Introduction The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows.1 Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors.1 These include: erosion by rivers, glaciers, or ocean waves creating oversteepened slopes; rock and soil slopes weakened through saturation by snowmelt or heavy rains; earthquakes that create stresses that make weak slopes fail; earthquakes of magnitude 4.0 or greater; volcanic eruptions producing loose ash deposits, heavy rain, and debris flow; and excess weight from accumulating rain or snow, stockpiling of rock or ore from waste piles or man-made structures that stress weak slopes to failure.1 Slope materials that become saturated with water may develop a debris flow or mud flow.1 The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path causing much damage.1 Landslides occur in every state and U.S. territory.1 The Appalachian Mountains, the Rocky Mountains and the Pacific Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems.1 Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides.1 Although the physical cause of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.1 Landslides are a serious geologic hazard common to almost every state in the United States.1 It is estimated that in the United States, they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year.1 Globally, landslides cause 100's of billions of dollars in damages and l00's of thousands of deaths and injuries, each year.1 The United States Geological Society (USGS) scientists continue to produce landslide susceptibility maps for many areas in the United States.1 In every state, USGS scientists monitor streamflow, noting changes in sediment load carried by rivers and streams that may result from landslides.1 In January 17, 1994, in Northridge, Calif., an earthquake-triggered landslides in an area of 10,000 km2.1 The landslide activity disturbed surface sediments in the Santa Susana Mountains, triggering renewed outbreaks of Valley Fever, a windborne soil fungus that causes a respiratory illness in people.1 Efforts to predict landslides may have prevented some of these occurrences. Materials and Methods An area of Southern California affected by the Northridge Earthquake of 1994 was used for this study. 100 m square grid cells were overlaid on a map of Southern California and 822 grid cells were randomly taken from this region. Many variables were measured from these grid cells.Dem100c This was the average elevation of the cell in meters calculated from a digital elevation model maintained by the USGS. The range is 250 to 1200 m approximately. Maxslope100 The maximum slope (in degrees) was measured at any point over the 100 m cell. In this region, the range was 0 to 65 degrees. Nrls10n100a A numerical value was assigned (0= no landslide and 1= landslide) that indicated whether a landslide occurred in the 100 m cell due to the Northridge Earthquake of 1994. Prenrls100 A numerical value was assigned (0= no landslide and 1= landslide) that indicated whether or not a landslide had occurred in the 100 m cell prior to the Northridge Earthquake of 1994. Included were landslides due to debris flow, earthquakes, or other. Sg100 The slope gradient (in degrees) was measured across the entire 100 m cell. In this region, the range was 0 to 40 degrees. Aspect100 The aspect (in degrees, 0= true North) was measured in the 100 m cell with the range being from 0 to 360 degrees. Aspect cagtegory All aspects were binned into 8 categories based on the 8 most common directions of the compass (North, Northeast, East, Southeast, South, Southwest, West, Northwest). For example, all aspects between 22.5 and 67.5 degrees were categorized as “Northeast”. Factor of safety The equation FS= c’/tγ sin α + tanφ’/tanα was used to obtain the static factor of safety where φ’ is the effective friction angle, c’ is the effective cohesion, α is the slope angle (sg100), γ is the material unit weight, and t is the slope-normal thickness of the failure slab.2 Histograms and outlier box plots were made for each of the variables listed above. The mean, median, and standard deviation were calculated for dem100c, maxslope100, sg100, aspect100, and factor of safety. For nrls10n100a and prenrls100, histograms were made and proportion of success were calculated. For aspect category, only a histogram was made. All statistics were calculated in JMP IN Version 5. Results Dem100c The dem100c histogram and corresponding 5 point outlier box plot expressed characteristics of slightly positive skew, unimodal, and had a CV calculation of 27.69% (Figure 1). A CV value of that level is consistent with low variability. The distribution expressed a mean of 586.2 m, median of 575.7 m, and standard deviation of 162.3 m.dem100c 255075100Count300 400 500 600 700 800 900 1000 Figure 1 Maxslope100 The maxslope100 histogram and corresponding 5 point outlier box plot were consistent with a normal distribution that is therefore unimodal and symmetric (Figure 2). This data approximately follows the empirical rule of >99% of all observations are within 3 standard deviations of the mean. The mean was calculated at 33.4 degrees and the median was 33.6 degrees with a standard deviation of 8.46. The CV calculated as 25.34% which was the lowest CV of all the datasets suggesting the least variability among all of the variables. maxslope100 255075100125Count10 20 30 40 50 60 Figure 2 Nrls10n100a Based on Figure 3, there were less 100 m grid cells where a landslide occurred due to the 1994 Northridge Earthquake. The proportion of success (where a landslide occurring = success), was calculated at 13.38%.nrls10n100a 250500750Count01 Figure 3 Prenrls100 Based on Figure 4, approximately half of the 100 m grid cells had a landslide that occurred within that area before the 1994 Northridge Earthquake. The proportion of success (where a landslide = success) was 48.54%. prenrls100 100200300400Count01 Figure 4 Sg100 The sg100 histogram and
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