Keywords

1 Introduction

Landforms with differential geometries, such as cuestas and hogbacks, are susceptible to frequent landslides. An asymmetrical cuesta terrain is featured by a steep escarpment on one side and a gentle slope on the other (Davis 1915; Peterek and Schrôder 2010). Multiple landslides occurred in a cuesta landscape with folded laminar structures during the 2004 Mid-Niigata Prefecture Earthquake, Japan (Chigira and Yagi 2006). Previous research on landslides in cuesta landscapes has primarily focused on rock falls on cuesta escarpments or small-scale slump-style slides (Cruden and Hu 1999; Grozavu. et al. 2010; Schmid and Meitz 2000). On the other hand, only a few studies, such as Kuroda (1964), addressed landslides on the cuesta’s back slopes. This is perhaps because landslides occurring on the escarpments of cuestas have been much more than those on the back slopes. However, translational slides on the cuesta’s back slopes are basically bedrock slides; they are fast, regardless of the cause, and can be as large as 1 km in width and length. Therefore, these landslides can pose significant risks of severe damage to their slopes and areas at their feet.

We focused on the intrinsic features of the cuesta’s back slopes, such as layers coming off and sliding down one after another along the rock’s bedding planes and cracks. In this study, we analyzed the relationship among geology, geological structure, cracks, and bedding planes associated with these landslides; the development of rivers and landslides; the triggers of landslides, such as earthquakes rainfalls, and snowmelt; and the recurrence of landslides. The primary aim of this study was to create valuable guidelines for engineering plans, prediction of landslides, and understanding the landslide mechanisms by focusing on the relationship between the landslide occurrence processes and the evolution of cuesta landscapes. This research shows that translational bedrock slides, which recur on the cuesta’s back slopes as part of the denudation processes, play an essential role in the evolution of cuesta landscapes.

2 Methodology

Cuesta’s back slope landslides of concern were surveyed between 1998 and today. They are landslides in the Hijiori region and Higashi-Naruse area, the Hijirigahana, Yokowatashi, and Nunomata landslides of Japan, the Goldau landslide of Switzerland, landslides along the Tamakoshi River in Nepal, and the Chiu-Fen-Erh-Shan and Tsaoling landslides in Taiwan. This study defines the cuesta’s back slope as the gentler slope of the asymmetric cuesta with an inclination of 5 to 30° (Cruden and Hu 1999, and other references).

During the survey, we primarily recorded the geologies, geological structures, strikes and dips of bedrock, slope inclinations, landslide movement types (Varnes 1978; Cruden and Varnes 1996), and the directions of movement. We measured the cracks in bedrocks in the landslides and their surrounding areas. We targeted cracks in bedrocks that extend more than 1 m and measured their strikes and dips. These values are presented on a rose diagram or a stereo net. The analysis of cracks in the Chiu-Fen-Erh-Shan landslide of Taiwan is based on Wang et al. (2003).

The widths and directions of visible cracks on an outcrop can vary remarkably. Therefore, when the outcrop area was limited, the entire extent of the outcrop was explored, whereas, in large outcrops, we targeted valleys and rivers where base rocks are exposed. Here, we assume that a crack direction at a particular site is a dominant direction on a Rose diagram of the crack azimuths observed there.

We analyzed the movement of the Yokowatashi landslides in the time domain using MIDAS GTS for BESSRA software to discuss the detachment sequence of elastoplastic landslide masses.

3 Results

3.1 Descriptions of Featured Landslides

This chapter shows case histories by their causes, i.e., rainfalls/ snowmelt and earthquakes.

3.1.1 Cuesta Landslides Triggered by Rainfalls and Snowmelt Landslides in the Hijiori Region, Japan

The Hijiori region encompassing Okura village in Yamagata prefecture is approximately 15 km long and 10 km wide, extending from the Quaternary Hijiori caldera (about 4 km in diameter) northward to the Mogami River. It is a zone of frequent landslides (Fig. 1).

