Coherent Landslides Research Articles

Increase in landslide activity after a low-magnitude earthquake as inferred from DInSAR interferometry

On August 16th, 2018, a Mw 5.1 earthquake struck the Molise region (central Italy), inducing 84 earthquake-triggered landslides that predominantly involved soil covers of clayey materials and flysch on gently dipping slopes. To quantify the spatiotemporal landslide activity in the months immediately after the earthquake, a differential SAR interferometry (DInSAR) analysis was performed for a time span from 2 years before to one year after the earthquake, recognising both first-time and reactivated landslides. The results showed a clear increase in landslide activity following the low-magnitude earthquake with respect to the activities recorded in the same months of the previous years. Several coherent landslides (earth slides and earth flows) were observed following seasonally recurrent rainfall events. Such increases were observed for both reactivated and first-time landslides, showing decreases in inactive periods and activity over longer periods. Furthermore, the spatial density distribution of the landslides was investigated in the postseismic time interval along transects perpendicular and parallel to the direction of the tectonic element responsible for the seismic event. An asymmetrical distribution was deduced parallel to the fault strike with a higher number of landslides located inside the compressional sector according to a strike-slip faulting mechanism.

Landslides induced by the 2017 Mw7.3 Sarpol Zahab earthquake (Iran)

Landslides are the main secondary effects of earthquakes in mountainous areas. The spatial distribution of these landslides is controlled by the local seismic ground motion and the local slope stability. While gravitational instabilities in arid and semi-arid environments are understudied, we document the landslides triggered by the Sarpol Zahab earthquake (November 12, 2017, Mw7.3, Iran/Iraq border), the largest event ever recorded in the semi-arid Zagros Mountains. An original earthquake-induced landslide inventory was derived, encompassing landslides of various sizes and velocities (from rapid disrupted rockfalls to slow-moving coherent landslides). This inventory confirms the low level of triggered landslides in semi-arid environments. It also displays clear differences in the spatial and volumetric distributions of earthquake-induced landslides, having 386 rockfalls of limited size triggered around the epicenter, and 9 giant (areas of ca. 106 m2) active and ancient deep-seated landslides coseismically accelerated at locations up to 180 km from the epicenter. This unusual distant triggering is discussed and interpreted as an interaction between the earthquake source properties and the local geological conditions, emphasizing the key role of seismic ground motion variability at short spatial scales in triggering landslides. Finally, the study documents the kinematics of slow-moving ancient landslides accelerated by earthquakes, and opens up new perspectives for studying landslide triggering over short (~ 1–10 years) and long-time (~ 1000–10,000 years) periods.

Statistical analyses of landslide size and spatial distribution triggered by 1990 Rudbar-Manjil (Mw 7.3) earthquake, northern Iran: revised inventory, and controlling factors

This study is a revised inventory of landslides triggered by the Rudbar-Manjil earthquake (Mw = 7.3), 1990, obtained through the study of Komak Panah and Hafezi Moghadas (1993), interpretations of the stereoscopic aerial photos, and field data. We mapped the 223 coherent landslides with a total area of 11.3 km2 and a calculated volume of 3.8 × 108 m3 using a digital elevation model (DEM, 12.5 m) within the geographical information system (GIS) software. The correlation between landslide occurrence (size and distribution) and the controlling factors was analyzed using the bivariate model and the linear automated modeling (LINEAR) procedure in the SPSS software. Both LINEAR models of landslide volume (LV, m3) and landslide area (LA, m2) indicate the distance from the fault surface rupture is the most influencing factor on size prediction of coseismic landslides. The LA is more dependent on seismic factors, whereas the LV is on the lithology and topographic factors. The result of principal component analysis (PCA) model, using the GIS, showed that most landslides occurred in the red band (including 95% of the variance of controlling factors). Thus, the selected factors to investigate the landslide occurrence are valid and the PCA can be used to identify regions prone to coseismic landslides. The curves of landslide number density (LND, landslides/km2), landslide area percentage (LAP, landslide area/total area%), LA, and LV have a multimodal distribution (several patterns of response) by the topographic and seismic factors. It is recommended that other models be applied to better investigate the local effects of factors on the coseismic landslides.

