Landslide Amplification by Liquefaction of Runout‐Path Material after the 2008 Wenchuan (M 8·0) Earthquake,China

Here, we propose that an earthquake can trigger the failure of a landslide mass while simultaneously triggering liquefaction of runout‐path materials before the arrival of the landslide mass, thus greatly increasing the size and mobility of an overriding landslide. During the 2008 Wenchuan earthquake, about 60 000 landslides were triggered, directly resulting in about 20 000 casualties. While these landslides mainly originated from steep slopes, some landslides with high mobility formed in colluvial valley deposits. Among these, the most catastrophic was the Xiejiadian landslide in Pengzhou city, which traveled hundreds of meters before coming to rest. Through field investigation and laboratory testing, we conclude that this landslide primarily formed from colluvial deposits in the valley and secondarily from failure of slopes in granitic rock located uphill. Much of the granitic slope failure was deposited in the upper part of the travel path (near the slide head); the remainder was dispersed throughout the main landslide deposit. Superposition of deposits at the landslide toe indicates that landslide debris derived from colluvial soil was deposited first. The deposits at the landslide toe displayed flow characteristics, such as fine materials comprising basal layers and large boulders covering the deposit surface. We hypothesize that the main part of the landslide resulted from seismogenic liquefaction of valley colluvium, rather than from liquefaction potentially caused by undrained loading from the granitic slope failures impacting the colluvium. To examine the likelihood that seismogenic liquefaction occurred, we took samples from different areas of the landslide deposit and performed undrained cyclic shear tests on them in the laboratory. The results showed that the sandy soils that comprise most of the deposit are highly liquefiable under seismic loading. Therefore, we conclude that liquefaction of the colluvium in the valley during the earthquake was the main reason for this rapid (~46 m/s) long‐runout (1·7 km) landslide. Copyright © 2012 John Wiley & Sons, Ltd.     

A preliminary study of the failure mechanisms of cascading landslide dams

Landslide dams commonly form when mass earth or rock movements reach a river channel and cause a complete or partial blockage of the channel.Intense rainfalls can induce upstream flows along a sloping channel that significantly affect downstream landslide dams.If a series of landslide dams are collapsed by incoming mountain torrents(induced by intense rainfall),large debris flows can form in a very short period.Furthermore,the failure of these dams can amplify the magnitude and scale of debris flows in the flow direction.The catastrophic debris flows that occurred in Zhouqu County,China on 8 August 2010 were caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies.Incorporating the role of outburst floods associated with the complete or partial failure of landslide dams is an interesting problem usually beyond the scope of analysis because of the inherent modeling complexity.To understand the cascading failure modes of a series of landslide dams,and the dynamic effect these failures have on the enlargement of debris flow scales,experimental tests are conducted in sloping channels mimicking field conditions,with the modeled landslide dams distributed along a sloping channel and crushed by different upstream flows.The failure modes of three different cascades of landslide dams fully or partially blocking a channel river are parametrically studied.This study illustrates that upstream flows can induce a cascading failure of the landslide dams along a channel.Overtopping is the primary failure mechanism,while piping and erosion can also induce failures for different constructed landslide dams.A cascading failure of landslide dams causes a gradually increasing flow velocity and discharge of the front flow,resulting in an increase in both diameter and percentage of the entrained coarse particles.Furthermore,large landslide blockages can act to enhance the efficiency of river incision,or conversely to induce aggradation of fluvial sediments,depending on the blockage factor of the landslide dams and upstream discharge.     

