Earthquake Swarm and Quiescence Precursory to the NE Himalaya 2011 Sikkim Earthquake (M 6.9)

Abstract The September 18, 2011 earthquake of M 6.9 has been critically examined and found to be associated with episodes of precursory swarms, quiescence, mainshock and aftershocks. This earthquake was clearly preceded by precursory swarm activity for almost one year followed by quiescence for one year. The precursory swarm and quiescence period consists of four earthquake swarms and one foreshock event of magnitude (mb ≥ 4.5) in the epicenter preparatory zone of the 2011 Sikkim earthquake. The 2011 Sikkim earthquake recorded about five aftershocks of magnitude (mb) ≥ 4.5 between 2011 and 2014 for the same region. This study shows that the 2011 Sikkim earthquake followed the same sequence of activities as observed for other major earthquakes in the northeast Himalaya.