Fig. 1
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The topography and landslide distribution in the Hijiori region (data added to a shaded relief map)

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This region can be roughly divided into two sections: an area of Neogene sedimentary rocks that make up most of the region’s northern half and a southern area consisting of pyroclastic-flow deposits formed by the ejecta of the Hijiori caldera. The area of the Neogene sedimentary rocks in the north is hilly, with ridges between 50 and 300 m in relative height. Their ridgelines and rivers align in north–south direction, following azimuths of fold axes. Cuesta topography is well developed here. The steep slopes on the western sides of the ridges incline 30° or more prominent, and the eastern sides have gentle slopes of 5 to 20°. The Toutazawa landslides are in this region (T in Fig. 1).

The activities of the Toutazawa landslides are dominant on the cuesta’s back slopes, where a homoclinal folding structure dipping to the east features the terrain.

The cuesta’s gentle back slope steps down. These steps control the movements of the landslide masses perching above ((a) in Fig. 2). The upper landslide masses on the stepped back slope move along these steps about 50 to 60° off the direction of the maximum dip. Due to erosive/depositional actions, these landslide masses can gradually be leveled ((b) in Fig. 2). Eventually, each transverse cross-section of these landslide masses has an asymmetric inverted triangle shape.

Fig. 2
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Model of the Toutazawa landslides (a) and the cross-section of the Toyomaki landslides (b)

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The maximum snow depth here exceeds 200 cm. Many of these landslides occur during the snowmelt season, perhaps due to increased pore water pressure associated with meltwater infiltration into the landslide masses with the asymmetric inverted triangle cross-sections.

The Toyomaki landslides are in the area containing Quaternary pyroclastic flow deposits (P in Fig. 1). The landslides with Pyroclastic flow deposit covers are more extensive than those with no pyroclastic soil cover (such as the Toutazawa landslides). The landslide masses with the pyroclastic soil covers are as thick as 150 m. The Toyomaki landslides consist of five landslides that occurred close to each other within a 5 km wide and 1 km long area. The entire terrain features and the estimated age of the pyroclastic flow deposits suggest significant displacements occurred first as a primary rockslide. Then, the pyroclastic flow covered the whole landslides (Abe et al. 2002), followed by repetitive sliding events. As revealed by borehole drillings on the sliding surface, the transverse cross-section of the landslide is an asymmetrical inverted triangle resting on the sliding surface dipping to the north. The landslide mass moves about 10° off the maximum dip of the strata (b in Fig. 2). All landslides in the Toyomaki landslides occurred during snowmelt, suggesting that a large amount of groundwater has played a role in the occurrence mechanism of landslides with asymmetrical cross-sections on their sliding surfaces.

3.1.2 Landslides in the Higashi-Naruse Region, Japan

Higashi-naruse, an area of frequent landslide occurrences, is located in the Ou Mountain range in the Tohoku region of Japan. Many landslides here occur as translational rockslides in Miocene siliceous mudstones formations. They are roughly divided into two groups: the Yachi landslides on the left bank of the Naruse River and the Yanagisawa landslides, the toe section of which is on the right bank of the Naruse River (Fig. 3). In the Yachi landslides, a cluster of multiple landslides is seen on the cuesta’s back slopes region, covering a 7 km by 3 km area. The adjacent Okamizawa and Yanagisawa landslides cover a 4 km by 2.5 km area on the cuesta’s back slopes (Fig. 3).

Fig.3
figure 3

Landslides of the Higashinaruse region (after Moriya et al. 2008)

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Large-scale landslide topography covering a 6 km by 3 km area is present in the Yachi landslides. The west side of the crown of this vast landslide topography is a cuesta scarp. The landslides are on the cuesta’s back slopes, dipping between 5 and 23°. Excavation of the head section of the landslide and drilling of test pits have identified the sliding surface of the Yachi landslide as a tuff layer within a siliceous mudstone layer. The Okamizawa and Yanagisawa landslides are translational rockslides that slide on a seam of tuff or black mudstone within a highly developed level containing siliceous mudstones.

