TECTONIC SETTING AND GEOLOGICAL EFFECTS OF THE MW 7.7 2001 BHUJ EARTHQUAKE

摘要

The January 26th, 2001 M_w7.7 Bhuj earthquake provides the first opportunity to document and evaluate a modern large magnitude earthquake that occurred in or near a stable continental region. An important issue with regard to the earthquake is related to the tectonic setting and origin of the event - was it an interplate event or an intraplate event? While much of peninsular India is a stable continental region (SCR) typical of continental interiors or intraplate environments, western and northwestern India have characteristics of more active tectonic environments. These regions have moderate rates of earthquake activity, and the ～M8 1819 Allah Bund, and M_w7.7 Bhuj earthquakes demonstrate that some structures, particularly in the northwest, are capable of producing large magnitude events. We believe the categorization of the Bhuj earthquake as either an interplate or intraplate event is at this time equivocal. The surficial geology and regional tectonic setting suggest that it is within a transitional part of the plate boundary, much like the distal reaches of the San Andreas system in eastern California and Nevada, or the Sierra Pompeanas, of eastern Argentina. However, the depth of the event and some of the seismological aspects suggest that the earthquake occurred in an intraplate environment. The Kachchh is perhaps best described as a region of fragmented continental crustal that has been underplated by mantle plume activity, and is now involved in the active deformation along the northwestern boundary of the India plate. As such, the Kachchh region has affinity to both continental regions and plate boundaries. There are three primary geological lessons to be learned from the Bhuj earthquake: (1) large magnitude, e.g. M>7.5, earthquakes can occur along blind faults and without producing primary surface fault rupture; (2) the geological characteristics of the region do not fit neatly into either the intraplate or interpolate tectonic models; and. (3) secondary effects, such as liquefaction occurred in highly susceptible deposits and could have been predictable using standard susceptibility mapping techniques.