From mass‐wasting to slope stabilization – putting constrains on a tectonically induced transition in slope erosion mode: a case study in the Judea Hills,Israel

Calcrete‐coated remnants of landslide debris and alluvial deposits are exposed along the presently stable hillslopes of the Soreq drainage, Judea Hills, Israel. These remnants indicate that a transition from landslide‐dominated terrain to dissolution‐controlled hillslope erosion had occurred. This transition possibly occurred due to the significant decrease in tectonic uplift during the late Cenozoic. The study area is characterized by sub‐humid Mediterranean climate. The drainage hillslopes are typically mantled by thick calcrete crusts overlying Upper Cretaceous marine carbonate rocks. Using TT‐OSL dating of aeolian quartz grains incorporated in the calcrete which cements an ancient landslide deposit, we conclude that incision of ~100 m occurred from 1056 ± 262 to 688 ± 86 ka due to ~0·3° westward tilt of the region; such incision invoked high frequency of landslide activity in the drainage. The ages of a younger landslide remnant, alluvial terrace, and alluvial fan, all situated only a few meters above the present level of the active streambed, range between 688 ± 86 ka and 244 ± 25 ka and indicate that since 688 ± 86 the Soreq base level had stabilized and that landslide activity decreased significantly by the middle Pleistocene. Copyright © 2012 John Wiley & Sons, Ltd.     

The paleoseismogenic activation of the Great Lakes segment of the Erzin-Agar-Dag fault

Seismogeological investigations conducted in the Erzin-Agar-Dag fault zone (the Great Lakes segment) revealed traces of paleoseismogenic deformation. A detailed on-land investigation of the Tatur-Tolga and the Hairakan paleoseismic dislocations (PSD) found a dominant left lateral type of movement. The amplitudes of one-act displacements on the faults vary between 2–3 and 6–7 m (horizontal) and within 1.5–2 m (vertical). Along with the parameters of one-act horizontal movements of a few meters, we found an accumulated amplitude of strike-slip movement of a few tens to hundreds of meters. Based on the parameters of seismotectonic deformations, the magnitudes of the respective paleoearthquakes are inferred to have been 6.6 to 7.9.     

The triggering of debris flow due to channel‐bed failure in some alpine headwater basins of the Dolomites: analyses of critical runoff

The debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff, triggering a water–sediment mixture flow known as debris flow. The routing of debris flow through human settlements can cause damage to civil structures and loss of human lives. The prediction of such an event, or the runoff discharge that triggers it, assumes an interest in risk analyses and the planning of defence measures. The object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel‐bed failure. Historical and rainfall data on 30 debris flows that occurred in six watersheds of the Dolomites (north‐eastern Italian Alps) were collected from different sources. Field investigations at the six sites, together with the hydrologic response to the rainfalls that triggered the events, were performed to obtain a realistic scenario of the formation of the debris flow there occurred. Field observations include a survey along the channel of the triggering reach of debris flow, with measurements of the channel slope and cross‐section and sampling of debris deposits for grain size distribution. Simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure. The results are discussed to provide a plausible physical‐based method for the prediction of the triggering of debris flow by channel‐bed failure. Copyright © 2007 John Wiley & Sons, Ltd.     

Isostatic Consequences of Giant Landslides on the Hawaiian Ridge

—Giant landslides, like melting glaciers, lead to a redistribution of mass which will have isostatic consequences. Three-dimensional numerical modeling experiments were devised to examine how this mass redistribution affects the isostatic flexural curve. A debris avalanche of 10–40% of pre-slide Oahu is required to account for the 1200–5000 km³ Nuuanu deposit, while only ～ 1% of pre-slide Hawaii Island is necessary to generate the 200–800 km³ Alika I and II avalanche deposits. Trials were run using 25, 30, and 40 km elastic plate thicknesses (T _e ). The island uplift resulting from the Nuuanu slide was calculated to be 23 m and 109 m for 10% and 40% volume slides, respectively, both using T _e = 25 km. A rebound of 10 m and 49 m was calculated for the same volumes, respectively, using T _e = 40 km. A greater amount of uplift is expressed direct lyover the failed flank, causing the edifice to tilt away from the calved-off portion. The landslide deposit depresses the plate several meters beneath the debris field itself. Smaller slides (e.g., Alika I and II) do not produce as much flexural response, with 17 m and 7 m uplift for T _e = 25 and 40 km, respectively. The effects of slow moving, intact slumps where the failed blocks remain relatively close to the island pedestal were examined for the case of the Hilina slump, making up approximately 10% of the Hawaii Island edifice. Perhaps more significant than the uplift for the Hilina slump, comparable to that for the 10% Nuuanu debris avalanche, is the 114 m and 56 m of downwarp beneath its massive slumped foot (T _e = 25 and 40 km, respectively). The landslide rebound process, in the case of a relatively large landslide, should be considered as an added component to the evolutionary course of oceanic islands.     

