to their homes shortly after the eruption, but about 2, people remained evacuated 10 days later.

Smaller earthquake swarms occurred January, then seismicity declined to about A- or B-type events per hour, generally with magnitudes of 0 or less. No additional explosions or major increases in seismicity had occurred as of mid-January. Further References. Katsui, Y. Naranjo, J. Thouret, J. Valdiri Wagner, J. Williams, S. Seismicity and deformation continued through early February, but there had been no eruptive episodes since the minor explosive activity on 4 January.

Weak ash emission occurred on some days, falling near the crater. The low-frequency seismicity also registered about 10 events daily without significant variations. Depths were between 2 and 6 km below a datum at 3.

In early February, events were concentrated in the NE part of the volcano [see also ]. An electronic tilt station W of the crater measured inflation values of the order of 0. A deformation event that started 18 January was measured by continuously recording tiltmeters 2 and 5 km NW of the summit and by EDM stations radiating W from the summit. Increased melting was observed but attributed to sunnier weather.

Melting of ice caused buckling of overlying tephra layers that were as much as 6 m thick near the summit, producing features reminiscent of tumuli that form on top of block lava flows. An improved alert system and additional hazard study has permitted some of the persons that had remained evacuated since 4 January to return to their homes.

Information Contacts: I. Okamura , HVO. Colombian geologists reported that microseismic activity diminished between mid-February and mid-March, with an average of 5 high-frequency and low-frequency events daily. Activity on 28 February was abnormally low, with only one low-frequency event. Epicenters were within km of the crater, and focal depths were at km below a datum at 4. The vapor column reached heights of m, without significant ash emission.

No significant changes were detected on the E flank. Additional seismic information comes from Jim Zollweg. Sites were chosen km from Arenas, Ruiz's active crater.

The first station was operating on 17 November, four stations were functioning by 28 November, and all six by 2 December, although some locations were changed later.

Recording is analog using pen-and-ink or smoked paper. There were about half as many low-frequency as high-frequency earthquakes during those months. About 1, earthquakes were counted in December countable events had coda lengths of at least 5 seconds on the Olleta telemetry station. In January, activity was considerably lower because of a major decrease in the rate of occurrence of high-frequency events. Only a few surface-type events gas emission signals or avalanches were recorded by telemetry stations in any month.

Instances of harmonic tremor sustained for more than a minute were also uncommon, particularly in December. The most important tremor recorded since the stations were installed occurred January, preceding and during a minor eruption January. This tremor was unusual because of its wide range of frequency content; frequencies of 7 Hz or more were recorded within a few hours of its onset, and frequencies as low as 0. More than high-frequency earthquakes have been located, and 81 of the better solutions are plotted in figure 6.

Focal depths of the high-frequency events are usually between 2 and 5 km beneath a datum at 4. The probable error range makes it difficult to say whether there are significant depth differences between events.

There is an interesting temporal pattern to the epicenters. Locations between 28 November and 5 December mainly fell in the central and eastern parts of the E-W zone, whereas vigorous swarms December were mainly confined to the W half of the zone.

There was a pronounced hiatus in high-frequency activity December, followed by a swarm of events December along the second zone, striking NW-SE. Between 26 December and 3 January, earthquakes reverted to a small area near the intersection of the two zones.

High-frequency activity was comparatively low in the weeks following the January eruption. First motions have been mostly compressions for nearly all events under Ruiz, suggesting a normal faulting environment. The data for the high-frequency sequences suggest the intrusion of dike-like bodies of magma along pre-existing fault zones. Deformation monitoring has been summarized by Barry Voight.

The data place constraints on the volume of rock susceptible to massive gravitational failure. They were first measured on 14 February.

Preliminary data from repeated distance measurements to 7 March suggest that the motions of glacier ice and underlying bedrock are decoupled. The fracture pattern observed on the surface of the summit plateau E of Arenas Crater dominantly reflects the propagation of crevasse patterns through the veneer of surficial deposits.