Mineralogical Analysis of Selective Melting in Partially Coherent Rockslides: Bridging Solid and Molten Friction

AbstractFrictional shearing along the basal slip surface is the main energy dissipation mechanism in coherent landslides, which undergo little internal deformation during runout. Most landslide bodies, however, deform and break down more or less extensively; thus, only a portion of their potential energy is dissipated through basal shearing. The mineral components of rocks and soils have individual melting temperatures. Therefore, as temperature increases, selective melting will occur in a defined sequence. The product of such melting is termed frictionite. Here, we exploited a sliding block model to investigate the role of selective melting in the evolution of the frictional resistance (and hence mobility) of partially coherent landslides. We recognized three frictional stages, characterized by weakening, strengthening, and melt lubrication. As melting proceeds, a gradual transition from solid friction to viscous flow occurs, which can be modeled in terms of the molten mineral fraction. The chemical composition also evolves and so does its viscosity. Rocks with different mineral proportions (e.g., mafic vs. felsic rocks) will exhibit different frictional evolutions. Our model results are consistent with the recognized role of landslide thickness in defining its mobility. Moreover, the frictional strengthening stage is shown to become irrelevant under high confinement, thereby facilitating higher mobility, or sometimes shifting slip surface if weak layer exists. We also discussed the relevance of some strengthening‐upon‐melting behavior observed at the experimental scale, pointing out the role of the energy conversion coefficient, which is a proxy for landslide coherence.

An Empirical Power Density‐Based Friction Law and Its Implications for Coherent Landslide Mobility

AbstractThe evolution of the shear resistance at the base of a coherent landslide body can effectively control its dynamic behavior. High‐velocity rotary shear experiments have allowed scientists to explore stress‐strain conditions close to those found in large landslides and faults. These experiments have led to two alternative models being proposed, which describe the evolution of the shear resistance through friction laws that depend either on normal stress or on velocity. Here, we discuss an integrated approach, first proposed to study seismic fault behavior, that reconciles these two models under a single parameter—the power density—which we utilize for the first time to investigate landslide dynamics. Using thermodynamic and process‐based considerations, different soil and rock types can be related to different weakening mechanisms, which in turn can determine different landslide behaviors.

Landslides triggered after the 16 August 2018 Mw 5.1 Molise earthquake (Italy) by a combination of intense rainfalls and seismic shaking

On 16 August 2018, a Mw 5.1 earthquake occurred in the Molise region (Central Italy) during an intense rainfall event which cumulated up to 140 mm in 3 days. Within 5 days after the seismic event, 88 landslides were surveyed and classified in disrupted and coherent as well as in first-time failures and reactivation. As it resulted by the inventorying, most of the surveyed ground effects were represented by coherent landslides involving clays, marly clays, and cover deposits on low dipping slopes. A spatial distribution analysis of landslides in relation to seismic action and rainfall intensity was carried out to evaluate the possible contribution of both the rainfall-induced saturation and the earthquake shaking to landslide triggering. Based on this analysis, in the epicentral area, it is not possible to clearly split the role of seismic and hydraulic destabilising actions, while, at greater distances, the joined contribution of rainfall and earthquake shaking could have promoted slope failures. Comparisons among collected data and existing worldwide catalogues allow to highlight, through the analysis of the spatial distribution of the surveyed landslides, the possible and not negligible effect of soil saturation during the seismic shaking both in epicentral distance vs. magnitude distribution and in number and spatial concentration of triggered ground effects.

An empirical power density-based friction law and its implications for coherent landslide mobility

An empirical power density-based friction law and its implications for coherent landslide mobility

Landslides Triggered by the 2016 Mw 7.8 Pedernales, Ecuador Earthquake: Correlations with ESI-07 Intensity, Lithology, Slope and PGA-h

We provide a dataset of the landslides induced by the 2016 Pedernales megathrust earthquake, Ecuador (Mw 7.8, focal depth of 20 km) and compare their spatial distribution with mapped bedrock lithology, horizontal peak ground acceleration (PGA-h) and the macroseismic intensity based on earthquake-induced environmental effects (ESI-07). We studied 192 coseismic landslides (classified as coherent, disrupted and lateral spreads) located in the epicentral area, defined by the VII to IXESI-07 isoseismals. Based on our findings, lahar deposits, tuffs and volcanoclastic units are the most susceptible to landslides occurrence. Alluvial plains with fluvial loose fine sand are the most susceptible setting for lateral spreading, with a maximum intensity of IXESI-07. The coherent landslides are frequently found in altered shale and siltstone geological units with moderate slopes (8°–16°), with typical intensity ranging between VII and VIIIESI-07. Our analysis draws a typical framework for slope movements triggered by subduction earthquakes in Ecuador. The most dangerous setting is the coastal region, a relatively highly urbanized area located near the epicenter and where liquefaction can trigger massive lateral spreading events. Coherent and disrupted landslides, dominating the more internal hilly region, can be triggered also in moderate slope settings (i.e., less than 10°). Indeed, the regression analysis between seismic intensity, PGA-h and landslide occurrence shows that most of the events occurred at PGA-h values between 0.4 g and 1.2 g, at a distance of 30 to 50 km from the rupture plane. Our database suggests that lithology and hillslope geometry are the main geological/geomorphological factors controlling coseismic landslides occurrence; while the distance from the rupture plane plays a significant role on determining the landslide size. Finally, we underline that coseismically-triggered landslides are among the most common environmental effects occurring during large subduction events that can be effectively used to properly evaluate the earthquake macroseismic field. The landslide inventory we compiled is suitable for assessing the vulnerability of physical environment from subduction earthquakes in Ecuador, and offers a primary data source for future worldwide analysis.

