The state of Kelantan is located in the northeastern corner of Peninsular Malaysia (Fig. 1). Yearly, several landslides occur during heavy monsoon rainfall in Kelantan River basin. In recent years, in particular, there have been many severe flooding events (in the year 2005, 2006, 2007, 2008, 2009, 2014 and 2015) which have led to significant damage to livestock, agricultural production, homes and businesses in the Kelantan River basin (Pradhan, 2009; Pradhan et al., 2009; Pradhan and Youssef, 2011; Tehrany et al., 2013; Nazaruddin et al., 2014). The problem stems from the inappropriate use of lands that are vulnerable to erosion, quick water runoff and slope failure. Two distinct wet seasons from September to December and February to May are reported by the Malaysian Meteorological Department (MMD) in Peninsular Malaysia (Mardiana et al., 2000). A flood episode in December 2014 caused several landslides, one of which occurred for each kilometer of the area of Kelantan state. Mapping landslides in tropical environments is difficult because landscapes are generally covered by thick tropical rainforest (Plank et al., 2016). In addition, the cloudy weather and rain affected the quality of the optical satellite remote sensing data (Melgani, 2006; Ju and Roy 2008; Ramli et al., 2009; Hashim et al., 2013; Razak et al., 2013; Eckardt et al., 2013; Shahabi and Hashim, 2015). The main target of this investigation is to identify high potential risk and susceptible zones in the Kelantan River basin for mapping landslide occurrence zone using satellite remote sensing technology and geographic information system (GIS) techniques.

Advances in remote sensing technology allow the application of synthetic aperture radar (SAR) data in the geological structural analysis for tropical environments (Shimada and Isoguchi, 2002; Morelli and Piana, 2006; Ramli et al., 2009; Pour and Hashim, 2014, 2015a, b; Pour et al., 2013, 2014). Structural field mapping is often difficult in heavily vegetated terrain. This is the case with the study area where dense vegetation cover, deep weathering and scarcity of bedrock exposure hamper geological structure mapping over a long distance. In addition, the use of optical remote sensing data is limited due to the persistent cloud coverage of the study area for the most part of the year (Ramli et al., 2009; Hashim et al., 2013). SAR data contain the potential to penetrate cloud and vegetation and unlike the optical data are dependent upon the surface roughness of the materials, ideally suited to mapping lineaments (faults and fractures) in tropical environments. Lineaments are related to large structural fractures, which represent zones of weakness in the brittle part of the lithosphere. The presence of linear tectonic structures is one of the most important factors in geological hazard occurrences (Bannert, 2000a, b). Lineaments are represented by faults (linear features), lithological contacts between rock units and drainage patterns in any area (van der Pluijm and Marshak, 1997). Observation from satellite images (Landsat Thematic Mapper) in the Himalayan Mountains of Nepal and China revealed a clear connection between active faults, associated with earthquakes and the occurrence of large landslides (Bannert, 2000a, b). Therefore, delineation of faults and fractures using advanced remote sensing technology in any region is a necessity to assess the potential for many natural geological hazards. Numerous investigations have used faults, drainage patterns and lithology factors as important factors to measure and map the susceptibility of geological hazard (Guzzetti et al., 1999; Dai and Lee, 2002; Suzen and Doyuran, 2004; Abdullah et al., 2013).

Landsat-8 was launched on 4 February 2013, carrying the Operational Land Imager (OLI) and the Thermal Infrared Sensor (TIRS) sensors. These two instruments collect image data for nine visible, near-infrared, shortwave infrared bands and two longwave thermal bands (Irons et al., 2012). They have a high signal-to-noise radiometer performance, enabling 12 bit quantization of data allowing for more bits for better land cover characterization by sharper color contrast. Landsat-8 provides moderate-resolution imagery, from 15 to 100 m of Earth's surface and polar regions (Roy et al., 2014).

