William Murphy

Patterns of movement in rotational and translational landslides

The movement of rotational and translational landslides occurs as a result of either sliding on discrete shear surfaces or ductile deformation within a shear zone. In this paper we examine the movement histories of a number of landslides that occurred in a variety of materials in a range of settings, and demonstrate that one of two movement styles is evident during accelerating phases for all landslides observed. The first style, which has previously been noted, has a linear form in a plot of 1/v against time (v is velocity). The second style has an asymptotic form in the same plot, trending toward steady-state movement rates. We propose that the linear form occurs in landslides in which crack propagation (i.e., shear surface generation) is the dominant process, whereas the second style occurs where movement is taking place across existing planes of weakness or as a result of ductile deformation processes. This study demonstrates that the evaluation of plots of 1/v against time for landslides is a useful technique for forecasting movement styles and rates, and for interpreting ongoing deformation processes in real time

Enhancement of physical weathering of building stones by microbial populations

Two limestones from Crete, Greece and a dolomite from Mansfield, UK were subjected to combined microbial and physical weathering simulation cycles, in an attempt to assess the contribution of each agent of decay. Sound stone discs were exposed to different temperature and wet/dry cycling regimes involving treatment with distilled water or solutions of sodium chloride or sodium sulphate. Before the weathering cycles, half of the discs were inoculated with mixed microbial populations (MMP), originally recovered from decayed building stone of Portchester Castle, Hampshire, UK. The presence of MMP greatly accelerated the rates of deterioration of stone of all treatments, measured by weight change and alteration of hydraulic properties of stone. A combination of physical and biological processes significantly enhanced the extent of decay when compared with the physical or biological agents acting alone. Populations of heterotrophic, sulphur-utilising, halotolerant and moderately halophilic bacterial populations remained large throughout the experiment. Biofilms formed by populations of microorganisms were visualised by staining and assessed by colorimetric measurement of total carbohydrate in the stone substrate. The relative contribution of microbial and physical weathering to the process is discussed.

USING GIS IN THE PROBABILISTIC ASSESSMENT OF EARTHQUAKE TRIGGERED LANDSLIDE HAZARDS

Summit Ridge, located in the North Santa Cruz Mountains, California, was heavily affected by landsliding during the 1989 Loma Prieta earthquake (Mw=7.0). Widespread shallow failures, along with 17 deep-seated landslides, caused extensive damage. In order to study possible methods of assessing landslide hazard a geographical information system (GIS) was utilised. Geological, geotechnical, geomorphological and seismological data were used in a deterministic analysis using two standard earthquake slope stability models (pseudo-static and Newmark displacement). The models were used to assess the landslide potential that existed during the Loma Prieta event. It was found that, of these two models, the Newmark displacement model proved most successful at predicting the location of shallow unstable slopes. However, because both models are deterministic in nature they did not take into account any of the error and uncertainty in the input parameters. Therefore, a probabilistic Newmark displacement method of analy...

The El Salvador earthquakes of January and February 2001: context, characteristics and implications for seismic risk

The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes Mw 7.7 and 6.6, respectively. The two events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very weak and present a major obstacle to risk mitigation. q 2002 Elsevier Science Ltd. All rights reserved.

Airborne remote sensing for landslide hazard assessment: a case study on the Jurassic escarpment slopes of Worcestershire, UK

This paper describes the application of airborne remote sensing to the study of landslides on the clay-dominated slopes of the Cotswolds Hills between the towns of Broadway, Worcestershire, and Snowshill, Gloucestershire, in the UK. The project involved an initial desk study, airphoto interpretation and field survey in order to provide detailed information about the nature and extent of the landsliding in the area. High-resolution Airborne Thematic Mapper (ATM) imagery was acquired by the NERC Airborne Remote Sensing Facility, which was subsequently processed in order to develop a remote sensing method for landslide identification using airborne multispectral data. A range of image processing methods are described including colour composite enhancement and thermal imaging, while the focus of the paper will be on the development of a semi-automated method of landslide identification using image classification and texture analysis. Results from the first stage of the study have shown that the use of image processing techniques such as colour composites and thermal imaging can provide information on the ground surface not visible in conventional aerial photography. In this case study, this has included more detailed geomorphological information on landsides in the area and the nature of the cambering and gulls at the top of the escarpment. The second part of the study has investigated the use of image texture enhancement as a method of landslide identification, applied in isolation and incorporated into an image classification scheme as a semi-automated method of landslide identification. Results from this investigation indicate that landslides can be identified automatically with a high degree of accuracy (83%) and that by using image texture, the image classification technique is able to successfully differentiate between areas of landslide activity and stable slopes in the airborne imagery. Its is clear from the results of this study that in order to identify landslides in these types of clay-dominated terrains, image texture must be used. Inland landslides, like those on the Cotswolds escarpment, do not have a spectral signature but they do exhibit a distinct spatial signature that allows them to be identified in airborne imagery using textural analysis. The semi-automated method of landslide identification described in this paper represents a rapid method of terrain evaluation and landslide hazard assessment, which can be undertaken prior to more detailed field mapping.

