Earthquake swarms precursory to moderate to great earthquakes in the northeast India region

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The northeast India region has seen ten M ⩾ 7 1 2 earthquakes since 1897, including two great earthquakes of M = 8.7 in 1897 and 1950. The last M ⩾7 1 2 earthquake occurred on August 17, 1952. With the exception of three earthquakes others are found to be associated with periods of background/normal seismicity, precursory swarms, quiescence and mainshocks (and the associated foreshocks and aftershocks). The dataset has been critically examined for completeness considering the current capabilities for defining locations and also considering other parameters. Regression equations relating the mainshock magnitude ( M m ), the average magnitude of the largest two events in the swarm ( M p ) and the time interval ( M p ) between the beginning of the swarm and the mainshock have been developed. These are: M m = 1.37 M p − 1.41 and M m = 3 log 10 T p − 3.27 It is important to recognize precursory swarms and quiescence in real time; indeed, we believe we have recognized one such sequence in the vicinity of the Arakan Yoma Fold Belt. On the basis of M p− and T p− values, the lateral extent of swarm activity and the fact that no event of M > 6 has occurred since 1975 in the preparation zone defined by the 1963–1965 swarm, we estimate that an M~ 8 earthquake could occur at any time in the area bounded by 21° and 25 1 2 ° N and 93° and 96° E. The focal depth of this impending earthquake is estimated to be 100 ± 40 km in view of the focal depths of the other events in the swarm.