The Advanced Land Observing Satellite-2 (ALOS-2) was launched on 24 May 2014, as the successor of ALOS-1 (launched on 24 January 2006 and decommissioned in May 2011) (http://global.jaxa.jp/press/2014/05/20140524_daichi2.html; Blau, 2014). The ALOS-2 is exclusively installed with the Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) using microwaves to maximized its ability compare to the ALOS-1, on which three sensors (one microwave and two optical devices) were on board (Igarashi, 2001; Suzuki et al., 2012).

In particular, L-band microwaves from PALSAR-2 have the ability to penetrate vegetation due to relatively long wavelengths (about 24 cm), making the data particularly useful for geological structural mapping in tropical environments (Igarashi, 2001; Rosenqvist et al., 2004; ERSDAC, 2006; Arikawa et al., 2010; Yamamoto et al., 2013; Pour and Hashim, 2014, 2015a, b; Pour et al., 2013, 2014; Shimada et al., 2015). The wavelength of the L band is relatively long among microwaves (C-band is about 6 cm and X-band is about 3 cm), allowing it to travel all the way down to the ground through vegetation (Woodhouse, 2006). Not only can information be obtained about vegetation but information of the ground surface can be obtained as well. Additionally, the L band is less affected by the growth of vegetation, which is useful for SAR interference analysis (interferometry). Therefore, the L band is capable of acquiring changes on the land more precisely compared to shorter wavelength SAR when some diastrophism takes place due to an earthquake or volcanic activity and floods or landslides caused by a natural disaster (Suzuki, 2014). Accordingly, a further increase in the amount of information for geological structural mapping could be derived from the recently launched PALSAR-2 data. Analysis of the data can provide completely new insights into heavily vegetated areas threatened by natural hazards of geological origin. Consequently, the advanced SAR remote sensing data are broadly applicable for geoenvironmental research to identify the causes of natural disasters and point the way to rehabilitation measures, especially in tropical environments.

In this investigation, multitemporal Landsat-8, the PALSAR-2 data and GIS techniques were integrated to detect and map landslide occurrence zones in the Kelantan state following massive rainfall in December 2014. The specific objectives that drive this investigation are (i) to identify high potential risk and susceptible zones for geological origin hazards using the recently launched ALOS-2 PALSAR-2 remote sensing data in the Kelantan River basin at regional and district scales; (ii) to produce accurate geological structure and topographical maps for the Kelantan River basin using Landsat-8 and PALSAR-2 data; and (iii) to compare the detected high potential risk and susceptible zones with highly damaged areas from recent flooding events in the Kelantan River basin.

The remainder of this paper is organized as follows. Section 2 briefly reviews geology of the Kelantan state. Section 3 explains the characteristics of satellite remote sensing data and their applications in landslide mapping and methodology used in the study. Section 4 describes results and discussion. Finally, Sect. 5 concludes with some accomplishments.

Peninsular Malaysia forms an integral part of the Southeast Asian continental core of Sundaland and comprises two tectonic blocks/terranes, the Sibumasu Terrane in the west and the Sukhothai Arc in the east, which were assembled by the Late Triassic (Metcalfe, 2013a, b). The state of Kelantan is located in the northeastern corner of Peninsular Malaysia (Fig. 1). Kelantan River is the major river in the region. It appears at the convergence of the Galas River and Lebir River near Kuala Kari and meanders over the coastal plain until it finally degrades into the South China Sea. Kelantan River basin covers 923 km2, which is about 85 % of the Kelantan state's surface area. It is composed of the flat slope to moderately sloping areas in the northern part and steep scraps and high slopes in the southern part of the river basin (Pradhan et al., 2009). A wide variety of rocks consisting of igneous, sedimentary and metamorphic rocks are distributed in a north–south trend in the Kelantan state. Typically, four types of rocks are classified in the region, including granitic rocks, sedimentary/metasedimentary rocks, extrusive rocks (volcanic rocks) and unconsolidated sediments (Fig. 2). Localized geological features comprise faulting and jointing in the granitic rocks and folding, faulting and jointing in the sedimentary rocks. Granitic rocks are distributed in the west (the Main Range granite) and east borders (the Boundary Range granite) of the state of Kelantan (Rahman and Mohamed, 2001; Department of Minerals and Geoscience Malaysia, 2003; Heng et al., 2006).

