Abstract
In this paper, we estimate the seismogenic energy during the Nepal Earthquake (25 April 2015) and studied the ground motion time-frequency characteristics in Kathmandu valley. The idea to analyze time-frequency characteristic of seismogenic energy signal is based on wavelet transform which we employed here. Wavelet transform has been used as a powerful signal analysis tools in various fields like compression, time-frequency analysis, earthquake parameter determination, climate studies, etc. This technique is particularly suitable for non-stationary signal. It is well recognized that the earthquake ground motion is a non-stationary random process. In order to characterize a non-stationary random process, it is required immeasurable samples in the mathematical sense. The wavelet transformation procedures that we follow here helps in random analyses of linear and non-linear structural systems, which are subjected to earthquake ground motion. The manners of seismic ground motion are characterized through wavelet coefficients associated to these signals. Both continuous wavelet transform (CWT) and discrete wavelet transform (DWT) techniques are applied to study ground motion in Kathmandu Valley in horizontal and vertical directions. These techniques help to point out the long-period ground motion with site response. We found that the long-period ground motions have enough power for structural damage. Comparing both the horizontal and the vertical motion, we observed that the most of the high amplitude signals are associated with the vertical motion: the high energy is released in that direction. It is found that the seismic energy is damped soon after the main event; however the period of damping is different. This can be seen on DWT curve where square wavelet coefficient is high at the time of aftershock and the value decrease with time. In other words, it is mostly associated with the arrival of Rayleigh waves. We concluded that long-period ground motions should be studied by earthquake engineers in order to avoid structural damage during the earthquake. Hence, by using wavelet technique we can specify the vulnerability of seismically active region and local topological features out there.
Highlights
The over-stressed earth generates energy in the form of earthquake and volcanoes
The buildings in Kathmandu valley are haphazardly constructed and do not follow the seismic design standards which specify the seismic vulnerability of Kathmandu valley
Comparing NS, East West (EW) and vertical motion, most of the high amplitude signals are associated with the horizontal motion because of high energy is released in that direction
Summary
The over-stressed earth generates energy in the form of earthquake and volcanoes. Earthquake is a global phenomenon cause by unexpected fault rupture within the crust. Kumar et al (2016) studied ground motion of Nepal earthquake occurred on 25 April 2015 using the data from sites (one rock site and three sedimentary sites) They found that the largest PGA was 241 cm/s2 recorded on the EW component at the rock site and that on horizontal component was 250 cm/s2. This analysis helps to understand the patterns relative to seismic events occurred on 25 April 2015 Nepal These figures show a time-frequency sketch of ground motion noted by TVU, THM, KTP and PTN stations, respectively. We can say that NS, EW and vertical motion as observed by four different stations where singularity present in signals observed during peak of aftershock It can be noticed in all figures that the highest wavelet coefficient amplitudes seems to be associated with the Rayleigh wave arrival. This can show how the energy transference takes place during the phenomenon
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