SUMMARY A seismic hazard map of the territory of India and adjacent areas has been prepared using a deterministic approach based on the computation of synthetic seismograms complete with all main phases. The input data set consists of structural models, seismogenic zones, focal mechanisms and earthquake catalogues. There are few probabilistic hazard maps available for the Indian subcontinent, however, this is the first study aimed at producing a deterministic seismic hazard map for the Indian region using realistic strong ground motion modelling with the knowledge of the physical process of earthquake generation, the level of seismicity and wave propagation in anelastic media. Synthetic seismograms at a frequency of 1 Hz have been generated at a regular grid of 0.2°× 0.2° by the modal summation technique. The seismic hazard, expressed in terms of maximum displacement (Dmax), maximum velocity (Vmax), and design ground acceleration (DGA), has been extracted from the synthetic signals and mapped on a regular grid over the studied territory. The estimated values of the peak ground acceleration are compared with the observed data available for the Himalayan region and are found to be in agreement. Many parts of the Himalayan region have DGA values exceeding 0.6 g. The epicentral areas of the great Assam earthquakes of 1897 and 1950 in northeast India represent the maximum hazard with DGA values reaching 1.2–1.3 g. The peak velocity and displacement in the same region is estimated as 120–177 cm s−1 and 60–90 cm, respectively.
We present GPS velocities in Kashmir valley and adjoining regions from continuous Global Positioning System (cGPS) network during 2008 to 2019. Results indicate total arc normal shortening rates of ~ 14 mm/year across this transect of Himalaya that is comparable to the rates of ~ 10 to 20 mm/year reported else-where in the 2500 km Himalaya Arc. For the first time in Himalayas, arc-parallel extension rate of ~ 7 mm/year was recorded in the Kashmir valley, pointing to oblique deformation. Inverse modeling of the contemporary deformation rates in Kashmir valley indicate oblique slip of ~ 16 mm/year along the decollement with locking depth of ~ 15 km and width of ~ 145 km. This result is consistent with the recorded micro-seismicity and low velocity layer at a depth of 12 to 16 km beneath the Kashmir valley obtained from collocated broadband seismic network. Geodetic strain rates are consistent with the dislocation model and micro-seismic activity, with high strain accumulation (~ 7e−08 maximum compression) to the north of Kashmir valley and south of Zanskar ranges. Assuming the stored energy was fully released during 1555 earthquake, high geodetic strain rate since then and observed micro-seismicity point to probable future large earthquakes of Mw ~ 7.7 in Kashmir seismic gap.
SUMMAR YSince the installation of three limited-aperture strong-motion networks in the Himalayan region in 1986, six earthquakes with M w =5.2±7.2 have been recorded up to 1991. The data set of horizontal peak accelerations and velocities consists of 182-component data for the hypocentral distance range 10±400 km. This data set is limited in volume and coverage and, worst of all, it is highly inhomogeneous. Thus, we could not determine regional trends for amplitudes by means of the traditional approach of empirical multiple regression. Instead, we perform the reduction of the observations to a ®xed distance and magnitude using independently de®ned distance and magnitude trends. To determine an appropriate magnitude-dependent distance attenuation law, we use the spectral energy propagation/random function approach of Gusev (1983) and adjust its parameters based on the residual variance. In doing so we con®rm the known, rather gradual mode of decay of amplitudes with distance in the Himalayas; this seems to be caused by the combination of high Qs and crustal waveguide effects for high frequencies.The data are then reduced with respect to magnitude. The trend of peak acceleration versus magnitude cannot be determined from observations, and we assume that it coincides with that of abundant Japanese data. For the resulting set of reduced log 10 (peak acceleration) data, the residual variance is 0.37 2 , much above commonly found values. However, dividing the data into two geographical groups, western with two events and eastern with four events, reduces the residual variance to a more usual level of 0.27 2 (a station/site component of 0.22 2 and an event component of 0.16 2 ). This kind of data description is considered acceptable. A similar analysis is performed with velocity data, and again we have to split the data into two subregional groups. With our theoretically grounded attenuation laws we attempt a tentative extrapolation of our results to small distances and large magnitudes. Our minimum estimates of peak acceleration for the epicentral zone of M w =7.5±8.5 events is A peak =0.25±0.4 g for the western Himalayas, and as large as A peak =1±1.6 g for the eastern Himalayas. Similarly, the expected minimum epicentral values of V peak for M w =8 are 35 cm s x1 for the western and 112 cm s x1 for the eastern Himalayas. To understand whether our results re¯ect the properties of the subregions and not of a small data set, we check them against macroseismic intensity data for the same subregion. The presence of unusually high levels of epicentral amplitudes for the eastern subregion agrees well with the macroseismic evidence such as the epicentral intensity levels of X±XII for the Great Assam
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