The Main Range Granite is located in the west of the state, which is stretched along western Kelantan up to the boundary of Perak and Pahang states and Thailand (Fig. 2). The dominant structural trend in Kelantan is along the N–S to NW–SE direction that derived from post-orogenic phase (Ghani, 2009). The Main Range Granite is located along the western margin of the Bentong–Raub Suture Zone (BRSZ) and extends north to Thailand (Schwartz et al., 1995; Metcalfe, 2000). The north–south-trending Bentong–Raub Suture (approximately 13 km wide) extends from Thailand through Raub and Bentong to the east of Malacca, Peninsular Malaysia. The BRSZ is characterized by a series of parallel topographic north–south-trending lineaments and the presence of small bodies of mafic to ultramafic rocks that are commonly serpentinized (Tan, 1996). The Lebir Fault Zone is located in the eastern part of Kelantan state (Fig. 2), which is one of the major lineaments in Peninsular Malaysia and considered to be post-Cretaceous and a sinistral strike-slip fault (Tjia, 1989; Harun, 2002).

The landscape in the state of Kelantan has been divided into four types: mountainous areas, hilly areas, plain areas and coastal areas (Unjah et al., 2001; Raj, 2009). Mount Chama (Gunung) is the highest point (2171 m) in the Kelantan state, located in Gua Musang district in the western part of the state, near the border of Perak state (Nazaruddin et al., 2014). Quaternary deposits consist of alluvium deposits or unconsolidated sediments. Triassic marine siliciclastics, volcaniclastics, sandstone and limestone are the other sedimentary rocks found in the Kelantan state (Nazaruddin et al., 2014).

A wide-swath ScanSAR observation mode of PALSAR-2 was used for comprehensive analysis of major geological structures, which shows mega-geomorphology and mega-lineaments in the Kelantan state. Figure 4 shows an RGB color composite of HH polarization channel in red, HV polarization channel in green and HH + HV polarization channel in blue for the ScanSAR resultant image. The RGB color composite yields an image with great structural details and geomorphological information. The different colors in the image indicate different backscattering signals from the ground. The Main Range granite located in the western part of the image and the Boundary Range granite on the eastern borders of Kelantan state appear as light green to green in color, which shows the regions with high altitude in the scene (Fig. 4). Major transcrustal lineaments such the BRSZ and Lebir Fault Zone are also detected. Quaternary deposits, Triassic marine siliciclastics, volcaniclastics, sandstone and limestone are manifested as pink to purple tones consisting of lands with low elevation (Fig. 4). Lakes and main river systems are portrayed as blue to dark blue in the image. In fact, the lines range from blue to dark blue on the image are faults and fracture zones occupied by streams. The river pattern is one of the most important factors contributing to the lineament because it reflects the nature of the existing fracture system (Suzen and Toprak, 1998).

Figure 5 shows a ScanSAR HV polarization image that is superimposed on a general topography map of the Kelantan state (Department of Minerals and Geoscience Malaysia, 2003). It is evident that the morphology of the study area is largely controlled by rock type and structure. High-elevation areas (500–100 and < 1000 m) in the Kelantan state are mountainous areas associated with Main Range granite (in the west) and Boundary Range granite (in the east). Hilly, plain and coastal areas with an altitude between 500 and 50 m are associated with sedimentary rocks.

