The Jarvis’s march proced ure. 

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... derived from the activity of the Los Humeros caldera, which is situated 35 km southwards of the studied area. These materials overlay both Mesozoic and Tertiary deposits. Moreover, this zone is characterized by the development of different superficial formations (regolith, old landslide deposits) that are easily remobilized by the frequent torrential rains affecting this tropical region. The behavior of different types of material can therefore be studied in this very limited zone. The reinterpreted geologic map (Ochoa-Tejeda, 2009; 2010) takes into account field observation and the information provided by INEGI (1994), Mooser (2000) and Angeles-Moreno and Sanchez-Martinez (2002). We reported (Fig. 2) landslide locations as the traces extracted from the Ikonos image for the 1999 event, and as points for the more recent mass movements). All the landslides surveyed in the field and their traces extracted as previously described from the satellite images, have been analyzed by using morphometric parameters in order to characterize and to classify them. Some parameters are directly related with the shape of studied landslides: for instance, the surface S , the perimeter P , the ratio between these two parameters, as well as the presence of holes that allows defining a porosity index. The simplest method used to measure the surface consists to calculate the total number of pixels N bp belonging to the connected component; but according to Pratt (1978), another approach consists to consider the surface as S   P S   P P 2 , where P S are the pixels that strictly belong to the surface and P P the pixels describing the perimeter. The surface in m 2 or km 2 is obtained by multiplying S by the value of the pixel surface. There are some more accurate methods that take into account the configuration of P p and the neighboring pixels in order to know exactly the portion of the pixel P p that belongs to the surface (Parrot, 2007). The perimeter P corresponds either to the total number of pixels ( N p ) describing the perimeter or to the length ( L p ) of this perimeter measured by using the length of the segment linking two successive pixel centers. Taking into account the values of the former parameters, it is possible to define various ratios such as for example the ratio perimeter/surface [in pixels]    N p N b p   100 , the ratio perimeter versus surface [in meters and square meters]    L p S   100 or the circularity index    P 2 S   100 . The notion of porosity characterizes topographically heterogeneous ensembles as encountered in some landslides, especially in the case of rotational landslides (graben-like structures). The calculation is as follow: where P s and P p are all the pixels describing the shape (surface and perimeter) and P h the pixels corresponding to the holes. Another way to describe the shape is to compare this shape and some plain shapes such as the rectangle, the circle or the smaller convex zone that circumscribes the studied item. It is then possible to measure the length and the width of the studied shape, to define the relation existing between these two values, etc... This approach requires to research the gravity center ( GC ) and to define the principal axis ( PA ) passing through the gravity center. The gravity center GC with coordinates X c , Y c and the second order moments  ,  and  are equal to: N bp is the total number of pixels and X i Y i the coordinates of a pixel i. The principal axis PA that passes through GC is equal to: tg  2    2  xy   yy   xx  if   yy   xx   0 . When the difference between  xx and  yy is equal to 0 , the connected component does not present any orientation. Otherwise, the PA orientation is calculated clockwise in degrees (from 0° to 180°, 0° corresponding to the north). It is also possible to compute all the distances between GC and all the perimeter pixels; in this case, PA corresponds to the perpendicular to the straight line that links GC and the closest pixel belonging to the perimeter. It is then possible to measure the length and width of the rectangle circumscribing the studied landslide surface and calculate its ratio. As the principal axis PA intersects the shape perimeter in two points (D 1 and D 2 ), the direction D of the landslide can be computed (Ochoa- Tejeda and Parrot, 2007). The relative hypsometric values of D 1 and D 2 permit to define a minimum (A min ) and a maximum (A max ). The direction D of the landslide corresponds to the straight line that links GC and A min . The measurement of the direction, especially in the case of shallow (hypersaturated) debris slides allows defining zones that present an uniform slope gradient related to the triggering of such superficial mass movements. There are many different procedures that allow defining a convex zone. Among them, the Jarvis’s march such as it is described by Akl and Toussaint (1978) is a simple algorithm that efficiently draws a convex contour (Fig. 3). The surface of the convex zone S c (or the total number of pixels N tc of this zone) and its perimeter P c calculated in pixels or in meters allows calculating several parameters: the ratio S/S c or the relation N bp /N tc that both correspond to two convexity indices based on the surface, the ratio P /P (perimeter convexity or ...