FINDING SEISMIC VELOCITY STRUCTURES OF NORTH-WEST HIMALAYAS USING S-WAVE DATA

Abstract

Ground vibrations caused by earthquakes were measured in the Garhwal Kurnaon region with an aim of seismic hazard microzonation for site response parameters. These parameters are resonance frequency' and ampliuication factor' i.e. amplification of ground motion at resonance frequency. From raw spectrum of ground motion data amplification of the horizontal motion with respect to the vertical motion (L-l/V) versus frequency was plotted to get the resonance frequency and amplification factor at various sites in Garhwal Kumaon region. The data show variation in these parameters from one area to another. This may he due to various reasons like variation in thickness and type of the soil layers. topography - of the area. etc. Contouring of resonance frequency, amplification factor and li/V values at various frequencies were also done. Our preliminary investigation shows that resonance frequency and amplification factor is affected by the presence of DIlR. which is a tectonic feature transverse to the Himalayas. In general resonance frequency increases and amplification factor decreases as we approach regions which lie along the trend of Dl IR. S wave velocity from the resonance frequency was also estimated by taking various cases of thickness. Further, inversion using resonance frequency values for more accurate estimation of velocity structure of shallow sedimentary deposits need to he carried out in the future.