Total Landslide Area Research Articles

The influence of different simplified sliding-block models and input parameters on regional predictions of seismic landslides triggered by the Northridge earthquake

Regional seismic landslide hazard maps are based on predictions of rigid-sliding-block displacement derived from estimates of earthquake ground shaking, topography, geology, and shear strength. The confidence in these predictions requires comparisons with field observations of landslide occurrence during previous well-documented earthquakes. This paper presents a comparison between observed landslides from the 1994 Northridge, California earthquake and predicted landslides based on sliding-block displacement estimates. Seven empirical displacement models, each of which uses a different combination of ground-motion parameters, are investigated to evaluate which models and associated ground-motion parameters best predict seismic landslides. Using best estimates of ground shaking and shear-strength properties from the Northridge earthquake, sliding displacements are calculated and compared with the locations of observed landslides. Only 20–40% of the observed landslides are captured and the total area of predicted landslides is much larger than observed. The ability to predict landslide occurrence accurately depends less on the displacement model and associated ground-motion parameters, and more on the uncertainty in the model parameters, particularly the assigned shear-strengths. Because current approaches do not take into account the spatial variability of shear strength within individual geologic units, the accuracy of the predictive models is controlled predominantly by the distribution of slope angles within a geologic unit. Assigning overly conservative (low) shear-strength values results in a higher percentage of landslides accurately identified but also results in a large over-estimation of the total landslide area. Making more accurate maps of seismic landslide hazards will require methods to define intra-formational variations in shear strength.

Characteristics of landslides in western Colorado, USA

Mass movement can be activated by earthquakes, rapid snowmelt, or intense rainstorms in conjunction with gravity. Whereas mass movement plays a major role in the evolution of a hillslope by modifying slope morphology and transporting material from the slope to the valley, it is also a potential natural hazard. Determining the relationships of frequency and magnitude of landslides are fundamental to understanding the role of landslides in the study of landscape evolution, hazard assessment, and determination of the rate of hillslope denudation. We mapped 735 shallow and active landslides in the Paonia to McClure Pass area of western Colorado from aerial photographs and field surveys. The study area covers ∼815 km2. The frequency–magnitude relationships of the landslides illustrate the flux of debris by mass movement in the area. The comparison of the probability density of the landslides with the double Pareto curve, defined by power scaling for negative slope (α), power scaling for positive slope (β), and location of rollover (t), shows that α = 1.1, β = 1.9, and t = 1,600 m2 for areas of landslides and α = 1.15, β = 1.8, and t = 1,900 m3 for volumes of landslides. The total area of landslides is 4.8 × 106 m2 and the total volume of the landslides is 1.4 × 107 m3. The areas (A) and the volumes (V) of landslides are related by V = 0.0254 × A 1.45. The frequency–magnitude analysis shows that landslides with areas ranging in size from 1,600 to 20,000 m2 are the most hazardous landslides in the study area. These landslides are the most frequent and also do a significant amount of geomorphic work. We also developed a conceptual model of hillslope development to upland plateau driven by river incision, shallow landsliding, and deep-seated large landsliding. The gentle slope to flat upland plateau that dominated the Quaternary landscape of the study area was modified to the present steep and rugged topography by the combined action of fluvial incision and glacial processes in response to rock uplift, very-frequent shallow landsliding, and less-frequent deep-seated landsliding.

Documenting five years of landsliding after the 2005 Kashmir earthquake, using repeat photography

The 8 October 2005 Kashmir earthquake triggered thousands of landslides at different scales through the Hazara–Kashmir region of northern Pakistan. A landslide inventory was prepared within a few months after the earthquake, which included detailed photographs and studies of landslides at 164 locations. Photographs were retaken in 2006 at all the 2005 locations and at selected 68 landslide locations in 2007. In 2010, 123 of the 2005 landslide locations in the inventory were reexamined and photographed again. Existing literature predicted that extensive landsliding, particularly under wet conditions, was likely to occur in the region in the years immediately following the earthquake. Surprisingly, the repeat studies revealed that the total landslide area increased only slightly over the five-year period of study, even given a particularly heavy monsoon rainfall season in 2006, with 46% of the locations showing little or no increase and 10% showing a noticeable increase in landsliding; in 44% of the locations vegetation growth was significant or complete within the exposed landslide slip area. Many of the new or reactivated failures occurred along roads and rivers, particularly along steeper slopes. We conclude that the landscape returned to equilibrium within only a few years after the earthquake. Nevertheless, a potential for future slope instability and landsliding in the region still exists. Hence continuation of landslide monitoring and risk assessment is still important for hazard mitigation in this region.