Topographic controls on coseismic rock slides during the 1999 Chi-Chi earthquake, Taiwan

Stability back-analyses of sixteen massive, disrupted rock slides triggered by the 1999 Chi-Chi earthquake in the Tachia Valley in Central Taiwan indicated that geological and geotechnical factors alone appear insufficient to explain the distribution of failures under regional shaking. This incidence of landsliding suggests site amplification. A strong topographic effect can be identified when slope morphology and topography are considered in conjunction with strong motion data and theoretical models of topographic amplification to assess the failures and their distribution. The landslides triggered under these conditions are shallow and with a crown located near the crest of the slopes. Slope orientation and slope height provide easily observable field indices that are particularly important, as data suggest that these parameters, together with the wavelength of the seismic waves, control the amplification of ground motions. These observations could provide key inputs to assess unexpectedly high ground accelerations for landslide and seismic hazard studies.

3D ground model development for an active landslide in Lias mudrocks using geophysical, remote sensing and geotechnical methods

A ground model of an active and complex landslide system in instability prone Lias mudrocks of North Yorkshire, UK is developed through an integrated approach, utilising geophysical, geotechnical and remote sensing investigative methods. Surface geomorphology is mapped and interpreted using immersive 3D visualisation software to interpret airborne light detection and ranging data and aerial photographs. Subsurface structure is determined by core logging and 3D electrical resistivity tomography (ERT), which is deployed at two scales of resolution to provide a means of volumetrically characterising the subsurface expression of both site scale (tens of metres) geological structure, and finer (metre to sub-metre) scale earth-flow related structures. Petrophysical analysis of the borehole core samples is used to develop relationships between the electrical and physical formation properties, to aid calibration and interpretation of 3D ERT images. Results of the landslide investigation reveal that an integrated approach centred on volumetric geophysical imaging successfully achieves a detailed understanding of structure and lithology of a complex landslide system, which cannot be achieved through the use of remotely sensed data or discrete intrusive sampling alone.

Understanding the recession of the Holderness Coast, east Yorkshire, UK: a new presentation of temporal and spatial patterns

Abstract The Holderness coastline is known to be one of the most rapidly retreating coastal regions in Europe. Previous studies on the recession of this coastline have often concentrated on providing a single annual value for the whole coast or for large subdivisions of it; however, relatively little attention has been given to the overall spatial and temporal variability. This paper summarizes and critically appraises the work previously undertaken in this region, presents the results of the former recession rate investigations and displays new interpretations of the data. This assessment found there to be a knowledge gap relating to the processes involved in the recession of this coastline, particularly with regard to frequency of high recession events, further knowledge of which could assist in the planning of the region. It is concluded that many of the former investigations are inadequate by todays standards, because of either the methods employed or the manner in which the results are displayed. Significant steps in gathering high-quality data relating to the erosion of this coastline have been made by the East Riding of Yorkshire Council with the initiation of their Erosion Post monitoring scheme and more recently by their dGPS monitoring. However, if further advancement is to be made in the understanding of the erosion of this region, this work will need to be supplemented with geomorphological monitoring of the cliff line, which will further resolve the processes occurring and aid the production of predictive models. These geomorphological data could be obtained through employment of traditional methods as well as new techniques such as laser scanning or digital photogrammetry. Supplementary Material A table giving the magnitude and variability in recession at each Erosion Post location and graphs showing the actual annual variations for each Erosion Post, calculated using a filtered version of the Erosion Post dataset, are available at http://www.geolsoc.org.uk/sup18345.

Uncertainty in ground motion estimates for the evaluation of slope stability during earthquakes

Three examples of landslides triggered by earthquakes have been examined to consider the natural variability of the slope forming materials, and the uncertainties surrounding input ground motions. These were the landslides at Villa Del Monte, California, Tachia Gorge, Taiwan and Las Colinas, El Salvador which were triggered by the Loma Prieta, (MW=7.0, 1989), Chi Chi, (MW=7.6, 1999) and El Salvador (MW=7.6, 2001) earthquakes respectively. The results of these analyses show a large scatter in the calculated factors of safety for earthquake conditions, some of which indicated stability when failure actually occurred. The models used in the calculation of seismic slope stability yield acceptable results. However, it is clear that the natural variability of slope forming materials has a strong influence on the range of results reported for the analysis of slopes subjected to strong shaking. This variability is exacerbated by limited knowledge about the interaction with seismic waves and slopes resulting in topographic amplification. Such problems are often further complicated by uncertainty and error associated with attenuation relationships.

The geomorphological controls on seismically triggered landslides during the 1908 Straits of Messina earthquake, Southern Italy

Abstract Investigation of landslides which have been located from contemporary accounts of the Straits of Messina earthquake of 1908 reveals that a failure envelope for the slopes may be drawn on the basis of slope height-slope angle relationships for static and dynamnic stresses. As would be expected there is a decrease in stability during the earthquake to a degree which is almost certainly linked to the generation of pore water pressures. However, a region of the failure envelope shows much greater decrease in stability; this section of the curve describes the stability of low slope height-high slope angle forms. The area for which this stability curve has been calculated is affected by a nearfield earthquake. It is suggested that these slopes are suffering from forced vibration from high-frequency accelerations. In support of such a hypothesis of a geomorphological control is the observations that many of the landslides along this section of coastline occurred at topographic irregularities.