During the weeks preceding 5 June, volcanic seismicity recorded by RESCO remained at low levels, showing only a few avalanches/day. Poor visibility prevented daily visual observations from the city of Colima, but sporadic observations from sites near the volcano (Yerbabuena and La Joya) have shown strong fumarolic activity (mainly vapor and a small grayish plume) and continued growth of the dome extrusion.

On 6 June between about 0000 and 0200, a tremor episode was clearly recorded by station EZV7 (at Volcancito, ~1 km NE of the summit), but was barely detectable at other stations. Activity then returned to previous levels. A second tremor episode, much stronger and clearly recorded by several stations, occurred between about 1800 and 2100, after which seismicity again returned to relative quiet. The activity was interpreted as probably being of phreatic origin, given recent rainfall in the region. Witnesses about 13 km from the volcano (in the Tonila area) reported conspicuous incandescence at the crater.

A third seismic episode, on 8 June between about 2000 and 2200, consisted of four large, complex shallow earthquakes followed by almost monochromatic harmonic tremor. The caretaker at nearby La Joya reported hearing four explosions followed by strong sustained whistling. On 9 June, a few small closely-spaced B-type earthquakes seemed to mark the onset of another tremor episode, but it did not materialize and no further tremor activity had been recorded as of 13 June.

During the evening of 9 June, there was an increase in both the number and duration of avalanche events, which remained of small magnitude. Long-duration avalanches continued as of 13 June, but their numbers had decreased. Geophysicists noted that the increased number and duration of avalanches on 9 June was similar to that observed before the 16 April dome collapse. No deep seismicity, indicating stress at depth, has been detected, but the tremor, not previously observed, suggested changes in activity requiring careful monitoring.

Geological Summary. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: F. Alejandro Nava, Francisco Núñez-Cornú, Gilberto Ornelas-Arciniega, Ariel Ramírez-Vázquez, G.A. Reyes-Dávila, Hector Tamez, and R. García, CICT, Universidad de Colima; Z. Jiménez, I. Yokoyama, and S. de la Cruz-Reyna, UNAM.