Occurrence of anomalous seismic activity preceding large to great earthquakes in northeast India region with special reference to 6 August 1988

All the major earthquakes with mb ≥ 6.1 occurred during 1963-1988 have been investigated for their association with anomalous seismicity/precursory swarms using the events with cutoff magnitude mb ≥ 4.5. Eleven such events have occurred in the region during the period except one earthquake of 29 May 1976. All the remaining 10 earthquakes were associated in some forms of anomalous seismicity epochs. Well-defined patterns of anomalous seismicity are observed prior to 1964-1965, 12 August 1976 and 30 December 1984 (mb 5.6). All these mainshocks are preceded by seismicity patterns in the order of low-high-low similar to that observed prior to the mainshocks from 1897 to 1962. The anomalous seismicity epoch is delineated with extremely high annual earthquake frequency, which was preceded and followed by extremely low seismicity epochs of background and gap/quiescence phases. Consequently, seismicity rates during anomalous seismicity epoch have always been above normal (1 event/year) whereas it is always below normal during the preceding and the following epochs. A prediction was made using the 1964 swarm based on the Mp and Tp values that a large earthquake with M 8 ± 0.5 with focal depth 100 ± 40 km could occur any time from 1986 to 1990 in an area bound by 21°-25 5°N latitudes and 93°-96°E longitude in Arakan Yoma fold belt. It is interesting to note that the 6 August 1988 earthquake with magnitude 7.5 and focal depth 115 km had occurred within the delineated zone. In addition, three consecutive swarm activities are identified in a limited area within the Eastern Syntaxis and these were not followed by any mainshock till date and could be potential zone for future earthquake.