HUMANS AS INADVERTENTLY HAZARDOUS GEOLOGIC AGENTS

Humans become inadvertently hazardous geologic agents when they unknowingly or uncaringly engage in activities that trigger landslides, debris flows, earthquakes, and land subsidence. In some cases the hazards are triggered out of ignorance by landowners who do not have the knowledge necessary to predict the consequences of seemingly harmless activities. A landowner in northern New Mexico, for example, triggered a 10,000 cubic meter landslide by irrigating a gravelly stream terrace while trying to establish a small orchard. The landslide mobilized into a debris flow that diverted a nearby stream and flooded neighboring property. A forensic investigation showed that the terrace rested on a buried soil, which impeded infiltration and perched groundwater, providing the pore water pressure needed for sliding and the volume of water necessary for subsequent debris mobilization. Landslides had not historically been a problem in that valley, and it is unclear whether an engineer or geologist would have predicted the consequences even if one had been retained prior to irrigation. In other cases, hazards can be triggered because professionals have misunderstood the local geology and its latent hazards, and therefore failed to anticipate the consequences of human activities. Misidentification of a dormant landslide as a moraine, for example, led to the 10 million cubic meter reactivation of the Costilla Dam landslide in northern New Mexico. Construction materials were stockpiled near the head of the reactivated landslide and a deep stilling basin was excavated across its toe. The reactivation resulted in a $12 million cost overrun and a dispute over responsibility that was finally settled in arbitration. There are at least two lessons to be learned from these and other examples of inadvertently triggered geologic hazards. First, the past may or may not be the key to the future when anticipating the geologic implications of human activities. This is particularly so when human activities give rise to unprecedented conditions. Second, the value of geologists can be diminished by the cookbook approach often used in geotechnical investigations. Therefore, creativity, curiosity, and critical analysis must be encouraged over standardization and stagnation.