As of late November 1999, microseismic activity had been occurring at Telica for more than a year. There were phreatic explosions in May and June 1999 (BGVN 24:06). An eruptive phase began in August 1999, generally producing only sporadic small and local ash falls. Intermittent gas-and-ash emissions continued to be reported through December 1999. One of the more vigorous events took place on 29 December, sending ash to several kilometers altitude and inducing falls detected 45 km away.

A noteworthy event began around 0200 on 10 August. Tremor and earthquakes increased abruptly. Small explosions took place in the crater, expelling gas and volcanic ash. Ash fell ~20 km WSW of Telica in the city of Chichigalpa. An interval of relative calm on 12 August lasted approximately one hour. It ended with the gas explosions and ash outbursts starting again at 1315 and continuing until 1515 with ongoing degassing afterwards. According to the summed seismic amplitudes (RSAM values), the greatest activity was between 2000 on 10 August until the morning of 11 August.

Observers saw a lava lake in the crater on 18 August. On that day, INETER's Wilfried Strauch and Armando Saballos, along with visiting North American specialists, climbed Telica to install GPS equipment. Taking advantage of periods of low degassing, they managed to observe the bottom of the new inner crater that had formed in the last few months (figure 11). To their surprise, they saw a lava lake there. In addition they listened to forceful jetting noises probably generated by the water contact with heated material.

Figure 11.Photograph from the crater rim at Telica showing the new inner crater, 18 August 1999. Courtesy of Wilfried Strauch, INETER.

On 21 August INETER's Virginia Tenorio and Julio Alvarez climbed the volcano and saw that the inner crater had enlarged; and, in addition they again heard jet-engine-like noises. Abundant escaping gases thwarted views into the inner crater so the visitors could not assess whether a lava lake remained. The same day between 0800 and 0900, residents who live on the SE flank of the volcano felt two rumblings from the volcano. Possibly, this caused the inner crater to enlarge even more.

Several days later seismic tremor increased, but the number of microearthquakes fluctuated, first dropping, then increasing again on the 25th. On 29 August seismic tremor began to drop substantially. Then, however, the number of microearthquakes increased. Telica's eruptive activity is typically associated with slightly increased tremor and over 200 to 300 microearthquakes per day.

During September, a month with 2,116 microearthquakes, gas emanations prevailed until the 10th. A seismic swarm at the beginning of October was followed by a series of explosions with tephra expulsions during 3-15 October. On the 5th, INETER staff on the crater's edge witnessed the discharge of both ash and lava (presumably in the form of bombs). The last similar lava-bearing explosion of this type was in 1988 (SEAN 13:01). On the 12th, W-flank residents reported that on the previous day (at about 1400 on the 11th) they had felt an unusually strong explosion shaking their houses. Later, they witnessed the fall of very fine gray ash. Observers also saw that the inner crater had grown wider than when seen in September. By 12 October the seismic amplitude had decreased to background levels. The number of earthquakes registered for October was 888.

During November the earthquake sum was comparatively low, 144, but that did not signify volcanic quiet. On 19 November, INETER's Julio Alvarez and Virginia Tenorio skirted the volcano along the León-Chinandega highway where they saw an ash column. Erminio Rojas, a farmer on Telica's S flank, told them that in the past few weeks the volcano had almost constantly been expelling gray ash. On 17 November he witnessed a very large explosion that caused an ashfall deposit reaching 2.5 cm thickness near his house, damaging his apples and beans. The observers further noticed that on the crater's SW a possible collapse feature had developed. Burned ash-covered plants lay in the area near the edge of the crater. Ash discharges on 17 November occasionally emitted a noise similar to a gunshot.

On 24 November, Civil defense of León reported a black cloud above Telica. An unusual seismic signal on 28 November prompted a visit to Telica by Tenorio and Strauch, along with Rafael Abelia of the Institute of Geomineras Investigations of Madrid, Spain. When they arrived at the volcano, the group found that a zone of disruption had spread over a great part of the N crater wall, and the edge of the crater was covered with a thick layer of fine dust. This indicated to them that there was no explosion and the cloud that the Civil Defense observed was due to the collapse of the N crater wall. COSPEC measurements conducted on 29 November indicated that the volcano was producing between 50 and 500 metric tons/day of SO₂ per day.

During December 1999 there were 1,085 volcanic earthquakes, of which, four were located. INETER's seismic network located several earthquakes that took place underneath the volcano on 14 December with magnitudes between 2 and 2.5. During December, tremor stayed low until the 24th, when it was punctuated by sporadic degassing and smaller ash-bearing discharges. On the 25th, tremor began to rise slowly; on the 28th there occurred an abrupt increase in the seismic signal, four-fold larger than seen during previous days. The morning of 29 December seismicity was high. The same day reports were received describing almost continuous ash-bearing explosions, with WSW-directed tephra falls.

Two large explosions at 0900 on 29 December sent ash to heights of more than 1,000 m above the crater. Besides affecting cities adjacent the volcano, ash was later known to have affected the cities of Posoltega (~16 km SW), Chichigalpa (20 km WSW), Quezalguaque (20 km SSW), Chinandega (35 km WSW), and Corinto (~45 km SW). INETER noted that civil-aviation pilots reported that ash rose up to 5 km, although whether this was an altitude or the height over the 1-km-tall volcano remained undisclosed. Tremor initially stayed high on 30 December but dropped on 31 December. Activity continued into January 2000.

Geological Summary. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).