Structure of Lō‘ihi Seamount, Hawai‘i and Lava Flow Morphology From High-Resolution Mapping

High-resolution bathymetric data were collected using AUV Sentry at the summit and at two hydrothermal vent fields on the deep south rift of L¯o‘ihi Seamount. The summit map records a nested series of caldera and pit crater collapse events, uplift of one resurgent block, and eruptions that formed at least five low lava shields that shaped the summit. The earliest and largest caldera, formed ~5900 years ago, bounds almost the entire summit plateau. The resurgent block was uplifted slightly more than 100 m and has a tilted surface with a dip of about 6.5° towards the SE. The resurgent block was then modified by collapse of a pit crater centered in the block that formed West Pit. The shallowest point on L¯o‘ihi’s summit is 986 m deep and is located on the northwest edge of the resurgent block. Several collapse events culminated in formation of East Pit, and the final collapse formed Pele’s Pit in 1996. The nine mapped collapse and resurgent structures indicate the presence of a shallow crustal magma chamber, ranging from depths of ~1 km to perhaps 2.5 km below the summit, and demonstrate that shallow sub-caldera magma reservoirs exist during the late pre-shield stage. On the deep south rift zone are young medium- to high-flux lava flows that likely erupted in 1996 and drained the shallow crustal magma chamber to trigger the collapse that formed Pele’s Pit. These low hummocky and channelized flows had molten cores and now host the FeMO hydrothermal field. The Shinkai Deep hydrothermal site is located among steep-sided hummocky flows that formed during low-flux eruptions. The Shinkai Ridge is most likely a coherent landslide block that originated on the east flank of L¯o‘ihi.

Topographic and geologic controls on landslides induced by the 2015 Gorkha earthquake and its aftershocks: an example from the Trishuli Valley, central Nepal

The devastating Gorkha earthquake (M w 7.8) on April 25, 2015 and its aftershocks triggered numerous landslides across the Lesser and Higher Himalayas of central Nepal. This study aims to characterize these landslides, based on the local topography and geology, and to develop data for landslide hazard zoning. This study focused on a mountainous catchment of the Trishuli River, where a digital elevation model was used to examine hilllslope and river profiles, aerial photos were used to identify 155 coherent landslides, and satellite images were used to map 912 earthquake-induced landslides. The topography of this area is mainly characterized by incised V-shaped inner gorges and steep (> 35°) SW-facing scarp slopes. Although most of the coherent landslides were not reactivated by the earthquakes, the Gogane landslide was affected by the earthquake and partly failed. A majority of the earthquake-induced landslides (91%) were new landslides, while the others were enlarged old landslides. The earthquake-induced landslides occurred mainly on the steep slopes of V-shaped inner gorges and scarp slopes, in gneiss and quartzite strata of the Lesser Himalayas, and they were primarily associated with fractured rock masses. This analysis provides a framework for zoning areas vulnerable to earthquake-induced landslides.

Assessment of Earthquake‐Induced Landslide Hazard in Greece: From Arias Intensity to Spatial Distribution of Slope Resistance Demand

Abstract The earthquake‐induced landslide hazard in Greece is evaluated by means of a parametric time probabilistic approach. First, by taking into account the characteristics of seismicity affecting Greece, we calculated occurrence probabilities for different levels of seismic shaking expressed in terms of Arias intensity for a time interval of 50 yrs. Then, by utilizing the linkage of Newmark displacement with Arias intensity and critical acceleration, we mapped the spatial distribution of the slope strength demand that imply a 10% probability that displacement will exceed critical thresholds for potential seismic triggering of incoherent and coherent landslides. The obtained results suggest that the areas of western Greece and the broader region of the Gulf of Corinth are the most exposed to conditions of seismic triggering of both kinds of landslides. The derived slope strength demand maps could provide the basis for the assessment of whether particular slopes have a significant failure probability by comparing the estimated strength demand with the actual critical acceleration values calculated from slope material properties and slope angle. We exemplified this approach at a coastal area of the western Gulf of Corinth, where, by considering situations expected in dry and wet periods, we also demonstrated that earthquake effects on slope stability depend on the season of earthquake occurrence.

