Seismic seafloor observations: a study of ‘anomalous’ intraplate seismicity in the northeastern Indian Ocean

Present-day intraplate tectonic activity in the northeast Indian Ocean, which manifests itself in high seismicity, is unusual for oceanic lithosphere plates. The contradictory points of the various tectonic hypotheses proposed to explain this ‘anomalous’ seismicity may be due to sparsity of ocean intraplate earthquakes of M > 4.5, which are recorded by means of networks of land-based permanent seismic stations. Seismological observations on the ocean floor by means of ocean bottom seismographs may provide additional and important data. Some results of such observations carried out in the Central Indian Basin simultaneously with deep seismic sounding surveys are analysed in this paper. About 118 seismic events were recorded at a site near 3°45′S and 80°E for a period of 7.3 days, and 24 events were recorded at a site near 0°50′S and 83°53′E for a period of 3.3 days. Magnitudes of these events range approximately from 2.8 to 4.0. Estimates of epicentre locations based on the Jeffreys-Bullen travel-time table show that over 90% of the earthquakes recorded at the first site were related to the Chagos Trough, during a period when no event was recorded there by land-based seismic stations. The most probable reason for these events is the Chagos Bank earthquake of 30 November 1983 ( M s = 7.6), which caused many aftershocks extending eastward from the main shock. Thus the high aftershock activity east of the Chagos Bank earthquake revealed by ocean bottom seismic observations complements the data on epicentres of larger earthquakes, which in turn provide the scope to extend the known seismicity of the Chagos Bank further east. In general, our results are in accordance with the most popular tectonic model of the Central Indian Ocean, in which a diffuse plate boundary divides the Australian plate from the Indo-Arabian or the Indian plate, initiating a new subduction zone.