One of the largest known mass wasting events in the Philippines happened in the province of Southern Leyte. On 17 February 2006, 10:26 LT, an estimated 15–20 million m3 of cascading rocks from a 700 m high mountain buried the entire village Guinsaugon in the municipality of Saint Bernard and killed 1221 people. Identified as a rockslide-debris avalanche , the fatal landslide happened on the slopes of Mt. Can-abag along a steep scarp defined by the Philippine Fault Zone or PFZ (Fig. ). The landslide was 4.1 km long, 1.5 km wide and had a planform area of about 3.3 km2.

The study area is part of the Southern Leyte segment of the sinistral PFZ, which has an average movement of 0.55 cmyr-1 up to 3.5 cmyr-1 (, as cited by ). This has led to formation of regional discontinuities and development of thick clay-rich gouge zones in the surface prominent in the entire province and other locations along the PFZ . As a result of active tectonic movement, poor rock mass quality is evident in many of the mountain ranges of Southern Leyte making the region prone to structurally controlled landslides. Other lithologies in the area are ophiolite basement, Paleogene sedimentary rocks and late Pliocene to Pleistocene volcanic rocks.

The Guinsaugon landslide was preceded by continuous downpour of rain from 8 to 14 February amounting to 683.6 mm . Measurements from the Otikon weather station, 7 km away from the landslide, show rainfall was highest during the period 10–12 February ranging from 131 to 171 mm although these are believed to be less than the actual values due to the orographic effect. Around the same time as the landslide, two earthquakes were recorded by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the United States Geological Survey (USGS) . Each were reported 21 km west and about 2 km north of Guinsaugon, respectively.

The topmost contributing slope of the Guinsaugon landslide is 50 to 60∘ with a post-landslide configuration of the crown exhibiting a distinct four-slip-plane failure. Based on the observed orientation of discontinuities and immense pore pressure developed from accumulated rainfall, a combined wedge, toppling, and planar slide is believed to have caused gravitational failure at Guinsaugon . At the foot slope of Mt. Can-abag are prehistoric landslide deposits, which were scoured during the 2006 landslide event contributing to the development of the massive avalanche which transformed into debris flows at its frontal edge.

A large part of Southern Leyte, Philippines, is susceptible to structurally controlled landslides. Old landslide scars and large-scale landslide deposits are apparent in the area with the most recent landslide being formed by the 2006 Guinsaugon debris avalanche event. Identifying manually specific areas where landslides may occur in the region is a challenge in terms of resources and scale of effort required. However, advanced methods in computer analysis of landslides may aid remotely sensed and field mapping in the province of Southern Leyte (, as preliminarily reported in). This study aimed to use numerical models for structurally controlled landslides, complemented by field mapping to identify danger zones. The method only considered translational slides, with either wedge, planar, or toppling type of failure due to presence of discontinuities. It does not encompass circular and complex types of landslides. Using high-resolution topography as base map for the simulations, hazard maps at barangay (village) scale were generated for use in disaster prevention and mitigation efforts in the Philippines.

For decades, advances in remote sensing have helped the field of natural hazards in terms of scale and workability. Reliability relies greatly on two factors: (1) how accurately the image captivates reality and (2) the approach to generate desired output from the data . In geology, these methods are extensively used considering both factors. Lineament patterns are one of the most observable features using a digital elevation model. They correspond to many fault and crustal fractures thus providing ideas on regional tectonics. In digital modeling, they are sets of relatively adjacent pixels having similar terrain values . Improvement in such a method comes in two different ways . First is the continuous enhancement of images for extraction through higher resolution imagery and better color or shading representation. Second is in automatic lineament detection using different algorithms .

