Analysis of Shallow Seismic Waves to Determine Geo-technical Characterization of Major Earthquake Affected Sites of Kathmandu Valley (original) (raw)
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Journal of Geoscience and Environment Protection
The present study was carried out to evaluate resonant frequency of the ground and to characterize subsurface ground based on shear wave velocity structure. For this, five sites were selected such as Pulchowk, Chhauni, Gaushala, Buddhanagar and Bhainsepati. About 20 data were recorded in each site and then shear wave velocity structure and graph of amplification ratio with their spatial distribution has been established with the help of software i.e. Seisimager/Seismodule Controller. The results of both analysis methods were then compared to the amplitude of the Gorkha Earthquake and borehole data. All these data and study indicates that the Kathmandu Valley sediments are dependent on the frequency of the seismic waves and the wave velocity is greater in the peripheral region than in the central part of the Valley. The result had also shown that the presence of silty-sand, clay and loose gravel soil with low bearing capacity and elastic modulus in most of the sites are responsible for devastation. It was also noted that apart from few limitations, a non-intrusive microtremor analysis can be adopted for earthquake site characterization in the Kathmandu Valley which can be readily applied and expanded upon in future seismic hazard and microzonation efforts for Kathmandu.
Geotechnical and Structural Aspect of 2015 Gorkha Nepal Earthquake and Lesson Learnt
Journal of the Institute of Engineering
An earthquake of moment magnitude (Mw ) 7.8 struck the central Nepal at 11:56 am on April 25, 2015. More than 9,000 people were killed and thousands of residential buildings, and hundreds other structures were also destroyed. An aftershock of moment magnitude (Mw ) 7.3 hit northeast of Kathmandu on May 12 after 17 days of main shock which caused additional damages. Immediately after the earthquake, authors undertook a field investigation and visited the affected areas. Strong motion records from both earthquakes and their impacts on structures as well as geotechnical issues are presented in this paper. Most of the structures in Nepal are made of adobe, unreinforced masonry, and reinforced concrete. Failure mechanisms of those buildings are briefly explained in this paper. Geotechnical aspects such as soil liquefaction, slope failures, settlement and lateral spreading, and site amplification effects that considerably influenced the damage patterns at many areas are briefly discussed...
Lateral Ground Spread at Kausaltar, Kathmandu that Appeared in the 2015 Gorkha Earthquake
2021
The April 25, 2015 Gorkha earthquake jolted the central region of Nepal causing extensive damage to buildings and earthen structures in both mountainous and urban areas of Nepal. Kathmandu-Bhaktapur road, one section of the Araniko Highway, crosses a small valley in the center of the Kathmandu Basin with an embankment. This embankment and adjacent area were deformed in the earthquake. To examine the cause of this ground deformation, several in-situ tests such as micro-tremor measurements, standard penetration tests (SPT), multi-channel analyses of surface waves (MASW), and C 14 dating were conducted. These tests show that a silty sand layer with low plasticity has most likely been liquefied 5 to 8 meters underground. It is also shown that groundwater lowering using existing wells can decrease the liquefaction-prone area by 81%.
