Limitations of the DeGraff Method

The DeGraff method has many advantages for producing a landslide hazard zone map— it is conceptually and computationally simple, and relies on the types of spatial data (topography, geology) that are commonly available.
However, the method has some inherent limitations that should be understood, in order to correctly interpret the maps of Landslide Susceptibility Classes.

The first limitation is that landslide susceptibility classes describe only the susceptibility of the local landscape to landsliding under some undefined triggering mechanism.
The temporal probabilities and spatial patterns of such triggering mechanisms constitute the opportunity for landsliding.
The potential for landsliding is calculated by multiplying a map of opportunity times a map of susceptibility.
From this concept arise two facts:
you cannot calculate a temporal probability or return period for landsliding from a susceptibility map, in the absence of an landslide opportunity map, and
the landslides that result from a given triggering mechanism (earthquake, hurricane, rainstorm) will probably occur in the highest susceptibility classes on the map, but only if the triggering event is similar to the typical triggering events that created all the landslides in the inventory.

For example, if most triggering events in the past several hundred years have brought near-horizontal rain in from the southwest and induced most landslides on slope with a southwest aspect, an anomalous storm from the northeast may produce most of its landslides in areas labeled moderate susceptibility on these maps.
Similar results could accompany earthquakes, where moderate-susceptibility areas fail near the epicenter, but only high-susceptibility areas fail far from the epicenter.

Second, the DeGraff technique relies solely on the landslide inventory, and the landslide densities computed therefrom, to define landslide susceptibility.
There is basically no room for either theory or common sense when the landslide densities seem to defy logic.
Any deficiencies in the inventory will be transmitted directly to the susceptibility map.
Such deficiencies can come from the mistakes or omissions of landslide investigators, or from the polyglot nature of the landslides preserved in today's landscape.
The landslides visible at any time are a complex mixture of large slides that range from very old to very young, intermediate-size slides that are young to very young, and small slides that are exclusively very young, often only visible for a few decades.
The landslide inventory is thus heterogeneous, and some scheme (such as the one employed in this study) is necessary to separate definite landslides from features that just might be landslides.