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

Abstract

Seismicity database from 1860 to 1985 of northeast India region bounded by the area 20°–32°N and 82°–100°E have been analyzed for the identification of precursory swarm/anomalous seismic activity preceding large to great earthquakes with M ≥ 7.5. It is observed that with the exception of three earthquakes (1908, 1912 and 1918), the large earthquakes of 1897, 1946, 1947, 1950 and 1951/1952 were preceded by well-developed epoch of swarm/anomalous seismic activity in space and time well before their occurrence. The seismicity is observed to fluctuate in the order of low-high-low ranging from 0–0.5, 01–33 to 0–0.7 events/year prior to these mainshocks during the epochs of normal/background, swarm/anomalous and gap/quiescence, respectively. The duration of precursory gap is observed to vary from 11 to 17 years for mainshocks of M 7.5–8.0, and from 23 to 27 years for M 8.7 and this period is dependent on the magnitude of the mainshocks. Using the values of magnitude of mainshock ( M m), average magnitude of swarm ( M p) and the precursory time gap ( T p), the following predictive equations are established for the region: M m = 1.37 M p − 1.40 M m = 3 log ⁡ T p − 3.27 All the major earthquakes with m b ≥ 6.1 occurred during 1963–1988 have been investigated for their association with anomalous seismicity/precursory swarms using the events with cutoff magnitude m b ≥ 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 ( m b 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 M p and T p 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.