River Courses Affected by Landslides and Implications for Hazard Assessment: A High Resolution Remote Sensing Case Study in NE Iraq–W Iran

The objective of this study is to understand the effect of landslides on the drainage network within the area of interest. We thus test the potential of rivers to record the intensity of landslides that affected their courses. The study area is located within the Zagros orogenic belt along the border between Iraq and Iran. We identified 280 landslides through nine QuickBird scenes using visual photo-interpretation. The total landslide area of 40.05 km2 and their distribution follows a NW–SE trend due to the tectonic control of main thrust faults. We observe a strong control of the landslides on the river course. We quantify the relationship between riverbed displacement and mass wasting occurrences using landslide sizes versus river offset and hypsometric integrals. Many valleys and river channels are curved around the toe of landslides, thus producing an offset of the stream which increases with the landslide area. The river offsets were quantified using two geomorphic indices: the river with respect to the basin midline (Fb); and the offset from the main river direction (Fd). Hypsometry and stream offset seem to be correlated. In addition; the analysis of selected river courses may give some information on the sizes of the past landslide events and therefore contribute to the hazard assessment.

Spatial Relationships between Landslides and Topographical Factors at the Liukuei Experimental Forest, Southwestern Taiwan after Typhoon Morakot

Typhoon Morakot produced copious amounts of rainfall and triggered enormous landslides in southern Taiwan when it hit Taiwan on 8~9 August 2009. The Liukuei Experimental Forest (LEF) managed by the Taiwan Forestry Research Institute was located in the peak rainfall region and inescapably incurred severe landslides and road destruction. This paper presents a statistical approach to study spatial relationships between landslides and their geographic factors in the LEF after typhoon Morakot. The normalized difference vegetation index (NDVI) of satellite images SPOT2 and SPOT4 photographed before and after typhoon Morakot were compared to extract landslide patches. Those extracted patches, consisting of 10 x 10-m digital elevation model (DEM) data and the related coverage of vegetation of LEF, were examined using the WinGrid computer software for the topographic analysis. Results indicated that the extremely heavy rainfall was the main factor that triggered landslides during the typhoon. There were 204 locations which experienced identifiable landslides, and their total area was about 804.49 ha. The elevation and types of vegetation cover were not factors related to the failure of the slopes. Steepness, aspect, and distances to roads and streams were factors that contributed to instability of the slopes. About 70.2% of the total area of landslides occurred on slopes with steepness in the range of 60~80%, and this showed that steep slopes are prone to collapse. Landslides with distance of the lower edge to a stream of ＜ 10 m occurred in 49 locations, and their areas accounted for 33.6% of the total landslide areas. In addition, 34 landslides (24.7% of all landslides areas) occurred within 50 m of a forest road, and the average area of those landslides was 5.79 ha. The deeply concentrated surface flows resulting from the extreme rainfall in streams and on road surfaces striking against the foot of hillslopes that contained saturated soil water were the main factors causing slides of large area. This also indicated that landslides in the LEF were closely related to stream systems, and forest road construction. The aspect of slope was not the main factor related to landslides in most cases. However, slopes with southerly and southeasterly aspects directly faced the strong winds and heavy rainfall brought by typhoon Morakot, and more landslides occurred on those slopes than on slopes with other aspects.

