Microearthquakes recorded by two surveys in the Shillong Plateau during December 1982-July 1983 and January-February 1984, respectively, have been analyzed for seismicity and tectonic study in the region. Of 1,000 events recorded during the surveys, 180 events with reliable P and S readings (which are observed within 50 km radius of the network), are relocated by the homogeneous station method. It is found that the events are mostly confined within a depth range of 10 to 35 km. The seismicity, in general, may be correlated with the major geological structure/lineaments in the area. High rates of seismicity are observed in the Shillong and Tura areas. On the other hand, an aseismic zone is delineated to the east of Shillong.
Composite source mechanisms of two groups of microearthquakes show thrust-fault mechanisms. One group of microearthquakes (1982 to 1983) in the Shillong area shows pure thrust faulting with a SE-NW compressive stress (P axis), whereas the other group of microearthquakes (1984) in the Tura area shows thrust faulting with a strike-slip component and a ENE-WSW compressive stress (P axis). The stress orientations are compatible with the known tectonics of the region: collision of the Indian plate with the Eurasian plate at the Himalaya and subduction of the Indian plate beneath the Burmese plate at the Arakan-Yoma range.