A comprehensive Survey of the 26 January 2001 Earthquake(Mw7.7) in the State of Gujarat, India

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Search for a surface fault associated with the earthquake was made over an epicentral area, especially along known active faults. But we could not find any surface faults that might have extended from the causative fault at depth. We found by trenching that the pressure ridge discovered in the north of Budarmora village was caused by lateral spreading of surface layer. A temporary seismograph network consisting of 8 stations was installed to monitor the aftershock activity. Recorded aftershocks are distributed in a depth range of 10 to 35 km ; i. e., the mid-to lower crust. It is inferred from this observation that the causative fault did not reach the the surface. The aftershocks are concentrated on a southerly dipping plane, which suggests that the earthquake was caused by a reverse fault dipping southward. An array of 14 GPS stations was set in and around the epicentral area to detect a post-seismic crustal deformation. Vertical displacements of a few centimeters were observed after a time interval of two months.Damage distribution was estimated investigating damage levels of different building types as defined by MSK intensity and questionnaire intensity survey, cause of damage in RC and masonry buildings, pattern of ground failure and lifeline structures, micro-tremor observation of site effects and buildings. Estimated isoseismals indicate maximum MSK intensity 10 in an area with a diameter of 50km with its center at Bhachau, and decrease to intensity 6 in Ahmedabad which is 210km distant from the epicenter. Fatality ratio was 6.5 % and highest in Bhachau district and 1.6 % in Bhuj district. The cause of the humanitarian disaster was mostly related with the collapse of masonry dwellings. In the case of RC structures, soft story effects and insufficient strengthening bars were found to have caused damages, while RC structures built after the 1956 Anjar earthquake in Ghandhidam indicated better quality and performance.