Landslides triggered by the 7 August 2009 Typhoon Morakot in southern Taiwan

Landslides triggered by Typhoon Morakot with an area over 576 m 2 were mapped from FORMOSAT-2 images. Recognition and calculation of the number and area of landslides show that at least 22,705 landslides with a total area of 274 km 2 were recognized in the 7811 km 2 study area. Among these landslides, 17,417, with a total area of 150 km 2, are recognized as new ones, and 5288, with a total area of 124 km 2, are classified as enlarged from pre-existing ones. Out of the 22,705 landslides, there were 22,221 recognized with an area smaller than 10 ha; 298 recognized in the range of 10 to 20 ha. Twenty-two landslides with an area of over 60 ha have also been identified, and the total area of these landslides is 20 km 2, which is equal to 7.37% of the total landslide area. With respect to exposed rock type, the landslides mainly occurred in Eocene-Miocene slate and Miocene sandstone and shale. In addition, Morakot-induced landslides mainly occurred in areas with cumulative precipitation in the range of 800 to 2600 mm, and the magnitude of landslide concentration is roughly linearly proportional to the amount of cumulative rainfall.

Landslides induced by the Wenchuan earthquake and the subsequent strong rainfall event: A case study in the Beichuan area of China

The Wenchuan earthquake on May 12, 2008 triggered massive landslides and a subsequent, strong rainfall prompted the development of new landslides as well as the reactivation of some pre-existing landslides. The highest seismic intensity zone of the Wenchuan earthquake in Beichuan, China was selected as a case study to analyze the influence of the earthquake and the subsequent, heavy rains on landslide evolution. We selected this study area (414 km 2) since it was close to the coseismic surface rupture and because it suffered strong ground motion. Based on the interpretation of high-resolution aerial photographs and remote sensing imagery combined with field investigation, 40 pre-earthquake landslides and 2221 coseismic landslides were identified with total landslide areas of 2.68 km 2 and 30.81 km 2, respectively. There were 134 large (over 50,000 m 2), co-seismic landslides that covered a surface area of 15.54 km 2 and represented approximately 50.4% of the total area of the co-seismic landslides. The coseismic landslides were mainly located on the hanging wall of the causative fault and on the steep, valley sides of the Jian River and its tributaries. A strong rainfall event occurred four months after the Wenchuan earthquake and induced 969 new landslides (which covered a 6.90 km 2 area) and enlarged 169 existing landslides (2.48 km 2). The landslides were identified in SPOT5 images. Ultimately, we found that earthquake tremors and the subsequent rainstorm severely disturbed the surface strata, resulting in a large number of landslides.

Seasonal landslide mapping and estimation of landslide mobilization rates using aerial and satellite images

We tested the possibility of using digital, color aerial ortho-photographs and monoscopic, panchromatic satellite images of comparable spatial and radiometric resolution, to map recent landslides in Italy and to update existing measures of landslide mobilization. In a 90-km 2 area in Umbria, central Apennines, rainfall resulted in abundant landslides in the period from September 2004 to June 2005. Analysis of the rainfall record determined the approximate dates of landslide occurrence and revealed that the slope failures occurred in response to moderately wet rainfall periods. The slope failures occurred primarily in cultivated terrain and left subtle morphological and land cover signatures, making the recognition and mapping of the individual landslides problematic. Despite the difficulty with the identification of the landslides without the use of stereoscopic visualization, visual analysis of the aerial and satellite images allowed mapping 457 new landslides, ranging in area 3.0 × 10 1 < A L < 2.5 × 10 4 m 2, for a total landslide area A LT = 6.92 × 10 5 m 2. To identify the landslides, the investigators adopted the interpretation criteria commonly used to identify and map landslides on aerial photography. The result confirms that monoscopic, very high resolution images taken by airborne and satellite sensors can be used to prepare landslide maps even where slope failures are difficult to detect, provided the imagery has sufficient geometric and radiometric resolutions. The different dates of the aerial (March 2005) and the satellite (June–July 2005) images allowed the temporal segmentation of the landslide information, and studying the statistics of landslide area and volume for different periods. Compared to pre-existing information on the abundance and size of the landslides in the area, the inventory obtained by studying the aerial and satellite images proved more complete. The new mapping showed 145% more landslides and 85% more landslide area than a pre-existing reconnaissance inventory. As a result of the improved mapping, the rate of landslide mobilization for the 2004–2005 landslide season was determined to be φ L = 27.1 mm year − 1 , 30% higher than a previous estimate for the same period. This seasonal rate of landslide mobilization is significantly larger than the long-term regional erosion rate in the central Apennines. The accelerated rate is attributed to agricultural practices that favor slope instability.

