Ryuichi Yatabe

A study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis

Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future. This may cause serious damage in the Kathmandu Valley, which sits on an ancient lake bed zone, with lacustrine sediments of more than 500 m depth. In this study, the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise. The results of the H/V ratio show two peaks in about 20 percent of the locations, which are distributed mainly in and around the center and northern part of the Kathmandu Valley. The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area, whereas the second resonance frequency varies from 4 Hz to 6 Hz in the center and northern part of the valley. This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast — one may be from the surface layer and the other may be from the layer underneath. These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.

A replication of landslide hazard mapping at catchment scale

Landslide hazard assessment is a primary tool to understand the basic characteristics of slopes that are prone to landslides, especially during extreme rainfall. In this study, weights-of-evidence modelling a bivariate statistical method, and a logistic regression model, a multivariate statistical method, were used for landslide hazard mapping in two catchments of the Siwaliks in the Nepal Himalaya. Two typical catchments, Charnath and Jalad of the Siwaliks in easternNepal, were selected for the landslide hazard mapping. Both modelling approaches were applied by considering 10 intrinsic factors and one extrinsic factor. Mainly digital elevation model-based causative factors and field data were used to prepare data layers of landslide causative factors. In many approaches formodelling of landslide hazard in GIS, the model validation process is always dependent, and landslide data, which are used to calculate a landslide hazard index (LHI), are applied for verification. However, in this study, the LHI was calculated in one catchment (Jalad) and the same index for a different class of causative factors was applied for another catchment (Charnath), and the LHI wasverified. The verification results were very promising, with an independent prediction rate of about 75%. This validates weights-of-evidence and logistic regression models for landslide hazard assessment in the Siwaliks Range of Nepal.

Areal distribution of large-scale landslides along highway corridors in central Nepal

Landslides are the most frequent natural disaster in Nepal. As such, the scientific study of the Nepal landslides has been in progress for several years, but no significant achievement has been made in preventing landslides and mitigating disaster damage yet. As one important aspect of dealing with landslides is understanding their distribution pattern based on geological and geomorphological formations, this paper addresses these issues, and attempts to present a scenario of large-scale landslide distribution along the road corridors of major highways in central Nepal. As a result of landslide mapping using aerial photographs, topographical maps and field verification, the following points were understood: (1) the distribution of large-scale landslides is relatively dense over the area close to tectonic thrusts; (2) slate and phyllite zones have a greater ratio of landslide distribution; and (3) topographies with a mean slope angle of about 27–36 degrees have denser distribution of large-scale landslides.

Effect of Landslide Factor Combinations on the Prediction Accuracy of Landslide Susceptibility Maps in the Blue Nile Gorge of Central Ethiopia

Database construction for landslide factors (slope, aspect, profile curvature, plan curvature, lithology, land use, distance from lineament & distance from river) and landslide inventory map is an important step in landslide susceptibility modelling. Using the frequency ratio model, the weights for each factor classes were calculated and assigned in GIS so as to add these factors and produce landslide susceptibility index maps based on mathematical combination theory. However, before combining them, their independence among each other should be ascertained. For this, the correlation matrix of logistic regression was applied and this showed that most of the correlations between factors were either absent or very insignificant suggesting that all landslide factors are independent. From a set of eight landslide factors, a total of 247 landslide susceptibility map combinations can be generated. However, for simplification, only 28 landslide susceptibility maps were chosen. Then the best landslide susceptibility map was selected based on high prediction accuracy. But, when there is similarity in the prediction accuracies of different combinations, the landslide susceptibility index difference values can be used as another selection criterion. Hence, the susceptibility map from a combination of all landslide factors except distance from river was found to be the best one. Among the 28 representative combinations, landslide susceptibility maps with the same prediction accuracy of 87.7% have been found in spite of their dissimilarity in their difference values. The combination, with a limited number of landslide factors and the highest prediction accuracy of 87.7%, was found from a combination of slope, lithology, land use and distance from lineament. In order to validate the prediction model, landslides were overlaid over the landslide susceptibility map and the number of landslides that fall into each susceptibility class was calculated. From this analysis 0.39%, 1.84%, 9.1%, 32.04% and 56.63% of the landslides fall in the very low, low, medium, high and very high landslide susceptibility classes respectively. Since 88.67% of the landslides fall in the high and very high susceptibility classes, the landslide susceptibility map can be considered reliable to predict future landslides.

Initiatives for earthquake disaster risk management in the Kathmandu Valley

Situated over the Himalayan tectonic zone, Kathmandu Valley as a lake in geological past has a long history of destructive earthquakes. In recent years, the earthquake risk of the valley has significantly increased due mainly to uncontrolled development, poor construction practices with no earthquake safety consideration, and lack of awareness among the general public and government authorities. Implementation of land use plan and building codes, strengthening of design and construction regulations, relocation of communities in risky areas, and conduction of public awareness programs are suitable means of earthquake disaster risk management practice. Kathmandu, the capital of Nepal, is still lacking earthquake disaster risk management plans. So, this paper highlights some initiatives adopted by both governmental and nongovernmental organizations of Nepal to manage earthquake disaster risk in the Kathmandu Valley. It provides some comprehensive information on recent initiatives of earthquake disaster risk management in the valley and also highlights the outcomes and challenges.

