Estudio de la sismicidad y deformación del Volcán de Colima durante la crisis de abril de 1991

Abstract

Increascd fumarolic emissions as well as rock sliding at Colima volcano during February 1991 wa~ caused by ascending new magma which cut through an older summit dome on March 1st and continued growing as a new lava lobe. The new lava lobe reached the summit edge on April 15th ancl on the 16th its advance caused a small partial collapse accompanied by avalanching and ash clouds. Afterwards, lava continued as a lava flow down the SW flank of the volcano. From April 12th to 24th 1991 seismicity and deformation were monitored al Colima volcano. A seismic network with 4 analogical stations as well as 3 dry tilt and 3 trigonometric levelling stations were established. Dry tilt stations were reoccupied on 3 occasions. Six trigonometriclevelling measurements of the new lobe were made from 3 different stations. Four types of seismic events were found: A, B, C, D (rock slides) and G (gas emission). The first 2 types (A and B) were classified according to Shimozuru (1971) and Minnbmi (1974) while D and G events were identified in the field. A-type event appcared only between April 13th and 15th whilc B. D and G events occurred during the whole period. Rock sliding events (O) increased on April19th and 20th. The frequency on the volcanic tremor increased from 3.0 to 9.0 Hz before April 16th and then decreased again to 3.0 Hz on that date after the small partial dome collapse. The resultant vectors of the 3 dry tilt stations (≤10 µ-rad) show small tilts mainly associated to del1ation during lava extrusion, although data is within the detection limits. Monitoring of the lobe growth showed a height of 36m and width of 109 m on April J4th with a reduction on April 24th to 13 m height and 66 m width, this was after the lobe had reached thc edge and changed into a lava 11ow down the SW flank (April 16th). The change in the lobe topography on April 15th indicates the stress it was under which was premonitory to thc partiatcollapse on the next day.