Post-disaster assessment of landslides in southern Taiwan after 2009 Typhoon Morakot using remote sensing and spatial analysis

Abstract. On 8 August 2009, the extreme rainfall of Typhoon Morakot triggered enormous landslides in mountainous regions of southern Taiwan, causing catastrophic infrastructure and property damages and human casualties. A comprehensive evaluation of the landslides is essential for the post-disaster reconstruction and should be helpful for future hazard mitigation. This paper presents a systematic approach to utilize multi-temporal satellite images and other geo-spatial data for the post-disaster assessment of landslides on a regional scale. Rigorous orthorectification and radiometric correction procedures were applied to the satellite images. Landslides were identified with NDVI filtering, change detection analysis and interactive post-analysis editing to produce an accurate landslide map. Spatial analysis was performed to obtain statistical characteristics of the identified landslides and their relationship with topographical factors. A total of 9333 landslides (22 590 ha) was detected from change detection analysis of satellite images. Most of the detected landslides are smaller than 10 ha. Less than 5% of them are larger than 10 ha but together they constitute more than 45% of the total landslide area. Spatial analysis of the detected landslides indicates that most of them have average elevations between 500 m to 2000 m and with average slope gradients between 20° and 40°. In addition, a particularly devastating landslide whose debris flow destroyed a riverside village was examined in depth for detailed investigation. The volume of this slide is estimated to be more than 2.6 million m3 with an average depth of 40 m.

Analysis of Ground Deformation Detected Using the SBAS-DInSAR Technique in Umbria, Central Italy

Ground deformation affecting the Umbria region (central Italy) in the 9-year period from 1992 to 2000 was investigated through multi-temporal Differential Synthetic Aperture Radar Interferometry (DInSAR). For the purpose, the Small BAseline Subset (SBAS) technique was adopted, which allows studying the temporal evolution of the detected deformation at two spatial scales: a low-resolution (regional) scale, and a full-resolution (local) scale. For the analysis, SAR data acquired by the European Remote Sensing (ERS-1/2) satellites along ascending and descending orbits were used. The detected deformation was analysed to investigate its relevance to geophysical, geomorphologic, and human-induced processes that may result in hazardous conditions to the population of Umbria. Low-resolution deformation data were used to: (i) determine the amount of displacement caused by the Umbria-Marche earthquake sequence from September 1997 to April 1998 in the Foligno area, (ii) determine the number and percentage of the known landslides that can be monitored by the DInSAR technology in the investigated area, and (iii) identify and measure subsidence induced by exploitation of a confined aquifer in the Valle Umbra. Results indicate that earthquakes moved through the Foligno area westwards up to 3.9 cm and with an uplift reaching 1.7 cm. Intersection in a GIS of the low-resolution deformation maps with a detailed landslide inventory map allowed the determination that the portion of landslides that can be monitored by the SBAS-DInSAR technique in Umbria ranges from 2.7% to 3.4%, and the percentage of the total landslide area ranges from 10.4% to 12.8%. In the Valle Umbra, a dependency was found between the time and the amount of detected ground deformation, and the record of water withdrawal. The full-resolution deformation data were used to investigate the movement of the Ivancich landslide, in the Assisi Municipality. Joint analysis of the spatial and the temporal characteristics of the ground displacement allowed the formulation of a hypothesis on the landslide geometry and deformation pattern.

