IG reported that Tungurahua's seismic network detected a significant increase in the number of long-period (LP) earthquakes on 12 September and small episodes of tremor beginning on 16 September. A swarm of 24 LP events were detected during 0408-0424 on 18 September. Starting at 1400 on 24 September the number of LP events again increased. Gas emissions were low, and together with the increased seismicity, possibly indicates a blocked conduit. IG noted that a possible large-scale eruption may happen within hours to days. In response, the Secretaría de Gestión de Riesgos (SGR) announced that the Alert Level was raised from Yellow to Orange (the second highest on a 4-color scale) on 26 September.

Geological Summary. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Sources: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN), Secretaría de Gestión de Riesgos (SGR)