Seismicity of the North-East India Region Part II: Earthquake Swarms Precursory to Moderate Magnitude to Great Earthquakes

Since 1897, ten earthquakes of M ≥ 7½ have occurred in north-east India region and its vicinity including the great earthquakes of June 12, 1897 and August 15, 1950, both of M 8.7. The last earthquake belonging to this category, occurred on August 17, 1952. With the exception of three earthquakes, others are found to be associated with the epochs of background/normal seismicity, precursory swarm, quiescence and mainshock (and the associated foreshocks and aftershocks). In our investigations, the earthquake data have been critically examined and taking due care of prevalent location - capabilities and other parameters. The mainshock magnitude (M_m) is related to the magnitude of the largest events in the swarm (̅M_p) and the time interval T_p (in days) between the beginning of the swarm and the main shock. The regression equations are:

                                 M_m = 1.37  ̅M_p - 1.41

and                            Mm = 3 log₁₀ T_p - 3.27

Installation of the World Wide Standard Seismograph Network during 1962-63 improved the earthquake detection and location capabilities considerably in the northeast India region and all earthquakes of m_b ≥ 4.5 are being located. Consequently, for the period 1963 through 1985, we have investigated all events of m_b ≥ 6.1 for occurrence of precursory swarms. Ten such events have occurred, four of which in a quick succession in the Arakan Yoma fold belt within a time span of six months only, and constitute a well defined swarm. Two other earthquakes are possibly gap events of Arakan Yoma swarm. Out of the remaining four, three are found to be associated with epochs of normal/background seismicity, precursory swarm, quiescence and main. shock (and the associated foreshocks and aftershocks). The latest killer earthquake in northeast India of Dec. 30, 1984 of m_b 5.6, which occurred in the vicinity of the Arakan Yoma fold belt and Shillong Plateau, was preceded by a well-defined precursory swarm and seismic quiescence.

The most important thing is to recognize the precursory swarms and quiescence in real time, before the occurrence of the main shock. We believe to have recognized one such sequence in the vicinity of Arakan Yoma fold belt. On the basis of M_p and T_p values, lateral extent of the swarm activity and the fact that no event of m_b ≥ 6.1 has occurred since 1975 in the preparation zone defined by the 1963-65 swarm, we estimate that an earthquake of M 8 ½ could occur any time from now in an area bound by 21°N and 25½°N latitudes and 93°E and 96°E longitude, with a focal depth of 100±40km.

We have also recognized a region in Eastern Syntaxis where repeated swarms have occurred during 1968, 1977 and 1980. The largest events in these swarms, however, were only of m_b 5 and the swarms were very localized. These may have been caused by magmatic emplacement at shallow crustal depths.