Landslide susceptibility assessment in the İzmir (West Anatolia, Turkey) city center and its near vicinity by the logistic regression method

A landslide susceptibility assessment for Izmir city (Western Turkey), which is the third biggest city of Turkey, was performed by a logistic regression method. A database of landslide characteristics was prepared using detailed field surveys. The major landslides in the study area are generally observed in the field, dominated by weathered volcanics, and 39.63% of the total landslide area is in this unit. The parameters of lithology, slope gradient, slope aspect, distance to drainage, distance to roads and distance to fault lines were used as variables in the logistic regression analysis. The effect of each parameter on landslide occurrence was assessed from the corresponding coefficients that appear in the logistic regression function. On the basis of the obtained coefficients, lithology plays the most important role in determining landslide occurrence and distribution. Slope gradient has a more significant effect than the other geomorphological parameters, such as slope aspect and distance to drainage. Using a predicted map of probability, the study area was classified into five categories of landslide susceptibility: very low, low, moderate, high and very high. Whereas 49.65% of the total study area has very low susceptibility, very high susceptibility zones make up 11.69% of the area.

Identification and mapping of recent rainfall-induced landslides using elevation data collected by airborne Lidar

Abstract. A high resolution Digital Elevation Model with a ground resolution of 2 m×2 m (DEM2) was obtained for the Collazzone area, central Umbria, through weighted linear interpolation of elevation points acquired by Airborne Lidar Swath Mapping. Acquisition of the elevation data was performed on 3 May 2004, following a rainfall period that resulted in numerous landslides. A reconnaissance field survey conducted immediately after the rainfall period allowed mapping 70 landslides in the study area, for a total landslide area of 2.7×105 m2. Topographic derivative maps obtained from the DEM2 were used to update the reconnaissance landslide inventory map in 22 selected sub-areas. The revised inventory map shows 27% more landslides and 39% less total landslide area, corresponding to a smaller average landslide size. Discrepancies between the reconnaissance and the revised inventory maps were attributed to mapping errors and imprecision chiefly in the reconnaissance field inventory. Landslides identified exploiting the Lidar elevation data matched the local topography more accurately than the same landslides mapped using the existing topographic maps. Reasons for the difference include an incomplete or inaccurate view of the landslides in the field, an unfaithful representation of topography in the based maps, and the limited time available to map the landslides in the field. The high resolution DEM2 was compared to a coarser resolution (10 m×10 m) DEM10 to establish how well the two DEMs captured the topographic signature of landslides. Results indicate that the improved topographic information provided by DEM2 was significant in identifying recent rainfall-induced landslides, and was less significant in improving the representation of stable slopes.

Remote Sensing Observations of Landslides and Ground Deformation from the 2004 Niigata Ken Chuetsu Earthquake

In recent years, major developments in remote sensing have made it possible to use these technologies to document the effects of earthquakes. Specifically, high-resolution satellite imagery and three-dimensional laser scanning (LIDAR) can provide important observations of earthquake damage that supplement traditional observations from field reconnaissance. The 2004 Niigata Ken Chuetsu earthquake provided an opportunity to use remote sensing to document the distribution of landslides in the epicentral region through the use of high-resolution satellite imagery and to document the detailed three-dimensional geometries of several failures using LIDAR. The satellite imagery was acquired the day after the earthquake, but at very large acquisition angles that resulted in image distortion. Nonetheless, the satellite imagery accurately identified the landslide distribution in the epicentral region, although the total area of landslides was underestimated by about 25% as compared with traditional aerial reconnaissance because of the large acquisition angle for the satellite imagery. Terrestrial LIDAR was used to collect three-dimensional data at several failure sites, including two large rock slides and a railroad tunnel portal affected by ground deformation. The LIDAR data allowed for precise measurement of failure deformations and geometries, and provided digital terrain models that could be archived and used in future analyses. In the future, satellite imagery and LIDAR, as well as other remote sensing technologies, will play an increasing role in documenting and understanding the effects of earthquakes.

Discussion of landslide self‐organized criticality and the initiation of debris flow

