The seismic b-value and its correlation with Bouguer gravity anomaly over the Shillong Plateau area: Tectonic implications

Clues to the understanding of intra- and inter-plate variations in strength or stress state of the crust can be achieved through different lines of evidence and their mutual relationships. Among these parameters Bouguer gravity anomalies and seismic b-values have been widely accepted over several decades for evaluating the crustal character and stress regime. The present study attempts a multivariate analysis for the Shillong Plateau using the Bouguer gravity anomaly and the earthquake database, and establishes a causal relationship between these parameters. Four seismic zones (Zones I-IV), with widely varying b-values, are delineated and an excellent correlation between the seismic b-value and the Bouguer gravity anomaly has been established for the plateau. Low b-values characterize the southwestern part (Zone IV) and a zone (Zone III) of intermediate b-values separates the eastern and western parts of the plateau (Zones I and II) which have high b-values. Positive Bouguer anomaly values as high as +40 mgal, a steep gradient in the Bouguer anomaly map and low b-values in the southwestern part of the plateau are interpreted as indicating a thinner crustal root, uplifted Moho and higher concentration of stress. In comparison, the negative Bouguer anomaly values, flat regional gradient in the Bouguer anomaly map and intermediate to high b-values in the northern part of the plateau are consistent with a comparatively thicker crustal root and lower concentration of stress, with intermittent dissipation of energy through earthquake shocks. Further, depth wise variation in the b-value for different seismic zones, delineated under this study, allowed an appreciation of intra-plateau variation in crustal thickness from ∼30 km in its southern part to ∼38 km in the northern part. The high b-values associated with the depth, coinciding with lower crust, indicate that the Shillong Plateau is supported by a strong lithosphere.