In this study we used high-resolution imagery derived from interferometric synthetic aperture radar (InSAR) in identifying structural features for numerical models. The InSAR used is from a 2012 nationwide digital terrain model (DTM) and digital surface model (DSM) data set with 0.5 m vertical accuracy and 5 m pixel resolution. The raw data were acquired using Intermap's STAR-3 InSAR system with an X-band InSAR in a Learjet 36A aircraft flying 4–12 km above mean sea level. DSMs were processed into bare earth terrain models by Intermap where vegetation and infrastructure are removed using filtering algorithms. Shaded relief images generated from these DEMs provided high resolution and shaded relief images that reveal well-defined lineaments and distinct changes in the face of mountain slopes.

After identifying important lineament patterns in the region, dip and dip direction of slopes corresponding to each trend were measured using Coltop3D. It is a pseudo-3-D topographic analysis software that represents slope aspect and slope angle. It gives unique color for a given combination of dip and dip direction using classical hue saturation value with a Schmidt–Lambert projected stereonet and depicts continuous and similar planar orientations with the same color . Slope aspect maps are effective tools for locating target sites that warrant structural investigation and have particularly significant implications for the stability of slopes . With Coltop3D, there is added capacity to delimit targets to those significant to the study. Recurring linear patterns are isolated with a particular tolerance and viewed separately. This step allows large amount of computer-generated point cloud data of discontinuity sets necessary for structural analysis that uses stereoplots and rose diagrams. It should be noted that this method assumed structurally homogeneous areas in regional scale as scope (precision) .

Rock strength and friction are factors that could resist the gravitational force pulling down the surface materials. Rock strength is defined by the physical and chemical property of a rock related to any kind of break or gap in a rock while friction is the force resisting the relative movement of materials . Friction increases with roughness or angularity of a rock, as well as the surface roughness. Slope angle also contributes to the determination of mass wasting occurrence. The closer a slope to being parallel to the downward direction of the material, the easier it is for gravity to overcome friction and rock strength resistance. Other factors that contribute to mass wasting include water, earthquakes as well as human activities. Fractures, joints, faults, and other structural features weaken rock strength. Having these gaps favors water seepage, which contributes to bond weakening through the weathering process.

In the identification of structurally controlled landslides, these discontinuities are treated as the primary cause of slope failure. Once determined, the discontinuity sets are subjected to kinematic analysis using Matterocking software, which in turn is used to identify unstable slopes. Matterocking is an open-source software which was used in this study to compute the mechanisms for planar, wedge, and toppling failure . The following definitions for the different failure types were observed in the analysis of unstable slopes for the region of Southern Leyte.

The Philippine Fault is classified as a left lateral strike slip fault. Segment of this 1200 km long geologic feature cuts Southern Leyte on its mountainous regions. Along this lining are conjugate shears and corresponding splays scattered in the whole province. General trends of these lines are identified as shown in Fig. . The study took into consideration four sets of regional discontinuities. We used four positions of light source: 315, 225, 135, and 45∘ all at an altitude of 45∘.

Guided by the lineaments produced in the earlier step, slopes to represent each trend are selected. Dip and dip direction of each slopes measured using COLOTP3d are shown in Table . These sets of discontinuities are considered for the stability analysis of the slopes in the study area. Measurements are of ±10∘ on both criteria as limited by the software. First is the obvious plane intersecting the ridge of the mountain trending southwest with dip and dip direction 41/061∘. This actually is of the same direction of one of the slip planes of the Guinsaugon landslide. Next is the dip and dip direction of the slopes following the trend of the Philippine Fault. It measures 32/250∘. Third is the lineament trending northwest cutting the point of landslide with 37/204∘. Fourth is the measured discontinuity of the southwest trending lineament with 36/307∘ and 32/111∘. This is of the same direction of another slip plane of the landslide. Last is the discontinuity set trending east with 31/185∘. We can verify these measured dip and dip direction to the actual field measurements considered in a post-disaster report by as shown in Table .

Selections of discontinuities along the Philippine Fault are also presented in rose diagrams. Shown in Fig.  are different segments of the lineament. The diagram is consistent with the usual rose diagram appearance of a left lateral strike slip fault with a main long axis parallel to the master fault (i.e., Philippine Fault) and two other minor orientation directions that represent associated faults such as Riedel shears.

