IGP reported six phreatic explosions from Ubinas during 1-3 September. The series of explosions decreased in both energy and length of the tremor signal associated with each explosion; tremor lasted 75 minutes for the first explosion and 5-10 minutes for the last one. Most of the explosions generated ash plumes that rose 1.5-2 km above the crater.

The first explosion occurred at 2246 on 1 September, lasted 110 seconds, ejected ballistics up to 2 m in diameter, and caused ashfall 2 km ENE. The second explosion, at 1552 on 2 September, lasted 292 seconds and ejected fewer ballistics. Details for the 3rd through the 6th explosions are as follows: the 3rd occurred at 2350 on 2 September and lasted 279 seconds; the 4th occurred at 0809 on 3 September and lasted 296 seconds; the 5th occurred at 1021 on 3 September and lasted 288 seconds; the 6th occurred at 1029 on 3 September and lasted 74 seconds.

Geological Summary. The truncated appearance of Ubinas, Perú's most active volcano, is a result of a 1.4-km-wide crater at the summit. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45°. The steep-walled, 150-m-deep summit crater contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one from about 1,000 years ago. Holocene lava flows are visible on the flanks, but activity documented since the 16th century has consisted of intermittent minor-to-moderate explosive eruptions.

Source: Instituto Geofísico del Perú (IGP)