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Mechanisms of Large Landslides and Seismic Ruptures

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Mechanisms of Large Landslides’ and Seismic Ruptures’ Formation – Basic and Applied Aspects

Tech Area / Field

  • OBS-GEO/Geology/Other Basic Sciences
  • ENV-MIN/Monitoring and Instrumentation/Environment

Status
3 Approved without Funding

Registration date
27.12.2000

Leading Institute
Institute of Seismology, NAS, Kyrgyzstan, Bishkek

Supporting institutes

  • Institute of Dynamics of the Geosphere, Russia, Moscow

Collaborators

  • Institute of Geological & Nuclear Sciences Limited, New Zealand, Lower Hutt\nArizona State University, USA, AZ, Tempe\nUniversity of Potsdam / Institute fur Geowissenschaften, Germany, Potsdam\nPatras University, Greece, Patras\nUniversite de Lille, France, Lille\nUniversity of Luton, UK, Luton\nUniversity of Maryland / Department of Geology, USA, MD, College Park

Project summary

Aims of Project are to study natural and artificial landslides and surface ruptures, to develop reliable model(s) of landslide formation and to reveal the criteria indicating fast or slow style of rupturing by extensive use of the data obtained in the course of underground chemical and nuclear explosions’ investigations.

Tasks of Project.

– to study the landslide scars structure, debris morphology and internal structure with such details, that will allow to select different types of such phenomena and to see if their peculiarities can be explained by the existing mechanical models;


– to develop the mechanical model(s) of landslide formation that will explain all basic peculiarities of natural and artificial landslides’ internal structure and morphology revealed in the frames of the Project and described in the literature;
– to single out geological, topographical and geodynamic factors favourable for the large landslide’s formation, that can be used as the grounds of landslide hazard assessment, and factors that govern the long-term and short-term stability of the landslide dams;
– to find typical features of the deformations that can be interpreted as the evidence of impulse (seismic) or slow (creep) surface rupturing with confidence.

Lead-in and review.

Large landslides and surface ruptures represent hazardous natural phenomena in mountainous regions, dangerous itself and tightly bounded with strong earthquakes.

Landslides often produce either ultra-mobile rock avalanches, that pose a threat to the vast areas at the mountain's feet, or high natural dams that lead to valley's submergence and which subsequent breach could cause catastrophic flood. At the same time they could be considered as the unique natural model of the behaviour of large masses of the non-uniform materials during high-speed motion and abnormally high loading. It should be pointed out that similar combination of forces and the affected volumes of rocks could be obtained artificially by large chemical or nuclear explosion only. Numerous models of the large landslide mobility were suggested (Kent 1966, Grigoryan 1979, Erismann 1979, 1986, Davies 1982, Campbell 1989, Melosh, 1979, 1983, 1986, Shaller, 1991, Xiaoning 1991, Kobayashi 1993) but most of them did not take into account all known peculiarities of their morphology and internal structure (Shaller, 1991, Strom, 1994, 1996, 1999). As the result, the mechanism of large landslide's formation is still under discussion (see, for example,. Erismann, 1986).

Recent surface ruptures which are the result of the source rupture propagation up to the surface, can be rather hazardous itself if they cross structure’s foundations, and are extremely important natural phenomena since they are interpreted as a rule as main geological effect of the past strong earthquakes. Their study and dating is the main modern technique of paleoseismological investigations and one of the grounds of seismic hazard assessment (McCalpin, 1996). Therefore a problem arises – if all such features can be interpreted in such a way or only some of them were really caused by impulse seismic rupturing while others – by significantly slower motions (tectonic creep).

Large landslides are accompanied by conversion of tremendous amount of the potential energy of rocks resting at high slopes to the other forms of the energy that could be compared with the energy release by nuclear explosion. Therefore some regularities of the behaviour of large amounts of rocks featuring high nonstationary loads, that were found out in the course of underground nuclear explosions’ investigations, can be applied to explain peculiarities of large-scale rock slides’ structure and motion. Large explosion can be considered as the artificial earthquake as well, that will help to understand conditions that support the formation of earthquake-triggered landslides – one of the most destructive effects of strong earthquakes – by comparison of the structure and morphology of the natural landslides and of the landslides triggered by large explosions, nuclear tests in particular.

Even bigger energy release is typical of the rapid tectonic movements which lead to the earthquakes accompanied by surface rupture’s formation. Despite the significant differences of the processes that take place in the large explosion and in the earthquake source, fractures generated above the source of powerful underground explosion can be considered as the analogue of the seismic surface ruptures. Therefore one more possible trend of the explosion mechanics application for the investigations of the hazardous natural processes deals with the search of the criteria indicating the stile of rupturing of the recent faults that are considered to be associated with past earthquakes.

