Southwestern Colombia and northern Ecuador were shaken by a shal-low-focus earthquake on 12 December 1979. The magnitude 8 shock, located near Tumaco, Colombia, was the largest in northwestern South America since 1942 and had been forecast to fill a seismic gap. Thrust faulting occurred on a 280- by 130-kilometer rectangular patch of a subduction zone that dips east beneath the Pacific coast of Colombia. A 200-kilometer stretch of the coast tectonically subsided as much as 1.6 meters; uplift occurred offshore on the continental slope. A tsunami swept inland immediately after the earthquake. Ground shaking (intensity VI to IX) caused many buildings to collapse and generated liquefaction in sand fills and in Holocene beach, lagoonal, and fluvial deposits.
Nevado del Ruiz, northernmost active volcano in the Andes, erupted on November 13, 1985, triggering lahars that killed at least 22,000 persons and destroyed more than $212 million in property. The eruption culminated a year of precursory earthquake and fumarolic activity that began in November 1984 and intensified in September 1985 with the start of phreatic eruption. Colombian and other international scientists, concerned that Ruiz might be building to a large eruption, initiated monitoring of Ruiz volcano on July 20, and in October completed hazard and risk assessments that correctly anticipated the disastrous effects of November 13. Although relatively small, the paroxysmal eruption at 9:08 P.M. produced pyroclastic flows and surges that scoured and melted part of the summit ice cap, generating lahars that swept down river valleys on the east, north, and west flanks of the volcano. A volcano observatory has been established in Manizales, Colombia, to monitor Ruiz's continuing activity.
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