Figure 6 shows the resultant image map for N–S, NE–SW and NW–SE (R: 0∘; G: 45∘; B: 135∘) directional filters. A major change in deformation style is obvious from the west to the east in Fig. 6. Structural analysis reveals four distinct parts from the west to the east, including (i) western part of the scene by ductile fabrics, (ii) western of the BRSZ affected mainly by brittle deformation, (iii) ductile–brittle deformation between the BRSZ and Lebir Fault Zone and (iv) brittle–ductile fabrics between Lebir Fault Zone and eastern coastal line. Lineament occurrence in Fig. 6 is mainly linked to the N–S trending of the BRSZ and Lebir Fault Zone. Generally, major faults are strike-slip with both dextral and sinistral movements, which trend N–S and NW–SE. NW–SE-trending strike-slip faults moved sinistrally in the Lebir Fault Zone. The sinistral movement along the Lebir Fault Zone is responsible for the formation of folding and reverse faulting adjacent to the fault and surrounding area. These structures characterized transpressive tectonic regime in Peninsular Malaysia (Richter et al., 1999; Harun, 2002).

The collision zone and compressional structures appear clearly in the west of the BRSZ in Main Range granite (Fig. 6). Deformation in this region shows the shortening zone oriented parallel to the BRSZ. Several faults, joints and fractures represent brittle deformation events in the region that mostly strike NW–SE. Generally, most of the short lineaments are clustered in the collision zone. Ductile deformation in the western margin of the image (Fig. 6) includes upright asymmetrical mega-folds with axial surfaces oriented W–E. Hence, the contractional strain direction affected by this domain is from NE–SW, followed by dextral shearing. The deformed area zone between the BRSZ and Lebir Fault Zone represents fault systems and folding area. NW–SE striking strike-slip faults and NE–SW normal faults are dominated brittle structural elements within this domain. Ductile deformation is demonstrated by several open upright folds, which have W–E to NE–SW axial plans (Fig. 6). Brittle–ductile fabrics in the eastern part of the image between Lebir Fault Zone and eastern coastal line illustrate curved shear zone that occupied several N–S and NW–SE striking faults, fractures and joints. A mega-concentric fold surrounds the shear zone with a WE striking axial surface. According to the orientation of the lineaments, sinistral movement along the Lebir Fault Zone has generated the tectonic features. Some N–S and NE–SW-trending normal faults and small curvatures are also identifiable near the eastern coastal line (Fig. 6).

Figure 7 shows a fine-mode merged image of the northern and southern parts of the Kelantan state. In this figure, HH polarization channel was assigned to be red, HV polarization channel to be green and HH + HV polarization channels to be blue for generating an RGB image for the study area. This color combination produced an image map contains important information related to water bodies, wetlands and geological structural features. River systems and lakes appear black especially in the northeastern part of the image and wetlands as mauve (Fig. 7). Smooth surfaces such as calm water bodies appear dark in SAR images due to the reflection of the radar signal. Hence, no returning radar signal could be detected in receiving antenna (Thurmond et al., 2006; Pour and Hashim, 2015a). Besides, the best soil moisture/wetness information is achievable from L-band microwaves because it comes out from deeper soils. In fact, vegetation is almost transparent and roughness effects are negligible using L-band microwaves for soil wetness mapping/monitoring (Jackson and Schmugge, 1989; Lacava et al., 2005). Soil moisture is one the most important factors in hydrogeological hazards, especially since the soil response is affected by its status of saturation. Accordingly, the identification of wetlands is very important for flood forecasting and prevention in the state of Kelantan. Wetlands are more distributed in the northern part of the study area (Fig. 7). A vast wetland (dark mauve) is represented clearly in the central north segment of the image. Several wetlands, such as light to dark mauve, are observable in central-east, south and southwestern part of the image (Fig. 7).