使用宽频带地震记录研究2017年6月24日茂县新磨滑坡的动力学过程

北京时间2017年6月24日5时39分左右,四川省茂县叠溪镇新磨村发生大型岩质滑坡.体积约4.3×10⁶ m³的巨型岩体从山顶脱落,顺坡滑行约2.6 km后破碎沉积;碎屑物掩埋了整个新磨村,造成了巨大的人员伤亡和财产损失.本文使用来自滑坡周围的10个地震台站的宽频带观测资料的长周期信号反演了这次滑坡的受力时间函数;同时使用逐步细化的格点搜索方法得到了滑坡的位置,与其真实位置一致;根据反演的受力时间函数计算了滑坡过程中滑体的运动学参数,得到的滑体运动轨迹与实际路径吻合.综合分析地震信号、受力时间函数和运动学参数表明,本次滑坡主运动的持续时间约为79 s;脱落岩体在5∶38∶50.2启动后持续加速,在5∶39∶37.2达到速度峰值,约为52.1 m·s^-1;这段时间内岩体没有明显的破碎;之后,岩体开始铲刮并裹挟古滑坡造成的碎屑沉积物,自身也开始破碎解体,总体开始减速运动,直到5∶40∶9.2主运动停止;此后,小规模的碎屑散落又持续了约10 s的时间.     

Lahars at Merapi volcano, Central Java: an overview

Merapi volcano, in Central Java, is one of the most active volcanoes in the world. At least 23 of the 61 reported eruptions since the mid-1500s have produced source deposits for lahars. The combined lahar deposits cover about 286 km² on the flanks and the surrounding piedmonts of the volcano. At Merapi, lahars are commonly rain-triggered by rainfalls having an average intensity of about 40 mm in 2 h. Most occur during the rainy season from November to April, and have average velocities of 5–7 m/s at 1000 m in elevation. A wide range of facies may be generated from a single flow, which may transform downvalley from debris flow to hyperconcentrated streamflow.Because of the high frequency and magnitude of the lahar events, lahar-related hazards are high below about 450–600 m elevation in each of the 13 rivers which drain the volcano. Hazard-zone maps for lahar were produced by Pardyanto et al. (Volcanic hazard map, Merapi volcano, Central Java (1/100,000). Geol. Surv. of Indonesia, Bandung, II, 4, 1978) and the Japanese–Indonesian Cooperation Agency (Master plan for land conservation and volcanic debris control in the area of Mt Merapi, Jakarta, 1980), but these maps are of a very small scale to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000- and 1/2000-scale) and risk assessments have been completed for a few critical river systems.     

Debris flow disaster at Larcha, upper Bhotekoshi Valley, central Nepal

Abstract   Damage, destruction and casualties related to landslide and debris flow are common phenomena in the Himalaya, especially during the summer monsoon. This fact was tragically illustrated on 22 July 1996, when Larcha, situated at the 109-km mark of the Arniko Highway, upper Bhotekoshi Valley, central Nepal, experienced a catastrophic debris flow powered by the Bhairab Kunda Stream. Of the 22 houses in Larcha, 16 were swept away, two were partially damaged and 54 people were killed in a matter of a few minutes. The event attracted attention when media linked it to a glacial lake outburst flood as a result of the fact that the source of the stream is a glacial lake. To understand the cause, initiation mechanism and deposition process, the basin area was studied from geological, geomorphologic and engineering geological points of view and the role of precipitation was evaluated. A combination of rainfall, runoff from cliff faces and stream undercutting triggered failure of the bedrock and colluvium, both on the dip and counter-dip slopes, 500 m upstream from the highway. The landslide debris dammed the channel, which was eventually breached, and deposited approximately 104 000 m³ of coarse debris, dominated by the metasediments of the Lesser Himalayan origin, and overwhelmed the village of Larcha. The debris deposit was studied for clast size, composition, texture and dimensions. Lack of sorting and the presence of abundant silt and clay in the source area helped in the initiation of debris flow. The abundance of the Lesser Himalayan metasediment clasts together with the absence of debris traces upstream from the landslide site ruled out the possibility of a glacial lake outburst flood. The disaster was a result of landslide damming triggered by precipitation and stream undercutting and sudden bursting.     

