Sensitivity Analysis of Soil Site Response Modelling in Seismic ... - ITC

and create more severe strong ground motions at the surface. Many earthquake prone cities are settled
over very susceptible areas with young deposits such as Mexico City (Seed, Romo et al. 1987), Loma
Prieta (Rodriguez-Marek and Bray 1999) and Kathmandu/Nepal. Having a high potential for earthquakes
and a susceptible local site environment, the situation in Kathmandu might turn a hazard into a
disaster in the future.
In microzoning the seismic hazard is assessed by means of expected response of the seismic waves for
a given (scenario) earthquake for a specific area. Since the local soil site has significant effects on the
seismic waves, “Ground Response” is one of the most important sections in microzoning. Additionally,
the subsurface and surface conditions in Kathmandu valley also are favourable for the occurrence
of secondary hazards such as landslides, liquefaction, and fires.
Ground response studies could be handled in different dimensions: one, two or three-dimensional.
One-dimensional ground response analyses are based on the assumption that the ground surface and
all material boundaries below the ground surface are horizontal and extend infinitely in all lateral directions
(Kramer 1996). The methods of one-dimensional ground response analysis are useful for level
or gently sloping sites with parallel material boundaries. The subsurface geology of Kathmandu Valley
is also assumed to be suitable for one-dimensional ground response analysis, as Kathmandu valley is
underlain by a thick deposit of alluvial and lacustrine deposits. In addition, the assumption that the
valley or another topographical feature could be one-dimensional is a major simplification.
For one-dimensional calculations of the ground response, SHAKE software is used widely since
1971(Ordonez 2002). Using SHAKE, the research problem then tends to focus on sensitivity analysis
of the seismic response modelling for Kathmandu city, which will try to define and simplify the information
needed and evaluate it within the concept of microzonation analysis. The main reason to
focus on sensitivity analysis is to try to get more information for the valley, where there is lacking in
data.
1.2. Problem Statement
In general terms, a seismic response analysis needs detailed geotechnical data, geology information,
thickness of the subsurface geology profiles and a specific earthquake accelerogram file. More details
on the required data will be explained in the coming section.
Many countries, which are vulnerable to earthquakes, are lacking in data for seismic response studies.
Estimating the soil site response for strong ground motion is costly. In order to quantify the expected
ground motion, we have to determine the manner in which the seismic signal is propagating through
the surface. Propagation is particularly affected by the local geology and the geotechnical ground conditions.
Borehole data, geotechnical, geological and geophysical parameters should be determined.
Such an investigation is not always easy to obtain hence you will need people with sufficient knowledge,
tools, and laboratories which is not always the case for many developing countries.
In Kathmandu the available data is not sufficient .To deal with this gap, numerical analysis could be
used additionally to the existing data. For seismic response analysis the readily available data was used
and then sensitivity analysis was applied. The aim of sensitivity analysis is to estimate the rate of
change in the output of a model with respect to changes in model inputs (Isukapalli 1999). A priority
order for the input parameters will be produced from the sensitivity assessment.
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Sensitivity Analysis Of Soil Site Response Modelling In Seismic Microzonation For Lalitpur, Nepal