These two landslides can be further subdivided into smaller slides. Data obtained from field mapping, borehole drilling, and geological investigations for the construction of catchment wells have revealed that cuesta’s steps separate these landslide masses, and each landslide mass moves individually on its sliding surface.

3.1.3 The Goldau Landslide, Swiss Alps

The Goldau landslide (Fig. 4) in the Swiss Alps occurred in 1806 on a cuesta’s back slope located 1500 m above sea level following heavy rainfall (Thuro and Hatem 2010). A massive volume of rock (10–50 × 106 m3) plunged into Lake Lauerz (Lauerzersee), generating a landslide tsunami that claimed 457 lives and left 206 missing persons (Zehnder 1988). Three such events are known to have happened from geomorphic and historical evidence (Fig. 4). Currently, a discernible displacement can be identified on the east side of the region, causing yet growing fear of another large-scale rockslide. The past slides dipping 20 to 30º are 200 to 700 m wide and 1000 to 2000 m long. The flank of each displaced mass shows a steep escarpment.

Fig. 4
figure 4

The Goldau landslide: (1) Prehistoric landslide, (2) the 1222 Rothen slide, (3) the 1806 rockslide, (4) future event? (Photo by Google Earth. History of landslide activity is after Thuro and Hatem 2010)

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The area’s geology alternates between conglomerate with sandstone and intercalated marl in a Miocene molasse. The sliding surface is presumed to have developed within the marl section, where fragmented marl has become clay.

The dips of the sliding surfaces are 20–30°. In the newest landslide that moved in 1806 ((3) in Fig. 4), the joints are mainly parallel or orthogonal to the direction of the landslide movement. The inclination of the sliding surface becomes steeper from 20 to 30° as we move up stepwise slopes from (1) to (3) in Fig. 4. The step between slopes (1) and (2) is approximately 1 m high, while the 0–20 cm high step between (2) and (3) inclines gently. These steps resemble the stepwise slide surfaces observed in the Higashinaruse area described above and the Hijirigahana landslides in the Niigata-ken Chuetsu-Oki Earthquake of 2007 in Japan. The prominent 3 to 5 m scarp is at the uppermost part of the landslide ((3) in Fig. 4). The stationary remains of the slide lie above this scarp. Many cracks observed on the soil remains can be an early sign of a coming landslide. In some places, the orientation of displacements obliquely crosses the maximum dip of the strata. Therefore, the surface water on this landslide mass flows towards the eastern flank of the landslide, where a stream emerges. Part of this water flows into the base of displaced material ((4) in Fig. 4) adjacent to the eastern side and may act as one of the triggering factors of the next sliding event.

3.1.4 Landslides along the Tamakoshi River, Nepal

Many landslides in Nepal are attributed to the steep Himalayan terrain as well as geological features such as the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), which are large-scale thrust faults with nappe structures (Abe et al. 1999). Charikot, on the Tamakoshi River, is situated in the Lower Himalayas of central Nepal. The area has an average altitude of approximately 1000–2000 m, and the bedrock is Precambrian to Paleozoic low-grade Augen gneiss (Fig. 5).

Fig.5
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Schematic diagram of the cuesta landscape on the banks of the Tamakoshi River and landslides

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Unlike the landslides in the fractured sections of the large-scale tectonic lines in Nepal, many landslides in this region are influenced by rock characteristics such as schistosity, joints, and cracks. There is an area of synclines with an east–west axis around Charikot, and cuestas with dip slopes of 10–20° form on the limbs of these folds, dipping north and south. The steps have developed in an east–west direction on the slopes.