A CASE STUDY ON POSSIBLE DEBRIS FLOW CAUSED BY WASTE SOLID IN JUNGAR COALFIELD AREA

Debrisflow,definedasakindofhighlycondensedtit!o-phaseflowofsolidandfluid,iscomposedofslurryandrocksofdifferentsizeswithageneraldensity'ofI.2to2.3t/m3.Theactivityliesbetweenfloodandlandslide.Itflowslikenormalfloodandsholvsturbulentstateaswithlesssolidcontentsandlowerviscosity,itisplasticviscousflowwithunstablegUstmovementwhellitisofhighsolidcontainsandhighviscosity.Debrisflowisformedunderthreebasicconditionsfprecipitousslopeoflandform,solidwasteoflargevolume,andenoughwatersupply.Theoccurrence,…     

Distinguishing volcanic debris avalanche deposits from their reworked products: the Perrier sequence (French Massif Central)

Debris avalanches associated with volcanic sector collapse are usually high-volume high-mobility phenomena. Debris avalanche deposit remobilisation by cohesive debris flows and landslides is common, so they can share textural characteristics such as hummocks and jigsaw cracks. Distinguishing original deposits from reworked products is critical for geological understanding and hazard assessment because of their different origin, frequency and environmental impact. We present a methodology based on field evidence to differentiate such epiclastic breccias. Basal contact mapping constrained by accurate altitude and location data allows the reconstruction of deposit stratigraphy and geometry. Lithological analysis helps to distinguish the different units. Incorporation structures, kinematic indicators and component mingling textures are used to characterise erosion and transport mechanisms. We apply this method to the enigmatic sequence at Perrier (French Massif Central), where four units (U1–U4) have been interpreted either as debris flow or debris avalanche deposits. The sequence results from activity on the Monts Dore Volcano about 2 Ma ago. The epiclastic units are matrix supported with an almost flat top. U2 and U3 have clear debris flow deposit affinities such as rounded clasts and intact blocks (no jigsaw cracks). U1 and U4 have jigsaw cracked blocks with matrix injection and stretched sediment blocks. U1 lacks large blocks (>10 m wide) and has a homogenous matrix with an upward increase of trapped air vesicle content and size. This unit is interpreted as a cohesive debris flow deposit spawned from a debris avalanche upstream. In contrast, U4 has large mega-blocks (up to 40 m wide), sharp contacts between mixed facies zones with different colours and numerous jigsaw fit blocks (open jigsaw cracks filled by monogenic intra-clast matrix). Mega-blocks are concentrated near the deposit base and are spatially associated with major substratum erosion. This deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation. We also identify two landslide deposits (L1 and L2) resulting from recent reworking that has produced a similar facies to U1 and U4. These are distinguishable from the original deposits, as they contain blocks of mixed U1/U4 facies, a distinctly less consolidated and more porous matrix and a fresh hummocky topography. This work shows how to differentiate epiclastic deposits with similar characteristics, but different origins. In doing so, we improve understanding of present and past instability of the Monts Dore and identify present landslide hazards at Perrier.     

