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. A small, 1.4-km-wide caldera cuts the top of Ubinas, Perú's most active volcano, giving it a truncated appearance. 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 degrees. The steep-walled, 150-m-deep summit caldera 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 of Holocene age about 1,000 years ago. Holocene lava flows are visible on the flanks, but historical activity, documented since the 16th century, has consisted of intermittent minor-to-moderate explosive eruptions.

Source: Instituto Geofísico del Perú (IGP)