The RGB image map (Fig. 7) was compared with the geological map of the Kelantan state. It is evident that most of the detected wetlands are associated with sedimentary and metamorphic rocks. A few of the wetlands are located in the granite bedrock areas in the west (the Main Range Granite) and east (the Boundary Range granite) of the state of Kelantan. The granite bedrock areas are all characterized by dissected hilly to mountainous terrain that gives rise to isolated highlands and mountain ranges (Fig. 7). These granitic areas represent the outcrops of a number of granite batholiths that are generally elongated along a N–S to NNW–SSE direction (Bignell and Snelling, 1977).

LSMs for before and after flood episodes were produced using related factors contributing to landslides event in the Kelantan state (Fig. 8). Overall, the number of landslides was 452. About 36.7 % occurred in the area of rubber plantation area, while 34.3 % occurred in forest areas. Location of landslides occurrences at different land use/cover type in Kelantan is given in Fig. 9. Forest is the dominant land use/cover in Kelantan besides rubber and palm oil.

The amplitude of the radar signal is highly sensitive to the physical properties of the ground surface, which produces the brightness of the surface to vary much more than optical or infrared images (Robinson et al., 1999). Hence, a combination of different polarization channels of fine-mode PALSAR-2 data contains considerable information for rock discrimination. In this study, an RGB color composite was produced by assigning HV + HH polarization channels to red, HV/HH polarization channels to green and HV polarization channel to blue for detailed rock discrimination in the Kelantan state. Figure 10 represents the RGB merged image for the northern and southern parts of the Kelantan state. Granitic rocks are manifested as light green (dark green in high-altitude area probably due to layover effect on SAR image; Gelautz et al., 1998; Franceschetti and Lanari, 1999), while sedimentary and metamorphic rocks appear as a variety of tones such as brown, light brown, dark green, light blue and light red in the image (Fig. 10). In comparison with the geological map of the study area, it seems that brown and dark green hues are metamorphic rocks and light brown, light blue and light red are sedimentary rocks (clay, shale, conglomerate, sandstone and limestone). River systems and lakes are observable clearly as black meandering streams and water bodies especially in the northern segment of the region (Fig. 10).

Four directionally filtered images of fine-mode observation, which contain enhanced information for a set of lineaments in N–S, E–W, NE–SW and NW–SE directions, were used for lineament mapping in the Kelantan state. Figure 11a shows a structural map for the Kelantan state, which is derived from the resultant image of directional filtering to HV polarization channel. It should be noted that the locations of wetlands are also portrayed in this figure. The most important structural features in the image map are fault zones, river systems and drainage lines patterns (Fig. 11a). Within the study area, two large fault zones are presented: BRSZ in the northwest and Lebir Fault Zone in the southeast. The N–S, NE–SW and NNE–SSW lineament trends are commonly dominant in the image map. The dominant lineaments tend to run in the N–S direction, which is mainly linked to the N-S trending of the BRSZ (in the west) and Lebir Fault Zone (in the east). Additionally, few short NW–SE-trending lineaments are detected in the western and eastern parts of the study area (Fig. 11a). The N–S and NE–SW striking system distributed in the southeastern segment of the region is particularly related to Lebir Fault Zone. The pattern of the lineament map in the northwestern part of the image map displays the occurrence of BRSZ, which contains lineaments in N–S and NNE–SSW directions. Most of the short and smaller faults follow the N–S, NE–SW and NNE–SSW trends as the major fault systems in the Kelantan state. It seems that the NW–SE faults are the youngest faults in the study area due to the low frequency of lineaments in this trend. The major N–S-trending faults are interpreted as oldest structures in Peninsular Malaysia and related to the amalgamation of Gondwana-derived terranes during the Permian–Triassic, and NE–SW and NW–SE-trending faults are interpreted to be Late Triassic to Jurassic in age (Metcalfe, 2013b).

