Activity during 15 June-10 July. "Harmonic tremor, which started on 4 May, still dominated the pattern of seismic activity at Ruiz as of 10 July. Several times, swarm-like single event sequences of mostly low-frequency earthquakes changed the pattern for hours or even days.

"On 5 July the tremor declined almost to background level. At the same time the frequency of events increased sharply, totaling ~300 (minimum 5-second duration) by 10 July at Olleta station (4.2 km W of Arenas Crater), the highest number for a 5-day period since the 13 November eruption. The maximum magnitude was 2.2 and most events were of the low-frequency type.

"A comprehensive analysis of all deformation data (EDM, electronic tilt, dry-tilt, and levelling) now clearly shows asymmetric behavior of the volcanic edifice. Most deformation data have been gathered in the NW sector and the least in the SE sector (where only one electronic tiltmeter is sensing deformation now, apparently with considerable electronic or geologic drift). However, comparison of the data sets suggests strongly that movements are most pronounced in the NW sector, somewhere between the main crater Arenas and the dormant crater La Olleta, on the W flank.

"dry-tilt station Molinos (3.2 km N of Olleta and 5.0 km NW of Arenas) recorded a fluctuating change of several tens of microradians, corresponding to upward movement in a direction E of Olleta (toward the summit) between mid-April and mid-June. There was a general agreement with this sense of deformation at Inderena electronic tiltmeter (1.1 km NE of Molinos dry-tiltmeter). The start of this anomaly corresponds roughly to the reactivation of seismicity (20 April). The deformation at Molinos reversed to downward movement around 17 June, while seismic activity continued.

"A comparison of all EDM lines also reveals that deformation is concentrated in the NW sector (line Arbolito). New EDM lines and dry-tilt stations are now being installed, to allow for more resolution of the deformation pattern.

"Ash emission seemed to be more or less continuous at one of the fumaroles in the crater. Samples of ash, taken from recent falls, were examined under the microscope. No signs of juvenile material were evident. The vapor column, although quite dense, was not seen at heights above 500 m. No correlation has been found yet between the fumarolic activity and the amplitude/period of the harmonic tremor."

Geological Summary. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: H. Meyer, INGEOMINAS, Manizales.