The Uttarakhand Himalaya which lies in the central seismic gap region of Himalaya is divided into the Garhwal and Kumaon Himalayas. Several studies by different workers confirm differential attenuation behaviour of shear wave in the Garhwal and Kumaon Himalaya. In the present study, the shear wave velocity structure has been estimated using strong motion data and ambient noise data recorded in these two regions. In this study, the SH-wave transfer function of the Haskell–Thompson matrix has been calculated using the initial shear wave velocity model selected through the genetic algorithm (GA) approach and is compared with the HVSR curve obtained from strong motion data and ambient noise data. The total number of 38 strong motion records together with the ambient noise record that lies between the MBT and MCT has been used in the present study. The obtained velocity model has been used to prepare three-dimensional velocity surrounding Askot which confirms the presence of klippe in the region. The average shear wave velocity at different depths has been calculated for all investigated data in Garhwal and Kumaon Himalaya. The comparison of the average shear wave velocity model obtained for the Garhwal and Kumaon regions shows that the shear wave velocity in Garhwal Himalaya is comparatively higher than that in the Kumaon Himalaya. The average shear wave velocity at 30 m depth in Garhwal and Kumaon Himalaya is found to be in the range of 274–577 and 233–531 m/s, respectively. The results show that the region of Garhwal and Kumaon Himalaya that was taken in the present study fall under the C and D type site category according to National Earthquake Hazards Reduction Program (NEHRP) site classification. Low-velocity stioff soil mostly covers the Kumaon Himalaya region while in Garhwal Himalaya, the region is covered with high-velocity soft rocks. Most of the regions in Kumaon Himalaya are covered by high predominant frequency values as compared to that in the Garhwal Himalaya which indicates the presence of thin sediment layers overlying the basement rocks in Kumaon Himalaya. The average predominant frequency in Garhwal and Kumaon Himalaya ranges from 2.4–15 and 2.5–18.4 Hz, respectively. The anelastic attenuation factor has also been calculated using obtained shear wave quality factor and obtained shear wave velocities at different depths which supports the high attenuation of seismic energy in Kumaon Himalaya as compared to the Garhwal Himalaya.

Site and topography effects are integral part of strong ground motion recorded during an earthquake. Site effects due to shallow subsurface velocity and topographic changes have been clearly seen in the Gorkha earthquake, 25th April, 2015 ($M_w$ 7.8) at Kapkot and Berinag stations, which lies at an epicentral distance of 507 and 485 km, respectively. The high peak ground acceleration was recorded at Kapkot station that is at valley, while comparatively low peak ground acceleration was recorded at Berinag station that is at hill. This paper investigates the effect of site topography and shallow velocity structure on ground acceleration generated due to propagation of SH wave generated by a finite far-field rupture. The propagation of SH wave in a shallow subsurface earth model with the vertical variation of velocity can be modelled by finite difference (FD) method based on staggered algorithm that can effectively model the propagation of the seismic wave in isotropic as well as heterogeneous elastic medium. This paper discusses the role of staggered algorithm in the generation of particle motion at the surface of modelled earth characterized by surface topography and vertical distribution of elastic constants. The developed software for FD modelling of the medium has been tested for SH wave propagation in a purely elastic medium in terms of numerical stability, dispersion and boundary conditions. Numerical experiments show that the method effectively models the topography and thin surface velocity layers in the model for varying cases. The obtained surface acceleration records from the propagation of SH wave at Kapkot and Berinag clearly show that both the site amplification and topographic effects have played a vital role in shaping the accelerograms at these stations.

This paper investigates the three-dimensional frequency-dependent attenuation structure of the Garhwal Himalaya in the Indian subcontinent. Based on the distribution of earthquakes and recording stations in the Garhwal Himalaya, the entire region of 152 ${\times}$ 94 km$^2$ is divided into 108 three-dimensional uniform rectangular blocks. These blocks are assumed to be of thickness 5 km that extends to 15 km depth. Each block represents the rock of different attenuation coefficients. The S-phase of strong motion records has been used to estimate the shear wave quality factor in each block by the inversion of spectral acceleration data. The inversion of spectral acceleration data is based on the modified technique of Joshi (2007) and Joshi et al. (2010) which was initially given by Hashida and Shimazaki (1984). The earthquake data of 19 events digitally recorded by 33 stations of the strong motion network between 2005 and 2017 have been used in this paper. The outcome of the inversion process is the shear wave quality factor at all frequencies present in the records. The three-dimensional attenuation structure at various frequencies is presented in this paper and is correlated with the regional tectonics of the Garhwal Himalaya. The correlation of attenuation structure at 10, 12 and 15 Hz with the tectonics of the region indicates that the shear wave quality factor has a strong relationship with the tectonics of the region. The values of the shear wave quality factor at different frequencies obtained from inversion have been used to obtain a relation of shear wave quality factor Q$_{\beta}$(f)=107f$^{0:82}$ for the region of Garhwal Himalaya for frequencies 10–16 Hz. The comparison of obtained shear wave quality factor with other studied relations clearly indicates that the obtained relation is close to what has been obtained in earlier studies and thereby indicates the reliability of obtained three-dimensional shear wave attenuation structure from inversion of spectral data.