The occurrence and concentration of geological structural features in any region are related to rock type, thickness of uppermost weathered mantle and brittleness of rocks (Harris et al., 1960). In the Kelantan state, high concentrations of lineaments are associated with granitic rocks in higher terrains and low concentrations of lineaments are usually associated with lower terrains of metamorphic and sedimentary rocks. Tjia and Harun (1985) analyzed regional structures of Peninsular Malaysia and reported that many lineaments are very well displayed in areas underlain by granite, whereas in other areas they are poorly shown. Moreover, Akhir (2004) identified that high lineament concentrations in the Upper Perak Valley, Perak state, Peninsular Malaysia are closely related to higher terrain which is formed by more-resistant rocks such as granite and sandstone, whereas low lineament concentrations are associated with lower terrain which is mainly formed by less-resistant rock types such as shale and slate. As well, volcanic rocks in the Upper Perak Valley encompass a low lineament concentration (Akhir, 2004).

The rivers in the study area are structurally controlled. They display zigzag patterns due to the presence of fractures, joints and faults with changes in orientation (Fig. 11a). The drainage system in the Kelantan River basin shows dendritic, sub-dendritic and rectangular patterns (Fig. 11a). It is evident that the drainage pattern is apparently being controlled by structure and lithology in the study area. However, several factors such as topography, soil type, bedrock type, climate and vegetation cover influence input, output and transport of sediment and water in a drainage system. The geological structures and lithologic variation have given rise to different drainage patterns (Summerfield, 1991). For instance, a dendritic and sub-dendritic pattern with a large number of tributaries is typical of drainage in areas of impermeable crystalline rock such as gneiss and/or sediment of uniform resistance (horizontal strata). The pattern is characteristic of essentially flat-lying and/or relatively homogeneous rocks and impervious soils with the lack of structural control. A rectangular pattern is usually caused by jointing or faulting of the underlying bedrock. It is usually associated with massive, intrusive igneous and metamorphic rocks (Summerfield, 1991). Therefore, it is assumed that the area with the dendritic and sub-dendritic pattern is subjected to hydrogeological hazards such as flooding because of low infiltration runoff. Rectangular drainage pattern is a susceptible zone that could be easily affected by landslide due to the slope of the land, lithostructural conditions and speed of runoff. In the Kelantan River basin, most of the dendritic and sub-dendritic drainage patterns are detected in central part of the river basin (Fig. 11a), which consists of sedimentary rocks. However, most of the rectangular drainage patterns are associated with igneous and metamorphic rocks in the western part of the Kelantan River basin (Fig. 11a). A structural and topographical feature map of the Kelantan River basin is shown in Fig. 11b. It is evident that most of the dendritic and sub-dendritic drainage patterns are located in lowlands and the rectangular drainage pattern dominates in highlands in the Kelantan River basin.

Precipitation influences the occurrence of landslides more than the distance to fault, which can be seen as having the lowest impact on landslide susceptibility. According to the landslide susceptibility map, 65 % of the whole area of Kelantan is found to be under the category of a low landslide susceptibility zone, while 35 % class located within high susceptibility zone. Table 4 shows information on statistical testing of landslides due to rainfall amount for the Kelantan state during the December 2014 flood episode. Two assumptions were used for statistical testing of the landslide in this analysis: (a) the value of rainfall is one of the major factors contributing to the landslide tragedy in Kelantan, which is assumed to be H0, and (b) the value of rainfall is not the major factor contributing to the landslide tragedy in Kelantan, which is determined to be HA. The ANOVA test shows that F calculated < F significance; thus the null hypothesis is accepted. The test statistic is the Fstatistic value of 0.0938. Using α of 0.05, F0.05;1,97 = 3.939. Since the Fstatistic value is much smaller than the critical value (p value = 0.7601), thus the null hypothesis, H0 is accepted at equal rainfall means and conclude that rainfall is one of the major factors contributing to the landslide tragedy in the Kelantan.

Before the flood, the mean amount of rainfall is much lower than mean of rainfall during flood episode, where few landslide points during 2014 fall under the mean of normal rainfall. During the flood episode, as the mean of rainfall is very high, many landslides points fall higher than the mean value. It is evident that a landslide is triggered when the amount of average monthly rainfall reaches ±400 and ±800 mm for daily rainfall.

