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people of Colombia for criticism. Few are able to recognize a hazard that, for all practical purposes, has not manifested itself for some 140 years. This lack of local experience with volcanic hazards, and, thus, the virtual absence of volcano risk perception among the Ruiz area residents, was likely a major obstacle faced by those scientists and officials who recognized the danger, engaged in preparedness for an emergency, and sought to ready the public to respond.
INITIAL THREAT DETECTION
The ability to predict the precise time and magnitude of a natural disaster depends on two factors: the current state of scientific knowledge about a particular disaster and local observations of precursor activities to the disaster. The earliest perception of threat associated with Ruiz was in December 1984. A landslide formed a dangerous natural dam. At about the same time, mountain climbers began to feel and report earthquakes and gas plumes. These activities persisted, and in February 1985 several geologists from INGEOMINAS (the National Institute of Geology and Mines) conducted a site visit. In March 1985, the Colombian Civil Defense (Defensa Civil de Colombia) and INGEOMINAS requested that the United Nations Disaster Relief Organization (UNDRO) send a scientist to study the volcano.
John Tomblin of UNDRO inspected Ruiz in March 1985. Tomblin concluded that the observed earthquakes and gas plumes could be indicative of an eruption and he recommended that INGEOMINAS install seismographs to monitor the volcano. In addition, he suggested that a risk map be prepared to illustrate the potential hazard associated with an eruption of the volcano. Further, he recommended that the Defensa Civil de Colombia draft an emergency plan to facilitate public warnings and evacuation for high-risk areas.
Thus, INGEOMINAS began a project to monitor and define the volcanic risk in the Ruiz vicinity. In May, UNDRO funded a reevaluation of Ruiz by Minard Hall of the Instituto Geofísico of the Escuela Politécnica Nacional of Ecuador. In May 1985, the U.S. Geological Survey initiated a cooperative effort with UNDRO to provide scientific equipment to monitor Ruiz. First steps began in July 1985 when four portable seismographs were installed on the mountain. At about the same time, the Ruiz Volcanic Risk Committee was formed. Its charge was to begin volcanic monitoring, local emergency planning, and public education about volcanic risk.
In August, the installed seismographs recorded 5-20 earthquakes each day (Herd, 1986). INGEOMINAS pointed out that, on average, such earthquakes have, on a global basis, accompanied large volcanic eruptions about 25 percent of the time throughout recorded history. A minor eruption of steam, ash, and rock occurred on September 11, 1985; this event captured government attention although no one was injured. Surveillance of the volcano contin-