The Tamakoshi River flows from north to south at the toe of these landslides. Streams flowing into the Tamokoshi River have cut both sides of each landslide down to lower elevations. Translational landslides along the schistosity and circular landslides are in the weathered layers on the cuesta’s back slopes. Similar landslides on the cuesta’s back slopes are also found in the Precambrian phyllite along the Sunkoshi River and the Roshi River, one of the tributaries of the Sunkoshi. Both are located around 50 km northwest and southwest of the said translational slides near Charikot.

3.2 Earthquake-Induced Landslides

3.2.1 The Yokowatashi Landslide, Japan

Japan’s 2004 M6.8 Mid-Niigata Prefecture Earthquake was a significant inland earthquake claiming 68 lives. Many landslides occurred in the vicinity of the epicenter of this earthquake (e.g., Chigira and Yagi 2006; Kieffer et al. 2006), which is located in a mountainous region 25 to 400 m above sea level. Geological features in this area include alternating beds of Pliocene sandstone, mudstone, and siltstone with a highly developed north–south trending composite fold structure.

The Yokowatashi landslide (Fig. 6), one of many that occurred during the 2004 Mid-Niigata Earthquake, is located on a cuesta 50 to 100 m above sea level, where an alluvial plain of Shinano River opens up at the western end of the hilly terrain.

Fig. 6
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Displacement model of the Yokowatashi landslide

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Within a lateral distance of 250 m, three landslides of Pliocene muddy siltstone moved together towards the Shinano River. The sliding surface is a seam of sandy tuff intercalated within siltstone. The surface stretching from A to F in Fig. 6 is terraced with 1 to 2 m high southeast trending steps. In addition to the displaced locations in the 2004 Mid-Niigata Prefecture Earthquake (B, D, and F in Fig. 6), other locations (A, C, and E in Fig. 6) also show planar dip-slopes that presumably resulted from past rockslides. Incidentally, no landslides have been identified on the cuesta scarp here. The conglomeratic strata that make up the topmost layer of the Yokowatashi landslide (see the conglomerate layer in Fig. 6) are key to understanding the history of past slide events here. The translational rockslides are assumed to have taken place in succession from north to south.

The northernmost stream initially developed along cracks and cut the slope down to the lower elevation to expose a loose plane along the creek. This process reduced the resisting force against the landslide mass south of the stream, which eventually caused the translational landslides.

A large planar piece of rock remains partially atop Slope B and over a large portion of Slope C. Large rock blocks on Slopes E and F are perhaps the remnants of the ridge section left by an earlier slide. As described later, the above is one of the most often identified features of the earthquake-triggered cuesta landslides, and Fig. 7 shows a chronological summary of the Yokowatashi landslides.

Fig. 7
figure 7

Schematic model of crack directions and the history of activity of the Yokowatashi landslide

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3.2.2 The Hijirigahana Landslide, Japan

Three years after the 2004 Mid-Niigata Prefecture Earthquake, the 2007 M6.8 Niigata-ken Chuetsu-Oki Earthquake occurred off the coast of Kashiwazaki near the epicenter of the earlier earthquake. This earthquake triggered a landslide on a cuesta’s back slope in hilly coastal terrain.

The Hijirigahana landslide move along the slope at high speed and are 100 m wide, 200 m long, and 10 m thick. Some of the sliding masses have remained at the crown of the slope, but most of them slid down as translational rockslides, reaching the foot of the slope in one movement. Their slip surface was a bedding plane between Pliocene siltstone and coarse sandstone layers.

3.2.3 The Nunomata Landslide, Japan

In the 1914 M7.1 Senboku Earthquake, another inland earthquake, eight rockslides were reported in Akita Prefecture, northeast Japan (Ohashi 1915). They included the translational Nunomata landslide on the cuesta’s back slope. It traveled 70 to 80 m down a slope of approximately 8°, creating a landslide dam. The landslide mass was 100 m wide, 300 m long, and 20 m thick. Our observations and analysis of this slide in a test pit at the head of the landslide confirm that the sliding surface lies within an intercalated sandstone between Pliocene siltstone layers. This sliding surface consists of a sandstone layer with the same softness as the sandy tuff in the Yokowatashi landslide.

