LANDSLIDE SUSCEPTIBILITY MAPPING, KINGSTON METROPOLITAN REGION, JAMIACA

(summarized from McCalpin, J.P., 1998, Landslide hazard zonation of the Kingston Metropolitan area, Jamaica: unpublished report submitted to the Department of Geology, University of the West Indies, Kingston, Jamaica by GEO-HAZ Consulting, Estes Park, CO, Feb. 13, 1998, 21 p. plus appendices and 8 oversized maps at 1:100,000 scale.)

          The Kingston Metropolitan Area covers roughly 554 square km and includes mountainous terrain north and east of urban Kingston. A landslide inventory of this area prepared by University of the West Indies, Mona, contains 2,321 landslides. Landslides are subdivided into active (landslides, n=46; zones, n=341), scarps (definite, n=613; probable, n=958; questionable, n=340), and deposits (definite, n=18; probable, n=4; questionable, n=1). Landslides cover 19.786 square km, or 3.57% of the entire study area. Excluding the Liguanea Plain, landslides cover 4.77% of the mountainous areas.

          GEO-HAZ performed a landslide susceptibility study using the susceptibility matrix approach pioneered by DeGraff and Romesburg (1980). First, we created factor maps for slope angle, slope aspect, downslope curvature, lithologic group, distance to faults, and distance to roads. The first three of these maps were based on a digital elevation model (DEM) created from 50 m contours on the published 1;50,000-scale metric base maps. Lithologic groups were simplified from the Provisional editions of 1:50,000-scale geologic sheets of the Blue Mountains and Kingston. Locations of faults and roads were derived from published maps.

          The digital data were analyzed by the geographic information system (GIS) software IDRISI for Windows v. 2.0, running under Windows NT 4.0. IDRISI is a raster-based program. Therefore, all the digital vector data provided to us by UWI-Mona was rasterized onto a 15 m pixel raster base. Raster maps of the study area rectangle contain roughly 3.7 million pixels, of which about 2.5 million are within the boundaries of Kingston and St. Andrew parishes. We adapted the original matrix technique of DeGraff to IDRISI in the PC environment, and our report to UWI-Mona contains a detailed account of the execution steps and files that must be created to produce the landslide susceptibility matrix and its accompanying maps.

          We performed separate DeGraff analyses for deep versus shallow landslides. Factor analysis for deep landslides shows they are sensitive (in order of decreasing importance) to lithologic group, distance to faults, slope angle, and slope aspect.

By overlaying maps of those four factors we created a map of 249 composite categories of slope-aspect-lithology-fault distance. Those categories possess landslide densities (defined as number of pixels containing landslides divided by total pixels) between 0% and 71%. Our classes of low, moderate, moderate-high, high, and very high susceptibility for deep landsliding are bounded by landslide densities of 2%, 3.5%, 4.5%, and 6% respectively. The highest susceptibility areas are in far eastern St. Andrew on Cretaceous volcanics, or on Eocene clastics or andesites within 60 m of faults. Granodiorite basement and limestones have low-moderate susceptibility.

Active landslides comprise only 16.7% of the number of total landslides and 4.5% of total landslide area. Most active landslides are small (200-300 square meters), shallow debris slides or debris avalanches that result from failure of colluvium. Controlling factors (in order of importance) appear to be lithology, slope angle, slope curvature, and aspect. Fault distance does not appear to affect shallow landslides. The overlay map contains 83 unique combinations of slope-aspect-lithology-curvature, which possess landslide densities between 0% and 0.25%. We subdivide this range into low, moderate, moderate-high, high, and very high susceptibilities. Highest susceptibilities are restricted to Eocene shales, sandstones, and andesites, particularly on north-facing slopes near ridge shoulders.

          The 2 maps of landslide susceptibility were then integrated into the Multi-Hazard Assessment for the Kingston Metropolitan Area, which is a component of the Caribbean Disaster Mitigation Project (CDMP). The CDMP is a cooperative effort between the governments of several Caribbean countries, the Organization of American States (OAS) and the U.S. Agency for International Development (USAID). For further information on the CDMP, see www.oas.org/en/cdmp

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