Slope instability induced by volcano-tectonics as an additional source of hazard in active volcanic areas: the case of Ischia island (Italy)

Ischia is an active volcanic island in the Gulf of Naples whose history has been dominated by a caldera-forming eruption (ca. 55 ka) and resurgence phenomena that have affected the caldera floor and generated a net uplift of about 900 m since 33 ka. The results of new geomorphological, stratigraphical and textural investigations of the products of gravitational movements triggered by volcano-tectonic events have been combined with the information arising from a reinterpretation of historical chronicles on natural phenomena such as earthquakes, ground deformation, gravitational movements and volcanic eruptions. The combined interpretation of all these data shows that gravitational movements, coeval to volcanic activity and uplift events related to the long-lasting resurgence, have affected the highly fractured marginal portions of the most uplifted Mt. Epomeo blocks. Such movements, mostly occurring since 3 ka, include debris avalanches; large debris flows (lahars); smaller mass movements (rock falls, slumps, debris and rock slides, and small debris flows); and deep-seated gravitational slope deformation. The occurrence of submarine deposits linked with subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches impacted on the sea. The obtained results corroborate the hypothesis that the behaviour of the Ischia volcano is based on an intimate interplay among magmatism, resurgence dynamics, fault generation, seismicity, slope oversteepening and instability, and eruptions. They also highlight that volcano-tectonically triggered mass movements are a potentially hazardous phenomena that have to be taken into account in any attempt to assess volcanic and related hazards at Ischia. Furthermore, the largest mass movements could also flow into the sea, generating tsunami waves that could impact on the island’s coast as well as on the neighbouring and densely inhabited coast of the Neapolitan area.     

Volcanic dry avalanche deposits — Identification and comparison with nonvolcanic debris stream deposits

A “volcanic dry avalanche deposit” is defined as a volcaniclastic deposit formed as a result of a large-scale sector collapse of a volcanic cone associated with some form of volcanic activity. Avalanche transport occurred in response to the gravitational field, in a manner similar to the transport of nonvolcanic debris streams (e.g. Hsü, 1975). Such deposits are characterized by megablock structure — deformed and fractured large blocks up to several hundreds meters in diameter. A megablock preserves original layering, intrusive contacts or weathered surfaces of the source volcanic edifice. Surface topography of the deposit is characterised with hummocky relief. Ratios of fell height to travel distance for volcanic dry avalanche deposits are between 0.18 and 0.06. This range is similar but smaller than the value of 0.58 to 0.08 for nonvolcanic debris stream deposit. This similarity suggests similar transportation mechanisms. Excessive travel distances as defined by Hsü (1975), calculated for volcanic dry avalanche deposits, give values larger than for debris stream deposits of the same volume. The difference is explained by lower rigidity of the collapsing mass due to the existence of soft pyroclastic layers, alteration around the vent, development of fractures owing to new cryptodome intrusion, and boiling of supercritical fluid contained within the collapsed mass.     

Tsunami Generated by the Late Bronze Age Eruption of Thera (Santorini), Greece

—Tsunami were generated during the Late Bronze Age (LBA) eruption of the island of Thera, in the southern Aegean Sea, by both caldera collapse, and by the entry of pyroclastic surges/flows and lahars/debris flows into the sea. Tsunami generated by caldera collapse propagated to the west producing deep-sea sedimentary deposits in the eastern Mediterranean Sea known as homogenites; open-ocean wave heights of about 1.9–17 m are estimated. Tsunami generated by the entry of pyroclastic flows/surges and lahars/debris flows into the sea propagated in all directions around the island; wave heights along coastal areas were about 7–12 m as estimated from newly identified tsunami deposits on eastern Thera as well as from pumice deposits found at archaeological sites on northern and eastern Crete.     

Overbank deposition along the concave side of the Red River meanders,Manitoba, and its geomorphic significance

Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada.     

Assessing marine debris in deep seafloor habitats off California

Marine debris is a global concern that pollutes the world’s oceans, including deep benthic habitats where little is known about the extent of the problem. We provide the first quantitative assessment of debris on the seafloor (20-365 m depth) in submarine canyons and the continental shelf off California, using the Delta submersible. Fishing activities were the most common contributors of debris. Highest densities occurred close to ports off central California and increased significantly over the 15-year study period. Recreational monofilament fishing line dominated this debris. Debris was less dense and more diverse off southern than central California. Plastic was the most abundant material and will likely persist for centuries. Disturbance to habitat and organisms was low, and debris was used as habitat by some fishes and macroinvertebrates. Future trends in human activities on land and at sea will determine the type and magnitude of debris that accumulates in deep water.     