Natural Hazards, 2013
The detrimental effects of an earthquake are strongly influenced by the response of soils subjected to dynamic loading. The behavior of soils under dynamic loading is governed by the dynamic soil properties such as shear wave velocity, damping characteristics and shear modulus. Worldwide, it is a common practice to obtain shear wave velocity (V s in m/s) using the correlation with field standard penetration test (SPT) N values in the absence of sophisticated dynamic field test data. In this paper, a similar but modified advanced approach has been proposed for a major metro city of eastern India, i.e., Kolkata city (latitudes 22°20 0 N-23°00 0 N and longitudes 88°04 0 E-88°33 0 E), to obtain shear wave velocity profile and soil site classification using regression and sensitivity analyses. Extensive geotechnical borehole data from 434 boreholes located across 75 sites in the city area of 185 km 2 and laboratory test data providing information on the thickness of subsoil strata, SPT N values, consistency indices and percentage of fines are collected and analyzed thoroughly. A correlation between shear wave velocity (V s) and SPT N value for various soil profiles of Kolkata city has been established by using power model of nonlinear regression analysis and compared with existing correlations for other Indian cities. The present correlations, having regression coefficients (R 2) in excess of 0.96, indicated good prediction capability. Sensitivity analysis predicts that significant influence of soil type exists in determining V s values, for example, typical silty sand shows 30.4 % increase in magnitude of V s as compared to silt of Kolkata city. Moreover, the soil site classification shows Class D and Class E category of soil that exists typically in Kolkata city as per NEHRP (Recommended provisions for seismic regulations for new buildings and other structures-Part 1: Provisions. Prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency (Report FEMA 450), Washington, DC, 2003) guidelines and thereby highlighting the seismic vulnerability of the city. The results
A ground investigation to inform earthquake hazard assessment in the Kathmandu Valley, Nepal
Proceedings of the XVII ECSMGE-2019, 2019
When designing ground investigations there is usually a requirement to fill large gaps in existing knowledge. In the developing world, specifically in seismic prone areas, this systematic lack of information affects the ability for practitioners to assess seismic risk. This paper presents the rationale and results of a ground investigation undertaken in Central Nepal to inform seismic hazard assessment, as part of the EPSRC funded project Seismic Safety and Resilience of Schools in Nepal (SAFER). Geological information and geotechnical parameters are presented. Downhole methodology has been used to derive an in situ shear wave velocity profile for seismic response analysis in a region which is currently reliant on seismic shear wave velocity correlations. This research presents discussion and comparison of velocity profiles obtained at other locations in the Kath-mandu Valley and introduces new information on the basin topography.
Soil Dynamics and Earthquake Engineering, 2018
Gandhinagar City (the Capital of Gujarat, India) falls under Zone III on the seismic zoning map of India where an earthquake of magnitude 6 can be expected. It is a well established fact that the site amplification/ shaking and damage is large in soil covered areas. To estimate the effect of soil on ground motion and to estimate the strong ground motion parameters at surface, soil modeling and the ground response analysis have been conducted along uniformly distributed 14 boreholes drilled upto a depth of 50 m. The methodology is divided into three parts (i) Estimation of depth of Engineering Bed layer (EBL) (a layer with a shear wave velocity 400 m/ s ≤ Vs ≤ 750 m/s, N value > 80 and minimum soil variation below it) through soil modeling, (ii) Estimation of Ground Motion at EBL due to scenario earthquake at nearby active fault and (iii) Estimation of surface strong ground motion using 1D ground response analysis through SHAKE 2000 program. The EBL is found at a depth of 21-33 m (shallower in central part and deeper in northern and southern parts). The Near-Field scenario earthquake (Eq.) of magnitude Mw 6.0 has been considered along East Cambay Fault (normal fault, 60°dip) located at about~20 km east and Far Field scenario Eq. of Mw 7.6 is considered along Kachchh Mainland Fault located~270 km west. The Peak Ground acceleration (PGA) of 0.172-0.237 g have been estimated at surface due to near field earthquake scenario. The mean spectral acceleration maps for 0.1-0.4 s, 0.4-0.7 s, 0.7-1.0 s and 1.0-1.5 s have also been computed. The mean spectral acceleration for the period of 0.1-0.4 s has been varying from 0.330 g to 0.508 g, for period of 0.4-0.7 Sec, it has been varying from 0.151 g to 0.161 g and for period between 1.0 and 1.5 Sec, it has been found from 0.83 g to 0.09 g. The PGA is found increased by 5-38% in the first subsurface soil layer in Gandhinagar city. The PGA of the order of 0.059-0.072 g and peak Spectral acceleration of the order of 0.187-0.259 g have been computed (with predominant periods of~0.1 s and 0.31 s) due to Far-Field Eq. scenario and are found less than Indian code. The PGA and Spectral acceleration (Sa) values are found higher than the Indian code in the period range of 0.1-0.4 s (one to four storey buildings) for Near Field Eq. Scenario.
