Preliminary determination of the interdependence among strong‐motion amplitude, earthquake magnitude and hypocentral distance for the Himalayan region

Summary Since 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 fixed distance and magnitude using independently defined 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 confirm the known, rather gradual mode of decay of amplitudes with distance in the Himalayas; this seems to be caused by the combination of high Q s 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 −1 for the western and 112 cm s −1 for the eastern Himalayas. To understand whether our results reflect 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 earthquake of 1897. Therefore, our results represent systematic regional effects, and they may be considered as a basis for future regionalized seismic hazard assessment in the Himalayan region. We see the location of earthquake sources/faults at a considerable depth within the relatively drier and higher‐strength shield crust as the main cause of the observed enhanced amplitudes for the eastern Himalayas events. Western Himalayas sources are shallower and occupy the tectonically highly fractured upper part of the crust, of accretionary origin. The low attenuation common to both subregions is due to the presence of cold, low‐scattering and high‐ Q shield crust.