AbstractThe study contains descriptions of landslides and debris flows in a small upland catchment in Taiwan between 1986 and 2004. This catchment, the Chushui Creek, is situated near the epicentre of the 1999 M7·6 Chi‐Chi earthquake. This is an area with high background erosion rate. The Chi‐Chi earthquake caused a sharp increase in the rate of mass wasting in the epicentral area, and the data presented in this study illustrates the geomorphic change associated with the earthquake. The measurements of the geometry of the trunk stream of the Chushui catchment also show its change due to the impact of a strong typhoon in 1996. Two channel reaches that were affected by separate debris flows during this event were identified. Each reach has three sections: scour, transfer and deposition. Cross profiles of these sections show a systematic change from V‐shaped bedrock channel in the scour areas to flat‐floored channel in the transfer and deposition areas. Debris flows also occurred on other occasions in this channel, and their frequency has increased since the 1999 earthquake. In addition, this study contains precise geographic and statistical descriptions of the landslides triggered by three typhoons and an earthquake that affected the catchment since 1996. The total landslide area is measured for different time intervals, and a marked increase in landslide incidence is found after the 1999 earthquake. Frequency–area plots of the 126 landslides in the catchment indicate a power‐law scaling with an exponent of about ‐1·5 of these quantities, as has been observed in other studies. We propose that a link exists between landslide frequency–area distributions and initialization of debris flow in the catchment. It may be classified as a self‐organized criticality process with a critical frequency–area distribution for the landslide population. In this case, the landslide frequency–area distribution is useful in quantifying the severity of the trigger and the contribution of landslides to debris flow. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Application of logistic regression for landslide susceptibility zoning of Cekmece Area, Istanbul, Turkey

As a result of industrialization, throughout the world, cities have been growing rapidly for the last century. One typical example of these growing cities is Istanbul, the population of which is over 10 million. Due to rapid urbanization, new areas suitable for settlement and engineering structures are necessary. The Cekmece area located west of the Istanbul metropolitan area is studied, because the landslide activity is extensive in this area. The purpose of this study is to develop a model that can be used to characterize landslide susceptibility in map form using logistic regression analysis of an extensive landslide database. A database of landslide activity was constructed using both aerial-photography and field studies. About 19.2% of the selected study area is covered by deep-seated landslides. The landslides that occur in the area are primarily located in sandstones with interbedded permeable and impermeable layers such as claystone, siltstone and mudstone. About 31.95% of the total landslide area is located at this unit. To apply logistic regression analyses, a data matrix including 37 variables was constructed. The variables used in the forwards stepwise analyses are different measures of slope, aspect, elevation, stream power index (SPI), plan curvature, profile curvature, geology, geomorphology and relative permeability of lithological units. A total of 25 variables were identified as exerting strong influence on landslide occurrence, and included by the logistic regression equation. Wald statistics values indicate that lithology, SPI and slope are more important than the other parameters in the equation. Beta coefficients of the 25 variables included the logistic regression equation provide a model for landslide susceptibility in the Cekmece area. This model is used to generate a landslide susceptibility map that correctly classified 83.8% of the landslide-prone areas.

Landslides triggered by the 23 November 2000 rainfall event in the Imperia Province, Western Liguria, Italy

From mid-October to 22 November 2000, the western Liguria Region of Italy experienced prolonged and intense rainfall, with cumulative values exceeding 1000 mm in 45 days. The severe rainfall sequence ended on November 23 with a high-intensity storm that dumped more than 180 mm of rain in 24 h. The high-intensity event caused flooding and triggered more than 1000 soils slips and debris flows and a few large, complex landslides. Slope failures caused three fatalities and severe damage to roads, private homes, and agriculture. Large (1:13,000) and very large (1:5000) scale colour aerial photographs were taken 45 days after the event over the areas most affected by the landslides. Through the interpretation of the 334 photographs covering an area of ∼500 km 2, we prepared a landslide inventory map that shows 1204 landslides, for a total landslide area of 1.6 km 2. We identified the rainfall conditions that triggered landslides in the Armea valley using cumulative- and continuous-rainfall data, combined with detailed information on the time of landslide occurrence. Landslide activity initiated 8 to 10 h after the beginning of the storm, and the most abundant activity occurred in response to rainfall intensities of 8 to 10 mm per hour. For the Ceriana Municipality, an area where the landslides were numerous in November 2000, we also collected information about a historical event that occurred on 8–11 December 1910 and triggered abundant landslides resulting in severe economic damage. A comparison of the damage caused by the historical and the recent landslide events indicated that damage caused by the 1910 historical event was more diffused but less costly than the damage caused by the 2000 event.