Seismic Microzonation of Densely Populated Area of Kathmandu Valley of Nepal using Microtremor Observations

The Kathmandu Valley with an estimated current urban population of 5 million has been shaken many times in the past by damaging earthquakes. The valley was formerly a lake and although it is clear that much of the historical damage in the valley arose from seismic amplification associated with thick lake sediments, which in places exceed 500 m, few studies have been undertaken to understand the variability of seismic site response throughout the valley. In this study, we map the spatial variability of the dominant period of amplification using a horizontal-to-vertical spectral ratio technique derived from recordings of ambient noise. The resulting map is a first step towards seismic microzonation. We find that the area near the center of the valley, corresponding to the thickest sedimentary deposits, is associated with dominant periods that approach 2 s. Long-period shaking during future earthquakes is expected to be especially severe in this part of the valley since it corresponds to the urban center where numerous tall structures and long-span bridges with relatively long natural periods are to be found.

Disaster risk reduction knowledge of local people in Nepal

BackgroundNepal is highly vulnerable to natural disasters. A high proportion of the national GDP is lost every year in landslides, floods, and many other forms of disasters. A high number of human casualties and loss of public and private property in Nepal due to natural disasters may be attributed to inadequate public awareness, lack of disaster preparedness, weak governance, lack of coordination among the concerned government agencies, inadequate financial resources, and inadequate technical knowledge for mitigating the natural disasters. In this context, quite a few awareness and training programs for disaster risk reduction (DRR) have already been initiated in Nepal and their impact assessments are also already documented. However, effectiveness of the various implemented DRR programs is not yet evaluated through an independent study.ResultsThe work presented in this paper explores local people’s knowledge on disaster risk reduction (DRR). Altogether, 124 local people from 18 to 74 years of age from randomly selected 19 districts of Nepal were interviewed focusing on various questions on disaster information, disaster knowledge, disaster readiness, disaster awareness, disaster adaptation, and disaster risk perception. The collected response data were statistically analyzed using histogram and independent sample t-tests to examine the DRR knowledge of people. An independent t-test analysis (Table 1) suggests that there is no statistically significant gender-based difference in disaster knowledge, disaster readiness, disaster awareness, and disaster risk perception of the surveyed people. Disaster adaptation capacity of the local people was evaluated and more than 60 percent of the respondents were determined to adapt state of disaster in the community.ConclusionsFindings of this independent research confirmed that the DRR education initiatives implemented in Nepal are not enough. The questionnaire survey results have pointed out at a few deficiencies in disseminating DRR knowledge in Nepal. We hope these findings will encourage the line agencies working in DRR issues in Nepal to modify their programs targeted for the local communities.

Residual-state creep behavior of typical clayey soils

Few researchers have studied the creeping displacement behavior of clayey soils using a triaxial compression cell and oedometer; however, in most cases, they have concentrated on the pre-peak state of shear. Clayey soil from a landslide is assumed to have already reached the residual-state, necessitating a study on residual strength to understand the creeping displacement behavior of clayey soils from landslides. In this work, an existing torsional ring shear apparatus was modified to understand the creeping displacement behaviors of typical clayey soil. The newly developed creep test apparatus is capable of measuring the displacement with respect to time under the application of a constant creep stress. This paper focuses mainly on residual-state creep behaviors of typical clayey soils. Residual-state creep failure prediction curves are also proposed, which may be used to predict failure time and displacement of creeping landslides in the future.

Vulnerability assessment and retrofitting of existing school buildings: a case study of Aceh

Purpose – As the widely held perception is that retrofitting is a complicated process and is expensive, the purpose of this paper is to analyze cost effectiveness of retrofitting existing buildings in order to make them safer against earthquake and also to add child friendly features. This case study focuses on the simple method of retrofitting on rectangular single storey existing school buildings. The school buildings are of different types, based on material, shape and size, number of storeys and their vulnerability to earthquake is different case by case. The paper also outlines the process of vulnerability assessment and approach to retrofitting.Design/methodology/approach – The paper is based on the data collected from a case study carried out in Aceh (Indonesia) as a part of an assessment of vulnerability of newly constructed school facilities and retrofitting them in order to increase their seismic performance.Findings – One of the key findings of this study is that retrofitting can be achieved th...

Effect of Shearing Rate on Residual Strength of Landslide Soils

The change in shearing rate due to the earthquake or volcanic activities may affect the residual strength of the slip surface soils of a reactivated landslide. Few studies have been performed to address the effect of shearing rate on residual strength of soils. However, the shearing rate effect and their mechanism on residual strength are not understood yet. In this study, the slow shearing rate effect on residual strength of landslide soils are studied using a ring shear machine. The shearing rate in the ring shear tests are varied in a range of 0.073 mm/min–0.586 mm/min (as 0.073 mm/min, 0.162 mm/min, 0.233 mm/min, 0.313 mm/min, 0.398 mm/min, and 0.586 mm/min) at an effective normal stress of 98.10 kN/m2. Test results show that a slight increased in residual strength after the shearing rate of 0.233 mm/min. Based on the test results, the shear-rate effect mechanism on residual strength of landslide soils are discussed.