Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Abstract

Full Text

Abstract

Seismogeological features in Kutch and adjoining region are mapped by using Indian Remote Sensing Satellite (IRS) images. These features included major faults with neotectonic movements, lithological units, statistical data of past seismic events and aftershock study to prepare various thematic maps. Studies suggest that faults identified as lineament zones are parallel to major geological structures such as Kutch rift, Cambay structure, Narmada-Tapti tectonic zone and Kukdi-Ghod lineament zone. Drilling data indicates that subsidence along the vertical boundary faults is continuing since Mesozoic Era Geomorphological evidences such as relief changes along straight lines, inundation patterns, extent of salt pans and dune fields etc suggest that the subsidence is also continuing presently. Regional studies in western India have indicated that these faults outline different crustal blocks which are moving either in vertical or strike-Slip direction in relation to each other. Higher seismicity is recorded in areas where lineament zones are intersecting each other. The study has revealed that obstruction of horizontally moving block by vertically moving block causes nucleation of stresses and sudden release of these stresses generally results in an earthquake. Epicential plots have also indicated that seismicity is conspicuously confined along faults related to major geological structures. The epicenters of all five major devastating earthquakes including the 26^th January 2001 earthquake are located very close to the intersection of Kutch Mainland fault, and NW-SE trending faults parallel to Cambay structure.

An interesting relationship has also emerged between the isoseismal lines due to the 26^th January 2001 earthquake and fabric of mapped faults. These lines show remarkable parallelism with the faults, related either to the Cambay structure or Kutch rift of the region. Preferential activation of faults parallel to Cambay structure seems to have caused damage along the linear zone passing through Ahmedabad and along the coast in the vicinity of Bharuch, Surat, Navasari and Billimora. The epicenters of aftershock following 26^th January earthquake are also plotted on a map depicting the faults. It has been found from that all the aftershocks of magnitude > 5 and 70% of those ranging between magnitude 3 to 5 are confined in an area resembling a horseshoe pattern with a pointed end towards NE. Other 20% of aftershocks of magnitude 3 to 5 are enclosed within almost parallel boundary. Only 10% are found beyond this limiting line. Furthermore, 50% of recorded aftershocks have taken place within first week of the main event and this study has revealed that the characteristic pattern of aftershock activity could emerge with only one week data.

A funnel shaped seismogeological model is proposed after the detailed study of major faults and seismic events in western India mainly covered by the Deccan volcanics. The widest area forming the rim is characterized by subterranean sounds and ground shaking at shallower depths. This is followed by a zone of periodic swarm type of seismic activity, marked by low magnitude (2-4) earthquakes. Less common, medium magnitude (4 1-6) earthquakes occur at depth range of 4 to 9 km. High magnitude (> 6) earthquakes are rare and generally occur at focal depth of 9 to 35 km and occupy the central part of the area.

Keywords

Full Text