3.2.4 The Chiu-Fen-Erh-Shan Landslide and the Tsaoling Landslide, Taiwan

The 1999 M7.3 Chi-chi Earthquake in central Taiwan triggered many landslides. Among those quake-induced landslides, the Chiu-Fen-Erh-Shan and the Tsaoling landslides are massive and occurred on the cuesta’s back slopes of Neogene sedimentary rock units.

The Chiu-Fen-Erh-Shan landslide is a 1100 m-wide, 1300 m-long translational rockslide that slid rapidly down the cuesta’s back slope at a dip angle of 20 to 35°. The slide occurred in Miocene sandstone and shale formations, and its sliding surface corresponds to a transition in layering from silty mudstone to clay (Wang et al. 2003).

The Tsaoling landslide is a high-velocity translational rockslide approximately 1000 m wide and 1000 m long on the cuesta’s back slope, dipping at 10 to 20°. This landslide mass is primarily Pliocene sandstone and shale, with its sliding surface on a seam of silty mudstone. The Tsaoling landslide is known to have moved four or five times after being triggered by earthquakes and precipitation (Chigira et al. 2003). At the time of our observations conducted seven years after the 1999 earthquake, about 2 m-deep longitudinal gullies had developed within the displaced material.

This cuesta landslide is rare in its cause, i.e., rainfall and earthquakes.

4 Discussion

4.1 Geological Properties and Geomorphic Features of Cuesta Landslides

Many examples of cuesta landscapes shown in this paper are in relatively hard Neogene mudstones with intercalated layers (a few centimeters to a meter thick) of soft sandstone, mudstone, marl, or tuff. Some are in gneisses with well-developed schistosity. This feature indicates that landslides on these cuesta’s back slopes occur as translational rockslides on sliding surfaces developed through the weak and thin layers of soft mudstone, marl, tuff, or sandstone. Moreover, as discussed below, the relationship between relatively systematic cracks closely related to the extent and direction of movement and occurrence history of landslides are often observed in such strata. These cracks are attributed to bedrock joints and folds as well as weathering.

4.2 Behaviors of Landslides, Dips of Strata, and Orientations of Rock Cracks

This section highlights the ranges and directions of rockslides’ movements and the relationship between the directions of valleys and the cracks in base rocks. We analyzed the directionality of the cracks in the bedrock in each landslide area (Fig. 8).

Fig. 8
figure 8

Rock cracks and landslide behavior a–d: landslides triggered by rainfalls and snowmelt, e, f: earthquake induced landslides

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In the Toutazawa landslides, where landslide masses move toward Mogami River, the orientation of the crown scarp is constrained by an E-W trending crack, while an N-S crack constrains the flank of the landslides. The direction of movement tends to be almost identical to the direction of the maximum dip of the strata. Conversely, for landslides moving eastward, the orientation of the escarpment is constrained by a crack trending NW–SE, while NE-SW cracks constrain the orientation of the frank. The direction of movement of the landslide is 50–60° oblique to the direction of the maximum dip of the strata. Therefore, the cross-section of each sliding landslide mass is asymmetric, deepening toward the north (a in Fig. 8).

The Yachi landslides are divided into three areas, with the E-W trending valley as the boundary. Each area has a direction of movement that agrees with the direction of the maximum dip of the strata. The orientation of the crown scarp is constrained by NW–SE cracks (b in Fig. 8).

In the Yanagisawa landslides, the orientation of the crown scarp is constrained by an NW–SE trending crack. An N-S trending fault partially constrains the flank orientation of the landslide. The direction of movement in the Okamizawa landslide agrees with the maximum dip of the strata but is partially oblique in the Yanagisawa landslide (c in Fig. 8).

The areas of the Goldau landslide of 1806 have the orientation of the crown scarp constrained by NW–SE trending cracks. An N-S trending crack partially constrains the flank orientation of the landslide. The landslide moved toward the most significant inclination at the center. However, the greater part of the landslide mass moved south 30–40° off the maximum dip of the strata (d in Fig. 8).

