THE FREQUENCY AND DISTRIBUTION OF RECENT LANDSLIDES IN 3 MONTANE TROPICAL REGIONS OF PUERTO-RICO

Landslides are common in steep mountainous areas of Puerto Rico where mean annual rainfall and the frequency of intense storms are high. Eac h year, landslides cause extensive damage to property and occasionally result in loss of life. Average population density is high, 422 peopl e/km(2), and is increasing. This increase in population density is acc ompanied by growing stress on the natural environment and physical inf rastructure. As a result, human populations are more vulnerable to lan dslide hazards. The Blanco, Cibuco, and Coamo study areas range in sur face area from 276 to 350 km(2) and represent the climatologic, geogra phic, and geologic conditions that typify Puerto Rico. Maps of recent landslides developed from 1:20,000-scale aerial photographs, in combin ation with a computerized geographic information system, were used to evaluate the frequency and distribution of shallow landslides in these areas. Several types of landslides were documented-rainfall-triggered debris flows, shallow soil slips, and slumps were most abundant. Hill slopes in the study area that have been anthropogenically modified, ex ceed 12 degrees in gradient, are greater than 300 m in elevation, and face the east-northeast, are most prone to landsliding. A set of simpl ified matrices representing geographic conditions in the three study a reas was developed and provides a basis for the estimation of the spat ial controls on the frequency of landslides in Puerto Rico. This appro ach is an example of an analysis of the frequency of landslides that i s computationally simple, and therefore, may be easily transferable to other settings. (C) 1998 Elsevier Science B.V. All rights reserved.