Further analysis has been carried out after the detection of landslides in Kelantan state and landslide susceptibility mapping through the GIS-based statistical model. Since the landslides occur mainly in the forest, information about what density of trees will cause slope failure is crucial any mitigating action plan to be undertaken in the future by the local authorities. Thus, ground surface displacement for all identified landslides occurrences and relation to forest density should be carried out. To know the impact of a landslide event that shifts the variety of different density of forest in an affected area, the area of the landslide before and after a flood episode must be analyzed by associating its size with forest density. Stock plots indicate that before a rainy season (Fig. 12a) many landslides in Kelantan tend to occur in moderate forest density (close to 0.50 NDVI value), while landslides are more likely to occur at a location with a slightly lower NDVI value (almost 0.40) during the flood episode (Fig. 12b). In contrast, large landslides are not highly dependent on how dense the forest is but are more likely due to other factors such as slope failure, geological structures (faults and fractures) and heavy rainfall.

Field observations were conducted to compare the detected high potential risk and susceptible zones with highly damaged areas in recent flooding events in the Kelantan River basin. The analysis of field investigation data indicates that many of flooded areas were associated with high potential risk zones for hydrogeological hazards such as wetlands, urban areas, floodplain scroll, meander bend and dendritic and sub-dendritic drainage patterns. Most of the hydrogeological hazards zones are located in flat topographic regions. Flat topography with impervious soils/surface has low infiltration runoff, which yields more storm runoff during heavy monsoon rainfall compared to other regions. Figure 13a and b show meander bends and floodplain scroll zones in the Kelantan River basin, which were flooded areas during the 2015 flooding event.

Numerous landslide-affected points were recorded in the high-altitude segment of the south and southwestern part of the Kelantan state. As mentioned above, high drainage density of the rectangular system is governed in this domain. The drainage density affects runoff, in that a high drainage density drains runoff water rapidly, decreases the lag time and increases the peak of the hydrograph. Consequently, the slope of the land in the south and southwestern regions increases the speed and extent of water and sediment transportation to the Kelantan River basin during heavy monsoon rainfall. Most of the landslide-affected points were located in the topographic slope of metamorphic and quaternary rock units. However, some landslides occurred in fracture zones of weathered igneous rock units. Some large landslide-affected zones were recorded in the intersection of longitude and latitude between 5∘08-02′′ N, 101∘58′53′′ E, 5∘08′14′′ N, 101∘59′06′′ E, 5∘08′24′′ N, 101∘59′21′′ E and 5∘09′03′′ N, 101∘59′41′′ E. These landslide points were associated with N–S, NNE–SSW and NE–SW-trending fault zones. Figure 13c and d show large landslide-affected zones in the southwestern part of the Kelantan state.

Results of this investigation indicate that Landsat-8 and ALOS-2 have proven to provide successful advanced remote sensing satellite data for disaster monitoring in tropical environments. Analysis of the Landsat-8 and ALOS-2 data provided significant information for identifying high potential risk and susceptible zones for natural hazards of geological origin in the Kelantan River basin, Malaysia. Wetlands, floodplain scroll, meander bend, dendritic and sub-dendritic drainage patterns and urban areas were identified as high potential risk zones for hydrogeological hazards. Landslide recurrence regions were detected in a high-altitude segment of the south and southwestern part of the Kelantan state, which is dominated with a high density of rectangular drainage pattern and topographic slope of metamorphic and quaternary rock units. Some of the large landslide zones were associated with N–S, NNE–SSW and NE–SW-trending fault systems. Structural and topographical geology maps were produced for the Kelantan River basin that could be used to facilitate the planning of geohazards mitigation. In conclusion, the results of this investigation have great potential assistance in terms of a total solution to flood disaster management in the Kelantan River basin by providing an important source of information to assess the potential for many natural hazards of geological origin.