The direction of movement of the Tsaoling landslide in Taiwan agrees with the maximum dip of the strata. The orientation of the crown scarp somewhat matches the direction of an E-W trending crack (e in Fig. 8). Wang et al. (2003) reported that though cracks constrained the mass movement of the Chiu-Fen-Erh-Shan landslide in the 1999 Chi-chi earthquake, the landslide mass moved toward the maximum dip of the strata.

In the Yokowatashi landslide, the orientation of the crown scarp is constrained by NE-SW trending cracks, while E-W trending cracks constrain the flank orientation of the landslide. The direction of movement of the landslide matches the direction of the maximum dip of the strata (f in Fig. 8).

These results indicate that cracks in the bedrock on the cuesta’s back slopes control the orientations of the crowns/heads and flanks of the landslides. The direction of landslide movement is generally in the direction of the maximum dip of the strata when caused by earthquakes. On the other hand, when the cause is rainfall/snowmelt, the direction of movement tends to be oblique to the direction of maximum dip (Table 1).

Table 1 The characteristics of the featured landslides
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The reasons why a landslide mass in a cuesta terrain can move oblique to the azimuth of the stratum’s maximum dip are the followings:

  • Difference in the azimuths of the maximum dip of the bedding plane, i.e., the slip surface and the stepwise offsets exposed on the strata.

  • Difference between the river’s flow direction scouring the slope toe to form a free surface at the slope’s toe and that of the maximum dip of the bedding plane, i.e., the sliding surface.

In short, regardless of the direction of the maximum dip of the strata, a landslide mass tends to move towards rivers, i.e., where there are free surfaces with less resistance. Also, steps on the cuesta’s backslope guide the landslide masses in their direction. Among landslides that moved off the direction of the strata’s maximum dip, the landslide masses at Toutazawa and Toyomaki had asymmetric reverse triangular cross-sections.

4.3 The Behavior of Pore Water Pressure in a Landslide with a Laterally Asymmetrical Slide Surface

To understand the effect of groundwater on rain/snowmelt-induced landslides at Totazawa and Toyomaki with asymmetrical cross-sections, we focused on the Yokomichizawa B landslide at the southern end of the Toyomaki landslide (Figs. 1 and 2). We drilled six boreholes to measure the pore water pressures. The results showed that the pore water pressure was highest where the sliding surface was the deepest (Fig. 9). Unlike ordinary landslides where the increasing pore water pressure causes the effective stress over the whole sliding surface to decrease, the pore water pressure reaches its most significant value at the deepest point of the sliding surface. Since no landslide occurred during our measurement, we could not capture the movement of the landslide in relation to the pore water pressure buildup. Perhaps, groundwater drainage from the base of the reverse triangular cross-section through the drainage tunnels in the adjacent Yokomichizawa C and Toyomaki landslides certainly lowered the amount of landslide movement.

Fig. 9
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Cross-sectional profile and the water head level of the Yokomichizawa B landslide ((b) in Fig. 2)

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4.4 Mechanical Behavior of a Landslide on the Cuesta’s Back Slope at the Time of an Earthquake

Tanaka et al. (2008) and Wakai et al. (2008) conducted 3D dynamic elastoplastic F.E.M. analyses on the Hitotsu-Minesawa landslide and Yokowatashi Landslides during the 2004 Mid-Niigata Prefecture Earthquake, where landslides are known to occur frequently. They reported that seismic acceleration was relatively high in ridges, and thus the shear stress was relatively high at the toes of these landslides.

An elastoplastic time-domain analysis was performed for the Yokowatashi landslides using MIDAS GTS for BESSRA software (Fig. 10). The underground input seismic motion for the 3D model (91 m below sea level) was obtained to adjust the observed strong ground motion at the Takezawa observatory (J.M.A.) (Wakai et al. 2008). The rock sliding sequence realized in the numerical simulations was:

Fig. 10
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The 3D dynamic elasto–plastic response FEM analysis of the Yokowatashi landslide area

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  1. 1.