Recurrence of major flank landslides during the last 2-Ma-history of Reunion Island

New detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of Reunion Island. These fan-shaped promontories are 20–25 km wide at the coastline and 70–150 km across the seafloor 40–50 km offshore. Their surfaces are characterized by a speckle sonar pattern, indicating the presence of large blocks up to several hundred meters across. Each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long. The submarine flanks of Reunion Island are thus mostly built by accumulation of debris avalanche fans. Morphologic and geologic evidence define large subaerial source areas for these mass-wasting events. In particular, inferred headwalls of most landslides having affected the Piton des Neiges massif generally coincide with the boundaries of its cirques (Mafate, Salazie, and Cilaos), whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the Piton de la Fournaise massif. Thus, about 15 slide events accompanied growth of the Reunion Island shield since 2 Ma.Editorial responsibility: J. Stix     

Volcanic tsunamis and prehistoric cultural transitions in Cook Inlet,Alaska

The 1883 eruption of Augustine Volcano produced a tsunami when a debris avalanche traveled into the waters of Cook Inlet. Older debris avalanches and coeval paleotsunami deposits from sites around Cook Inlet record several older volcanic tsunamis. A debris avalanche into the sea on the west side of Augustine Island ca. 450 years ago produced a wave that affected areas 17 m above high tide on Augustine Island. A large volcanic tsunami was generated by a debris avalanche on the east side of Augustine Island ca. 1600 yr BP, and affected areas more than 7 m above high tide at distances of 80 km from the volcano on the Kenai Peninsula. A tsunami deposit dated to ca. 3600 yr BP is tentatively correlated with a southward directed collapse of the summit of Redoubt Volcano, although little is known about the magnitude of the tsunami. The 1600 yr BP tsunami from Augustine Volcano occurred about the same time as the collapse of the well-developed Kachemak culture in the southern Cook Inlet area, suggesting a link between volcanic tsunamis and prehistoric cultural changes in this region of Alaska.     

Morphology,structure and kinematics of a rainfall controlled slow‐moving Andean landslide,Peru

The large slow‐moving landslide of Maca is located in the upper Colca valley (southern Peru), a region characterized by a well pronounced rainy period, and intense and recurrent sustained seismicity. The landslide, developed in deep lacustrine deposits, has recently accelerated, threatening the Maca village. This work aims at understanding the rupture mechanism and the causes of the recent landslide reactivation/acceleration. We present a multidisciplinary characterization of the Maca landslide that includes: (i) geological and morphological mapping in the field; (ii) remote sensing analysis using an historical aerial photograph of 1955 and the Pléiades satellite images (2013); (iii) global positioning system (GPS) including time‐series of surveys over 13 years, and continuous measurements over 14 months; (iv) a geophysical campaign with deep electrical resistivity tomography profiles acquired across the landslide mass. Our study shows that this 60 Mm³ landslide, which can be classified as a clay/silt compound landslide, moved by 15 m between 2001 and 2014 with a large inter‐annual velocity variation (up to a factor of 500) depending on the rainfall intensity. We suggest that these dramatic changes in velocity are the result of the combination of a threshold mechanism and the short intense rainy season in Peru. This study reveals three main driving factors acting at different timescales: (i) over several decades, the river course has significantly changed, causing the Maca landslide reactivation in the 1980s due to the erosion of its toe; (ii) at the year scale, a minimum amount of rainfall is required to trigger the motion and this amount controls the landslide velocity; (iii) transient changes in slide velocity may occur anytime due to earthquakes. This study particularly highlights the non‐linear behaviour of the motion with rainfall. Copyright © 2016 John Wiley & Sons, Ltd.