Large rock slides millions to billions cubic meters in volume and seismic ruptures in Kyrgyzstan would be used for such comparison. Features of both types are widely spread in Tien Shan, many of them ere easily attainable, with good outcrops (for example in the Chong-Kemin, Naryn, Kokomeren, Aksu, Karasu river valleys) and could be studied in details. Technique of such studies was developed during previous investigations that were carried out by the Project participants (Kyrgyz Institute of seismology - KIS and Institute for dynamics of geosperes -IDG) (Korjenkov, 1997; Korjenkov and Charimov, 1993; Korjenkov and Chedia, 1986; Korjenkov et al., 1994, 1999a, b; Strom, 1994, 1996, 1999a,). Data on large chemical and nuclear explosions will be provided by IDG.

Expected results.

– New qualitative and quantitative data on the large landslide’s morphology and internal structure that will supply field constraints necessary to test and validate theoretical models and provide data for the numerical modelling of the landslide formation and motion.


– Mechanical model(s) of the landslide formation and motion taking into account all basic peculiarities of their morphology and internal structure that will be found out in the frames of the Project and are described in the literature.
– New quantitative grounds of the landslide hazard assessment including high landslide blockages breach hazard, that can be applied for the assessment of stability of hazardous blockages such as the Karasu, Sary-Chelek and others in Kyrgyzstan, as well as the Sarez lake dam in the Pamirs.
– Constraints that should be taken into account for the blast-fill dams (artificial landslides) design.
– Description of rupture’s peculiarities, indicating the stile of rupturing motion (rapid, associated with the earthquake, or slow creep motion)

Volume of work.

The Project is concentrated on the mechanical analysis of the natural hazardous phenomena.

The preliminary stage includes:

– elaboration of the joint scientific program and the Project schedule;


– preparation of the personal programs of the participants;
– preparation of the agreement with the ISTC.

The main stage includes:

1. Field and laboratory investigations of the landslides and recent ruptures in Tien Shan mountains.

– compilation of the large scale geomorphic maps and digital elevation models (DEM) of the landslides and recent ruptures on the basis of the remote sensing data – aerial and high resolution space images and detailed topographic maps;
– reconstruction of the pre-landslide relief for some of landslides by use of appropriate software;
– detailed mapping of landslides/rock avalanches bodies and their scars by the laser total station;
– details field study of minor geomorphic features that can shed a light on the mechanism of landslide and rapture formation;
– large scale geological mapping of the selected landslides, localisation of different lithologic and petrologic units both at the landslide scars and in the landslide deposits with use of data of the seismic reflection survey;
– estimation of physical properties (density, strength of the uniaxial compression (Rc) and tension (Rp) at the sample scale) of different lithologic units both in scars and landslide bodies and in the loose material, displaced by recent ruptures by use of analogues and by direct tests, and also determination of the some parameters in the natural conditions with use of the seismic reflection survey;
– study of grain size composition of each unit that would be identified in the landslide body by use of the “linear measurements” method and “weight analysis”;
– laboratory study of fine fraction’s content, mineralogy and particle’s shape;
– trenching of the selected seismic ruptures of the historical earthquakes and prehistoric recent faults and detail description of deformations in their fault zones;
– shallow seismic reflection survey across the rock avalanches/landslides bodies, and also across the fault scarps with the aim of recognition of their internal structures.


2. Analysis of the data available on the effects of large chemical and nuclear explosions on the host rock massifs and on the neighbouring slopes stability.

– data obtained at the Novaya Zemlia and Semipalatinsk nuclear tests sites;
– data on the 1989 Uch-Terek experimental explosion of approximately 2000 tons of explosive material in Kyrgyzstan.


3. Modelling and analyses.

– comparison of the “initial” (in the scar) and final (in the landslide deposits) rock sequences;
– comparison of the peculiarities of the natural and artificial (caused by explosions) landslides;
– analysis of the previously proposed models of the landslide motion, their conformity and discordance with the results of the field and laboratory studies;
– development of the mechanical model of the landslide mechanism and numerical modelling of the transfer from the geotechnical model of the initial rock massif to the final landslide deposit geotechnical model;
– comparison of the peculiarities of the natural and artificial surface ruptures;
– selection of the deformation criteria indicating either rapid (seismic) or slow (tectonic creep) sense of faulting;
– numerical modelling of the natural and artificial large landslide dams development with time with due regard to the data on their internal structure and geotechnical properties obtained in the frames of the Project and selection of the main factors, determining dam’s long-term and short-term (in the case of overtopping) stability.


4. Analysis of the further practical/commercial application of the results, obtained in the course of the Project realisation.


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