EDM deformation monitoring of the 1997–2000 activity at Volcán de Colima

The 1997–2000 activity at Volcán de Colima began in November 1997 with a series of seismic swarms and deformation of the summit lava dome. This activity reached a climax on 20 November 1998 with the extrusion of lava, whose downslope movement was accompanied by pyroclastic flows. Since the major explosion on 10 February 1999, explosive activity has continued up to the time of paper preparation (August 2001). Summit deformation was detected by electronic distance measurement (EDM) surveys. Slope distance measurements were carried out from three base stations utilizing nine fixed reflectors grouped on the volcano edifice at three different altitudes (3250, 3450, and 3850 m). After the destruction of two summit reflectors in July 1998, the EDM surveys were continued, primarily using five reflectors remaining on the north flank of the volcano. Slope distance measurements taken from November 1997 to July 1998 show a maximum cumulative distance change of 0.5 cm/day to the summit reflectors, which is interpreted to result from inflation of the volcanic edifice in response to magma movement towards the surface. EDM variations recorded in August 1998–February 1999 from reflectors on the north flank of the volcano suggest further inflation before extrusion of lava began on 20 November 1998. Subsequent deflation was observed after lava effusion ceased in February 1999. Inflation was measured five days before the 10 February 1999 explosion. No significant deformation of the upper cone has been detected since then.