Failure zones generated by Matterocking are shown in Fig. . The province of Southern Leyte is highly susceptible to structurally controlled landslide as clearly presented. This includes the analysis of a planar and wedge failure that may occur due to discontinuities considered. Generally steep slopes in the region and presence of discontinuities due to the segment of PF caused this high susceptibility. Looking closer at the Guinsaugon landslide area, we can expect more failure as for the presence of the scarp before the mass movement happened. This is the case of wedge failure that could trigger most rockslides. This is due to the persistence of intersecting discontinuity sets in the area. About 10 % of the slopes in the province have failure potential according to the model.

Kinematic analysis was performed for several slopes of the Mt. Can-abag east-facing facade as shown in Fig. . Slopes covered in red are those that were identified as a rockslide source. These were chosen just to verify and further analyze the mode of failure that could occur as shown in Table . Present discontinuity sets in the area were used in the analysis. Results have shown the probability of wedge and planar failure in almost all cases. Intersection of discontinuity sets 41/61 and 41/185 most probably would cause wedge failure on this side of the mountain. Toppling failure could not be taken into account in this case due to lack of very steep recorded discontinuity. We considered the limitation of DEM's resolution that dip angles would always be less than the actual values on the field. Planar slide in S1 follows the configuration slide in one slip plane of the Guinsaugon landslide.

It should be noted that this method assumed structurally homogeneous areas in regional scale as scope (precision) . To put into scope the parameter used in determining the friction angle for the region, the slope family concept presented by different studies by , , , , and is a suitable tool due to prominent slope families produced by active movement in a large part of the province . The result of Histofit , a program for computing Gaussian fittings for a slope angle distribution, shows a threshold value for a family of steep slopes of 38∘ (Fig. ), which corresponds to the internal angle of friction. Direction of discontinuities produced by this movement is also as prominent. Thus, an incorporated effect of critical slope and discontinuity sets is enough for identifying the potential source of failure.

Landslide hazard mapping does not end with identifying failure zones. It also requires the projection of possible runout extent to which the landslide mass will propagate. These areas are as exposed to landslide hazards as the failure zones and are important to locate to get communities out of harm's way. Here we show the method used to determine the likely areas exposed to the hazard of structurally controlled landslides.

A number of models use an angle from source to extent of a landslide to determine possible runout within the area of interest . Studies differ on what H/L ratio to use in determining this limit angle as shown in Fig. , either the highest portion of the foot slope or the top failure plane. suggested that the minimum shadow angle is preferable (H is the lowest portion of the failure block). To come up with an acceptable value of this angle, a certain amount of landslide events' runout should be considered. A study by Onfri and Candian (1979) used 98 rockfalls to suggest a minimum Fahrböshung in Italy from structural instability to be 28.34∘. used 16 profiles of rockfall paths suggesting a minimum shadow angle of 27.5∘ in British Columbia. Several studies also reported lower values (i.e., 22–24∘) for shadow angles in smooth surfaces, such as those in snow- and grass-covered areas . The use of this approach is widely accepted in terms of capturing real-event scenarios.

This study uses a 20∘ shadow angle. This is a conservative approach based on the literature presented but is a reasonable choice considering that debris avalanches have a long runout. Conefall uses this shadow angle to compute the runout zone in a GIS environment given a digital elevation model (DTM) according to the following relationship (Eq. ): 0<△x2+△y2-tanπ2-ϕp2×(z0-z)2, where z must be less than the surface height. The estimated runout in 3-D is the cone envelope (Fig. ) covered by the aperture angle from the source point .

Since the sources of rockfall/slide were already identified, zones where it will propagate were also mapped using Conefall. The study utilized a runout angle of 20∘ as discussed above. Ratio of “safe” zones from structurally controlled landslide to high susceptibility area is 2 : 1. Shown in Fig.  is the combined map for the source and propagation extent of Southern Leyte.