Medium-term forecast of the 1988 north-east India earthquake

Northeast India is seismically one of the most active intra-continental regions in the world. It has been a site of 10 earthquakes of magnitude (M) greater than or equal to 7.5 during the last 100 years. Gupta and Singh [J. Geol. Soc. India, 28 (1986) 367-406] systematically analysed the time series of earthquakes associated with several main shocks in an area bound by 20 degreesN and 32 degreesN latitude and 87 degreesE and 100 degreesE longitude and concluded: (1) Moderate magnitude to great earthquakes in the north-east India region are found to be preceded, generally, by well defined earthquake swarms and quiescence periods. (2) On the basis of an earthquake swarm and quiescence period, an area bound by 21 degreesN and 25.5 degreesN latitude and 93 degreesE and 96 degreesE longitude is identified to be the site of a possible future earthquake of M = 8 +/- 0.5 with a focal depth of 100 +/- 40 km. This earthquake should occur any time from now onwards. Should it not occur fill the end of 1990, this forecast could be considered as a false alarm. This medium-term earthquake forecast came true with the occurrence of the M = 7.3 earthquake on August 6, 1988, within the specified spatial and temporal parameters. The forecast of this earthquake was based on the concept of precursory swarms and quiescence preceding main-shocks. It is worthwhile noting that in the entire region under study, the August 6, 1988 earthquake was the largest since the August 17, 1952 earthquake of M = 7.5. This lends support to the worldwide effort to study and identify spatial and temporal variation of seismicity and recognise patterns that precede major earthquakes. (C) 2001 Elsevier Science B.V. All rights reserved.