Landslides triggered by the 20 April 2013 Lushan, China, Mw 6.6 earthquake from field investigations and preliminary analyses

On April 20, 2013, a Mw 6.6 earthquake struck Lushan County of Sichuan Province, China, about 80 km southwest of the epicenter of the 12 May 2008 Wenchuan Mw 7.9 event. It triggered a large number of landslides in a broad area, causing causality and injuries and damages to roads, drains, dwellings, and infrastructures. Field investigations of coseismic landslides provide a basis for better understanding and illustrating the spatial distributions and hazards related to the Lushan earthquake-triggered landslides. Therefore, this paper presents results of field investigations of the Lushan earthquake-triggered landslides, which can be classified into two main categories of disrupted landslides and coherent landslides, and constitutes seven types. The disrupted landslides include five types: rock falls, rock slides, rock avalanches, soil falls, and soil slides; and the coherent landslides have two types: soil slumps and slow earth flows. All of the seven types of landslides can be observed in many field photos, aerial photographs, and/or satellite images. Among them, seven typical large-scale landslides, distributed in a northeast trending line consistent with the Shuangshi-Dachuan fault, which is presumably related with the earthquake, are described in detail. We prepared an original, emergency-based coseismic landslide inventory map based on visual interpretation of available post-earthquake aerial photographs and pre-earthquake satellite images, combined with results from emergency-based field investigations. The original inventory map registered 3884 landslides. Later, 11,761 more landslides were identified by much more careful and time-consuming visual interpretation based on the same aerial photographs and satellite images. Subsequently, all the 15,645 coseismic landslides were correlated with six common landslide impact factors, including elevation, slope angle, slope aspect, topographic position, stratum/lithology, and peak ground accumulation (PGA). These observational and compiled coseismic landslide data allow us to make subsequent compiling of a detailed and completed landslide inventory by visual interpretation of remote sensing images as well as further studies of landslides induced by the Lushan earthquake.

A PROBABILISTIC APPROACH FOR THE WAVES GENERATED BY A SUBMARINE LANDSLIDE

A numerical study aimed at probabilistically assessing the coastal hazard posed by tsunamis induced by one-dimensional submarine rigid landslides that experience translational failure is presented. The numerical model here utilized is the finite difference recreation of a linear, fully dispersive mild-slope equation model for wave generation and propagation. This recreated model has the capability to simulate submarine landslides that detach into multiple rigid pieces as failure occurs. An ad-hoc formulation describing the combined space-time coherency of the landslide is presented. Monte Carlo simulations are employed, with an emphasis on the shoreward-traveling waves, to construct probability of exceedance curves for the maximum dimensionless wave height from which wave statistics can be extracted. As inputs to the model, eight dimensionless parameters are specified both deterministically in the form of parameter spaces and probabilistically with normal distributions. Based on a sensitivity analysis, the results of this study indicate that submarine landslides with large width to thickness ratios and coherent failure behavior are most effective in generating tsunamis. Failures modes involving numerous slide pieces that fail in a very compact fashion, however, were observed to induce bigger waves than more coherent landslides. Rapid weakening in tsunami generation potential for some of the parameter combinations suggests that the hazard posed by submarine landslide tsunamis is strongly dependent on source features and local conditions and is only of concern for landslides of substantial dimensions.

SURFACE RUPTURE OF THE NORMAL SEISMIC FAULTS AND SLOPE FAILURES APPEARED IN APRIL 11<sup>th</sup>, 2011 FUKUSHIMA-PREFECTURE HAMADOORI EARTHQUAKE

On April 11th, 2011, Iwaki region of Fukushima prefecture was jolted by Fukushima-Prefecture Hamadoori Earthquake. Surface ruptures were observed along causative Idosawa and Yunotake normal faults. In addition to numerous small slope failures, a coherent landslide and building structures of Tabito Junior High School, bisected by Idosawa Fault, were found along the causative faults. A precise digital elevation model of the coherent landslide was obtained through the ground and air-born LiDAR surveys. The measurements of perimeters of the gymnasium building and the swimming pool of Tabito Junior High School have shown that ground undergoes a slow and steady/continual deformation.

