SHALLOW SHEAR WAVE VELOCITY MODEL FOR THE KUMAON AND GARHWAL HIMALAYAS

Abstract

The region of Kumaon and Garhwal Himalayas that has been used for the present research work falls in central Himalayan seismic gap. Despite of high seismicity in the Himalayas, the quantitative study of the amplification of seismic waves is still lacking. Limited database available in the rough and inaccessible terrain of Kumaon and Garhwal region have resulted in limited research work related to the estimation of site amplification. The work presented in the thesis includes several locations in Kumaon and Garhwal Himalayas. In the present analysis, the site amplification functions have been estimated for Kumaon and Garhwal Himalayas using earthquakes and ambient noise data recorded locally at 27 sites from Kumaon and Garhwal region. Various methods have been applied on the records to generate shallow shear wave velocity models in Kumaon and Garhwal Himalayas. Later, the results derived from various methods are compared. No such in-depth study has been done in India yet, primarily because data acquisition in the terrains of Kumaon and Garhwal Himalayas is very tough. Following are the sections and methods used in the present research work: Multi- channel Surface Wave Analysis (MASW): The data has been acquired at nine fields of the Kumaon and Garhwal Himalayas. The data acquisition, processing and interpretation of recorded data in the Kumaon and Garhwal Himalaya using MASW have been discussed in detail in the research. The final outcomes of the method are 2D shear wave velocity profiles (VS30) and the approximate bedrock thickness 30 m beneath the surface. Geology of the stations considered in the present work has been correlated with the results obtained. Later in this study, the records are analyzed using three different methods: (a) Non- Reference site method : the horizontal to vertical (H/V) spectral ratio on ambient noise records, (b) Non- Reference site method : the horizontal to vertical (H/V) spectral ratio on strong ground motion records, and (c) Reference site dependent method : Standard Spectral ratio, Non- Reference site method: the horizontal to vertical (H/V) spectral ratio on ambient noise records (Nakamura’s Technique): The data has been regularly recorded and collected every year. The present study is done using the data from twenty seven locations of the Kumaon and Garhwal Himalayas. The recorded data at 27 stations has been processed for the research purpose i.e. ambient noise. Whenever the density of the data is more, best twenty records are selected considering good data quality at each station. The amplification functions ii i.e. predominant frequency of various sites and amplification levels are estimated using Horizontal to Vertical Spectral Ratio (HVSR) technique. The inversion of shear wave velocity model using global optimization technique was carried out. The shear wave velocity models up to 500 m have been estimated using ambient noise records. These shear wave velocity models after many genetic algorithm (GA) runs at individual stations can be used for seismic hazard studies in the area. Non- Reference site method: the horizontal to vertical (H/V) spectral ratio on strong ground motion records: Total 248 strong ground motion records at 22 stations has been processed for the research purpose. The shear wave velocity models up to 500 m have been estimated using strong ground motion records. The amplification functions i.e. predominant frequency of various sites and amplification levels are estimated using HVSR technique. The comparison between ambient noise data and strong ground motion data has been done. The inversion of shear wave velocity model using global optimization technique was carried out. The shear wave velocity models up to 500 m have been estimated using strong ground motion records and then compared with ambient noise records. The two dimensional shear wave velocity profiles at nineteen stations of Kumaon Himalaya and eight stations of Garhwal Himalaya respectively are generated after inverting H/V curves using genetic algorithm for both ambient noise records as well as strong ground motion records. Shear wave velocity of Kumaon and Garhwal Himalayas i.e. VS500 varies from 462 ms-1 to 1348 ms-1 and Vs500 vary from 557 ms-1 to 1651 ms-1 respectively. These velocity values can be employed in various studies related to seismic hazard of the region. Velocity information is also important for simulation of earthquake strong ground motions. Reference site dependent method: Standard Spectral ratio (SSR): Such analysis has been carried out for total ten stations using strong ground motion data. Sediment to bedrock spectral ratio estimates between pairs of stations are compared with the ratio obtained using non- reference site estimate i.e. horizontal to vertical component spectral ratio of strong ground motion records and ambient noise. The site amplification factor for the region varies between 0.23 and 7.91 using HVSR (strong ground motion records) and between 0.3 and 6.01 using HVSR (ambient noise records). The estimated predominant frequency using ambient noise records and using strong ground motion data are found to be varying from 8.7 Hz to 11.6 Hz and from 9.8 Hz to 14.5 Hz, respectively.