International Society for Soil Mechanics and Geotechnical Engineering
2019
Sri Lanka has been considered as an aseismic country in the past considering the large distance from the island to the active plate boundaries. However, with the increased degree of urbanization the possible impact of intraplate earthquakes on population centres within the island has become important. In this context, deterministic and probabilistic seismic hazard assessments have been carried out by few researchers so far. However, these studies haven’t considered the influence of variation of bedrock profile on the seismic wave propagation. In the study presented here, a numerical simulation is carried out to investigate the effect of variation of the bedrock profile on the seismic wave propagation through Sri Lanka. The acceleration time histories of seven real time earthquake record, selected from the PEER data base were used as input in a numerical finite difference model simulating the two dimensional bedrock response. The resultant spectra at the bedrock surface thus obtained...
2017
An earthquake of moment magnitude (Mw) 7.8 struck Nepal at 06:11 UTC on April 25, 2015. A field reconnaissance was carried out immediately after the main shock. This paper presents the accelerograms and the geology of Nepal. The acceleration response spectra of the motions at the valley show a prominent amplification at the period of 5 sec. This paper describes the effects of local geology and topography on the damage severity during the earthquake. The damage patterns illustrate the strong influence of local geology conditions on the severity of the damage at many places, like soil amplification in Gongabu, Machhapokhari, Ramkot, Purano Naikap, areas along the major rivers in Kathmandu Valley with loose alluvium deposits, and ridge effects on the Swayambhu Nath hill and Chautara. The effect of low frequency amplification caused by the Kathmandu Valley basin is evident from the severe damage to well-designed tall buildings in Kathmandu. Severe damages including ground fissures and l...
Seismic site characterization is the basic requirement for seismic microzonation and site response studies of an area. Site characterization helps to gauge the average dynamic properties of soil deposits and thus helps to evaluate the surface level response. This paper presents a seismic site characterization of Agartala city, the capital of Tripura state, in the northeast of India. Seismically, Agartala city is situated in the Bengal Basin zone which is classified as a highly active seismic zone, assigned by Indian seismic code BIS-1893, Indian Standard Criteria for Earthquake Resistant Design of Structures, Part-1 General Provisions and Buildings. According to the Bureau of Indian Standards, New Delhi , it is the highest seismic level (zone-V) in the country. The city is very close to the Sylhet fault (Bangladesh) where two major earthquakes (M w [ 7) have occurred in the past and affected severely this city and the whole of northeast India. In order to perform site response evaluation, a series of geophysical tests at 27 locations were conducted using the multichannel analysis of surface waves (MASW) technique, which is an advanced method for obtaining shear wave velocity (V s ) profiles from in situ measurements. Similarly, standard penetration test (SPT-N) bore log data sets have been obtained from the Urban Development Department, Govt. of Tripura. In the collected data sets, out of 50 bore logs, 27 were selected which are close to the MASW test locations and used for further study. Both the data sets (V s profiles with depth and SPT-N bore log profiles) have been used to calculate the average shear wave velocity (V s 30) and average SPT-N values for the upper 30 m depth of the subsurface soil profiles. These were used for site classification of the study area recommended by the National Earthquake Hazard Reduction Program (NEHRP) manual. The average V s 30 and SPT-N classified the study area as seismic site class D and E categories, indicating that the city is susceptible to site effects and liquefaction. Further, the different data set combinations between V s and SPT-N (corrected and uncorrected) values have been used to develop site-specific correlation equations by statistical regression, as 'V s ' is a function of SPT-N value (corrected and uncorrected), considered with or without depth. However, after considering the data set pairs, a probabilistic approach has also been presented to develop a correlation using a quantile-quantile (Q-Q) plot. A comparison has also been made with the well known published correlations (for all soils) available in the literature. The present correlations closely agree with the other equations, but, comparatively, the correlation of shear wave velocity with the variation of depth and uncorrected SPT-N values provides a more suitable predicting model. Also the Q-Q plot agrees with all the other equations. In the absence of in situ measurements, the present correlations could be used to measure V s profiles of the study area for site response studies.