On far field occurrence of seismically induced landslides

Earthquakes may induce landslides at large distances from the epicentral area. In the last two decades, there have been many studies of this phenomenon to determine the causes that contribute to the occurrence of landslides at very long distances from earthquake epicenter. In this study, which is based on previously published works, a worldwide database comprising 270 earthquakes, including 150 during the instrumental period, was compiled to analyse the features of those landslides that occur at much further distances than maximum expected distances. From the analysis of the compiled data, it was observed that susceptible slopes can be grouped into five broad geological categories: jointed rock, marly–clayey (cohesive) soils, granular alluvial and/or colluvial sediments, volcanic soils, and residual soil slopes. These categories were equally likely to be associated with far field disrupted landslides, whereas far field coherent landslides were more frequent on cohesive soil slopes. Other factors, along with slope susceptibility may also contribute to far field landslides and increase the size of the area affected. Among these factors, site effects, antecedent rain and occurrence of seismic series have been cited. The analysis of available data also showed that for events of the same magnitude, far field disrupted landslides may occur at greater distances than coherent ones. The same database has also been employed to determine, for the first time, the upper bound curves for the maximum observed epicentral distances of several types of landslides and the maximum area observed to be affected by landslides, both based on the epicentral intensity of the events.

Seismically-induced landslides in the Betic Cordillera (S Spain)

A database of seismically-induced landslides in the Betic Cordillera is presented. Data included were classified according to landslide typology. Most of them (≈80%) correspond to small size, disrupted landslides (including rock/earth falls and earth slides that disorganize as mass-movement progresses) and the remaining consist mainly of coherent landslides (slumps in soils and rock-slides). Deep seated induced landslides are uncommon in the study zone and have occurred only after the few events of large magnitude reported in the Cordillera. Data available show that events of small magnitude ( M w<5.0) can induce instabilities in the study zone for comparatively large distances (>10 km) when compared with available upper bound curves for maximum epicentral distances for seismic induced landslides, that concentrate along areas prone to landsliding, like river banks or slopes on soft materials, which points out the importance of the role of slope susceptibility on the occurrence of instabilities during earthquakes. Landslides in the database are then analyzed and a power-law relationship that relates earthquake size, measured as epicentral intensity ( I o), to maximum distance of induced landslide valid for the study zone is proposed. Although included data represent a clear partial and incomplete dataset, they show the landslide state of knowledge for this region.

Case History of Landslide Movement during the Northridge Earthquake

We document a case history of Northridge earthquake induced movement of a coherent landslide mass comprised principally of weathered, previously sheared siltstone. Information on the residual shear strength of the material through which the sliding occurred is available both from backanalysis of a static (rainfall-induced) failure on the same slope and laboratory test data. Postearthquake field reconnaissance data establishes the seismic slope displacements near the slope crest. A yield coefficient is estimated for the slope based on residual shear strengths and measured groundwater levels at the time of the Northridge earthquake. This yield coefficient is used with four horizontal equivalent acceleration time histories (that approximate the seismic demand within the slide mass) in Newmark sliding block displacement analyses. The calculated displacements range from approximately 20 to 90 mm, which compares well with observed displacements at the slide scarp of approximately 50 mm.

Comparative studies of gravity tectonics in Quaternary sediments and sedimentary rocks related to fold belts

Summary Two different modes of gravity tectonics have been active as deformation mechanisms in the Quaternary of Denmark, namely ‘gravity gliding’ and ‘gravity spreading’. Examples of gravity gliding can be found in the fairly slowly moving coherent landslides which characterize many coastal cliffs with lithologies intercalated with Tertiary marine clay or Quaternary interglacial clay. The gravity spreading mechanism generated folds and dislocation structures characteristic of glacial tectonics, mainly during the Weichselian glacier advances of the Pleistocene glaciation. Imbricate dislocation structures formed by diapiric rise of water-saturated clay into sand occur. This structure type is proposed to be characteristic for the gravity spreading model. Analysis of these gravity deformed Quaternary deposits serve as a model for structures occurring in large-scale tectonic settings. Among the megascopic tectonic settings mentioned, two regions are discussed. Firstly, the transverse thin-skinned thrust fault belt in the southern part of the North Greenland Fold Belt is demonstrated to contain the characteristics of a gravity gliding system. Secondly, the gravity spreading structures of the glacio-dynamic deformations in Denmark are suggested to have a megascopic equivalent in the plate-tectonic convergence zone of the Barbados Ridge.