The steady outward motion of the summit area as deduced from observation of surficial fracturing and EDM monitoring mainly reflects movements in glacier ice rather than motion of underlying bedrock. Although my current impression is that all of the headwall is probably stable with respect to deep-seated rock failure, the potential for mass movement involving a portion of the crater remains to be thoroughly evaluated. Fabian Hoyos P. Isotopic compositions of 8 July hot spring samples were similar to those reported in and to CHEC.

Other samples collected until just before the 13 November eruption are being analyzed. Table 2. Data courtesy of Fabian Hoyos P. Rodriguez , and N. Rojas , Univ Nacional, Manizales; N. Voight , Penn State Univ; S. Williams , Louisiana State Univ. Ash contents of the plume were low. Seismicity during the period generally remained similar to the previous month.

Depths of high-frequency events were as much as 8 km below a datum at 4. Epicenters were dominantly in the S part of the volcano. Around 21 March there was a small seismic crisis with 24 low-frequency and 13 high-frequency events that were associated with a small ash emission. On 6 and 7 April, two significant low-frequency events were registered, but they were not accompanied by any other activity.

Colombian geologists noted that regional earthquakes could have some influence on the increase in low-frequency seismicity. Dry and electronic tilt measurements have not shown significant changes. Rates of movement of ice near the summit have slowed since January to less than half the December rates. Information Contacts: A. Six weeks of harmonic tremor; increased SO 2 ; light ashfalls.

Activity during 15 April May. A substantial increase in seismic activity began in late April and was continuing in mid-May. Seismic records suggested that a minor ash emission occurred on 4 May, and ashfalls were reported within 3 km of the summit. People forgot about the destruction and devastation and Armero was built on the same site, growing to a town of 30, by The lahar began when twenty million cubic meters of hot ash and rocks erupted from the vent of the volcano and rained down or flowed across the ice and snow-covered summit.

The hot ash and gases moved across the snow pack in avalanches of hot volcanic debris pyroclastic flows and fast-moving, hot, turbulent clouds of gas and ash pyroclastic surges. The hot pyroclastic flows and surges caused rapid melting of the snow and ice. This released a lot of water and debris that swept down canyons from the summit.

Seismicity decreased during September with respect to August. A minor ash emission on 20 September drifted W from the summit at 5.

A possible emission on 23 September drifted NW at 6. Two emissions were reported drifting WNW of the summit on 26 September at 5. Continuous volcanic tremors were registered throughout September, with the higher energy activity during the second half of the month. One episode of drumbeat seismicity on 15 September lasted for 38 minutes and consisted of 25 very low energy earthquakes.

Steam and gas plumes reached 1, m above the crater rim during September figure Five emissions of ash were confirmed by the webcams and park officials during the month, in spite of difficult meteorological conditions; three of them occurred between 15 and 20 September. Seismicity increased during October with respect to September. A few of the LP and tremor seismic events were associated with small emissions of gas and ash, confirmed by web cameras, park personnel, and the Washington VAAC.

The plumes rose to 5. Steam plumes were visible most days of the month figure Only a few were visible in satellite data, but most were visible in the webcams. Several episodes of drumbeat seismicity were recorded on 13, , and 27 October, which were characterized by being of short duration and consisting of very low energy earthquakes.

The tallest plume during the month rose about 2 km above the crater rim on 18 October. Ash emissions were recorded eight times during the month by SGC. During November , the number of seismic events decreased relative to October, but the amount of energy released increased. Some of the seismicity was associated with small emissions of gas and ash, confirmed by webcams around the volcano. The Washington VAAC reported ash emissions on 22 and 30 November; the 22 November event was faintly visible in satellite images and was also associated with an LP seismic event.

They rose to 5. Various episodes of drumbeat seismicity registered during November were short- to moderate-duration, very low energy, and consisted of seismicity associated with rock fracturing VT. Multiple steam plumes were visible from communities tens of kilometers away figure Seismic activity increased in December relative to November.

It was characterized by continuous volcanic tremor, tremor pulses, long-period LP and very long-period VLP earthquakes. Some of these signals were associated with gas and ash emissions, one confirmed through the webcams. The first rose to 5. The second rose to 6. A single discrete cloud was observed 35 km W of the summit; it dissipated within six hours.