The Mw 7.7 Bhuj Earthquake: Global Lessons for Earthquake Hazard in Intra-Plate Regions

The Mw 7 7 Bhuj earthquake occurred in the Kachchh District of the State of Gujarat, India on 26 January 2001, and was one of the most damaging intraplate earthquakes ever recorded. This earthquake is in many ways similar to the three great. New Madrid earthquakes that occurred in the central United States in 1811-1812. An Indo- US team is studying the similarities and differences of these sequences in order to learn lessons for earthquake hazard in intraplate regions Herein we present some preliminary conclusions from that study Both the Kutch and New Madrid regions have rift type geotectonic setting. In both regions the strain rates are of the order of 10⁹/yr and attenuation of seismic waves as inferred from observations of intensity and liquefaction are low. These strain rates predict recurrence intervals for Bhuj or New Madrid sized earthquakes of several thousand years or more. In contrast, intervals estimated from paleoseismic studies and from other independent data are significantly shorter, probably hundreds of years. All these observations together may suggest that earthquakes relax high ambient stresses that are locally concentrated by rheologic heterogeneities, rather than loading by plate-tectonic forces. The latter model generally underlies basic assumptions made in earthquake hazard assessment, that the long-term average rate of energy released by earthquakes is determined by the tectonic loading rate, which thus implies an inherent average periodicity of earthquake occurrence. Interpreting the observations in terms of the former model therefore may require re-examining the basic assumptions of hazard assessment.