SLOPE AND LITHOLOGY OF LANDSLIDE SOURCE AREA IN A LOW RELIEF TOPOGRAPHY, NEW JERSEY
Landslide point locations were obtained from a database of 181 historical landslides from 1782 to 2010 compiled by the New Jersey Geological Survey. The database includes 17 landslides that caused 68 fatalities and 53 injuries. The majority of the landslides (54%) in this area were triggered by heavy rain, and April is the month with the greatest number of historical landslides. We used GIS to overlay landslide point locations on the New Jersey state geological map, and maps of local relief and slope computed from the National Elevation Dataset. New Jersey is divided into four topographic provinces: Coastal Plains, Piedmont, Highlands, and Valley and Ridge. The majority of the slides occur in the Piedmont and Highlands, which are dominated by igneous and metamorphic rocks. We lumped bedrock lithology into five categories: clastic sedimentary, nonclastic sedimentary, igneous intrusive, igneous extrusive, and metamorphic. Our analysis shows that igneous intrusive rocks have the highest incidence of landslides per unit area, therefore they are the most susceptible rock-type for landslide occurrence.
The majority of landslides (53%) occurred on slopes of 10° or less. By aggregating slope from a 30-m DEM (using a standard 3X3 slope formula) to maximum slope in 90-m, 240-m and 480-m grid cells we confirmed that most landslides did occur in areas of low slope. Our studies of the database revealed that human activity, such as, construction, mining and quarrying contributed to 32% of landslides on gentle slopes (≤10°). Our analysis of landslides in areas of gentle slope indicates that areas of New Jersey where landslide frequency is less 1.9X10-5 km2yr-1 have negligible landslide hazard, based on the number of landslides statewide in areas of gentle slope and for the period of record.