No new eruptive activity has been reported since the 4-day ash eruption that started 9 June. Seismic activity declined after the eruption and has continued at moderate levels since mid-June. During July, 162 high-frequency, 445 low-frequency, and 68 shallow explosion events were recorded. The actual number of events was obscured by tremor that filled the records on some days. Major high-frequency swarms occurred on 17 July (57 events) and the night of 31 July-1 August (120 events). Strong tremor occurred 14-16 and 23-26 July, with peak-to-peak amplitudes of 9-10 mm. Additional earthquake swarms were recorded on 6 August (high-frequency) and 9 August (low-frequency).

Deformation measurements were more stable in July than in June. On 7 July, all stations showed inflationary trends, with a maximum value of 10 µrad. The SO₂ content of the plume as measured by COSPEC decreased to the lowest values yet measured, 170 t/d.

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: E. Parra, INGEOMINAS, Manizales.