    The amplified acceleration forcibly shook the cuesta ridge.

  2. 2.

    Shear stress was first concentrated around the toe part of the landslide mass.

  3. 3.

    Then the intense shear stress developed along the base of each exposed step (escarpment) of the cuesta.

  4. 4.

    Then rapid rockslides followed.

As seen in the cases of Yokowatashi, Hijirigahana, and Chiu-Fen-Erh-Shan, earthquake-triggered translational landslides can leave parts of their moving masses atop the cuesta’s back slopes. This finding suggests that shear stress acting at the toe and flanks of the landslide, rather than horizontal acceleration, is most strongly associated with sliding. The step-like offset along the neighboring landslide masses’ edges is essential for understanding the retrogressive sequence of sliding events (Fig. 7). The stream cutting down the northern end of the Yokowatashi landslide area likely initiated the primary landslide.

4.5 Movement of Landslides and the Cuesta Landscape

Many landslides on the cuesta’s back slopes of well-developed sedimentary rock units -especially mudstone- that dominate cuesta landscapes occur as translational rockslides. These landslides touching each other can form a step-like landscape. Bedding, cracks, and stepwise boundaries of these masses can control the movements of the landslide masses, causing directions of these mass movements to differ from each other and thus affect the detaching sequence. River erosion at the slopes’ toes also contributes to the landslide mass movement (Fig. 11).

Fig. 11
figure 11

Schematic models of the landslide history induced by the development of cracks and streams

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As the fluvial erosion develops, retrogressive detaching of landslide masses occurs repeatedly, and intact sliding surfaces appear one after another.

When a landslide mass has an asymmetric inverse triangular transverse cross-section, underground water gathers around the deepest vertex of the triangle. Thus, the landslide initiation can differ from a generally observed landslide. Earthquake-induced large landslides on cuesta’s back slopes may have repeated over a long geological time. Therefore, knowing landslide histories is an essential subject of study to be prepared for the next.

According to the current view, cuesta landscapes are the product of differential erosion. However, the cyclic processes of geomorphological evolution, i.e., the development of cracks and the consequent formation of a river, followed by denudation and landslides, also have an essential role in the processes that lead to the formation of cuesta landscapes.

5 Conclusions

In this study, we focused on the mechanisms of landslides on the cuesta’s back slopes, which are the most commonly occurring as large-scale landslides in cuesta landscapes. We also examined the role of these landslides on the geomorphological evolution of cuesta terrains. We came to the following conclusions:

  1. 1.

    The extents, forms, and movement directions of many landslides in the cuesta’s back slopes depend on the bedding of strata, bedrock cracks, and rivers formed along the cracks. The landslides occur as slab-shaped translational slides moving on a bedding plane.

  2. 2.

    In general, landslides occur on slopes where the same mass moves over a long time and undergoes weathering and fracturing. Whereas most landslides on the cuesta’s back slopes recur, causing an intact bedding plane to appear as a new sliding surface, thus maintaining the overall form of the cuesta’s back slopes.

  3. 3.

    Direction of a rain/snowmelt-induced landslide mass movement often differs from the direction of the maximum dip of the strata. Conversely, an earthquake-induced landslide mass tends to move in the same direction as the strata’s maximum dip.

  4. 4.

    When a landslide mass has an asymmetric reverse-triangular cross-section, underground water gathers the deepest vertex of the triangle, affecting the detaching process. Earthquake-induced landslides are often a sequence of detaching events of landslide masses touching each other. The sequence is affected by the river erosion of the landslide toe and intense shear forces localized at the toes and lower flanks of landslide masses.

The results suggest that these translational bedrock landslides occurring on the cuesta’s back slopes have repeatedly denuded intact sliding surfaces over the long term while maintaining the cuesta landscape.