The impact of landslides in the Umbria region, central Italy

Abstract. The Umbria Region of Central Italy has a long history of mass movements. Landslides range from fast moving rock falls and debris flows, most abundant in mountain areas, to slow moving complex failures extending up to several hectares in the hilly part of the Region. Despite landslides occurring every year in Umbria, their impact remains largely unknown. We present an estimate of the impact of slope failures in the Umbria region based on the analysis of a catalogue of historical information on landslide events, a recent and detailed regional landslide inventory map, and three event inventories prepared after major landslide triggering events. Emphasis is given to the impact of landslides on the population, the transportation network, and the built-up areas. Analysis of the available historical information reveals that 1488 landslide events occurred at 1292 sites in Umbria between 1917 and 2001. In the same period 16 people died or were missing and 31 people were injured by slope movements. Roads and railways were damaged by slope failures at 661 sites, and 281 built-up areas suffered landslide damage. Three event inventories showing landslides triggered by high intensity rainfall events in the period 1937–1941, rapid snow melting in January 1997, and earthquakes in September–October 1997, indicate the type, abundance and distribution of damage to the population, the built-up areas and the transportation network caused by typical landslide-triggering events. Analysis of a geomorphological landslide inventory map reveals that in some of the municipalities in the region total landslide area exceeds 25%. Of the more than 45 700 landslide areas shown in the geomorphological inventory map, 4115 intersect a road or railway, and 6119 intersect a built-up area. In these areas slope failures can be expected during future landslide triggering events.

Depositional and erosional landforms in the Lothar Khola watershed, Central Nepal

The high intensity rainfall of 19 and 20 July 1993 triggered off a large number of mass movements in the Lothar Khola watershed of Central Nepal. Most of the slides were reactivated on highly fractured and weathered rocks during the downpour. Among more than 40 landslides encountered in the watershed, the large rockslides were found at Purbangkhani, Karse, and Loling. A detailed study of landslide distribution in the watershed revealed that mos t of the landslides (about 65 % of the total landslide area) occurred on slopes ranging from 26 to 40°. These slopes were nearly equal to or a little steeper than the internal friction angle of constituting soil or rock mass. According to land use pattern, more than two thirds of the landslide area fell in the forestland and there were no landslides on the grassland. Similarly, rockslides were concentrated (about 53% of the total rockslide area) on slopes covered by slates and phyllites. On the other hand, an overwhelming majority of soil slides (more than 85% of the total soil slide area) occurred on residual soils. Debris flows were also very common in the Lothar Khola watershed. During the debris flow, from 1 to 3 m deepening of the riverbed was observed in many erosional zones whereas the sediment accumulation reached up to 4 m in the depositional zones.

Earthquake-induced landslides: 1980–1997

A database of earthquake-induced landslides has been compiled which extends the work of Keefer (Keefer DK. Landslides caused by earthquakes. Bulletin of the Geological Society of America 1984;95:406–421) who covered the period 1811–1980 to 1997. A total of 36 earthquakes world-wide are included, the new database having about the same number of earthquakes as reported by Keefer. Correlations evolving from the new database are compared with those of Keefer. Generally the results are very similar, though the presence of extreme outliers in some of the correlations emphasises the need to be aware of special cases, particularly those involving quick clay landslides. Seismological features, including multiple earthquakes and simultaneous arrival of different phases of seismic waves, also influence the outliers. The correlations between earthquake magnitude and total landslide area, however, differ somewhat from Keefer's. For the intermediate magnitude range 5.3–7.0, a modified correlation is suggested. The scatter of the data from which the correlations are derived is greater than found by Keefer. This is ascribed to the different geographic locations of the earthquakes in the two data sets.

Evaluation of Landsat Data for Landslide Identification as a Means for Watershed Management

Identification of landslides by Landsat MSS digital data was attempted to evaluate the data as a means for watershed management. The maximum likelihood classifier identified about 10 percent of the landslides, which occupied nearly 50 percent of the total landslide area. The threshold size was approximately one-third of the pixel size, or in this case about 2000m2. If the landslide can be successfully separated from the flood plain, the data may be utilized, based on the area estimate, for computing failed volume and debris discharges in a given area for the management.