Mapping of b-value Beneath the Shillong Plateau

The seismic parameter 'b' has been computed over rectangular grid of dimension 0.3° ' 0.8° at four depths range: 0-13 km (first layer), 13.1-26 km (second layer), 26.1-39 km (third layer) and 39.1-52 km (fourth layer) beneath the Shillong Plateau area. The four depths were carefully selected based on the crustal structure and distribution of hypocentres. The dimension of each grid was chosen so as to have enough events that can represent the b-value at the respective layer. Finally, two-dimensional mapping was done at these depth-levels considering the respective b-value over each grid. This analysis includes viz., low b-value all through the first layer, and a trend of increasing b-value, which was initially towards north, changes to northwest. Eastern and western parts of the second and third layers document almost moderate b-values, whereas the north-south-oriented central part of layer second is apparently dominated by low b-values, which seems to divide the area broadly into three parallel zones based on b-values. In the deeper part (fourth layer) beneath the Shillong Plateau a moderate b-value that was initially trending towards north becomes high near the northeastern part. This phenomenon may be associated with higher heterogeneity of the medium, and interestingly, this region lies between the lower crust and upper mantle, possibly documents lower degree of seismic coupling, where the Shillong Plateau is being supported by the strong Indian lithosphere at these depths. In addition, minima were noted towards the southern parts of layers first, second and third, which may presumably be related with steeply Bouguer gravity anomaly. It is thus less clear that the occurrence of earthquakes beneath the Shillong Plateau whether is attributed to faults or lineaments at intermediate to deeper level. However, a correlation between high b-values in few parts of each layer and deep-seated minor faults cannot be ruled out.