Cooperative Institute for Research in Environmental Sciences

Science Rendezvous > 2009 Posters
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Listric earthquake faulting and triggering in the Mach/Shiragh/Quetta earthquakes 1931-5

Walter Szeliga, D. Schelling, and R. Bilham

Between 1931 and 1935, three major earthquakes occurred between the Bolan Pass and Quetta in what is now Pakistan. The first two earthquakes struck within 66 hours of each other and caused 120 deaths. Less than four years later, and 85 km to the west, the Mw7.7 sinestral Quetta earthquake caused 35,000 deaths. All three earthquakes were located in the transpressively deforming western region of the Indian-Eurasian plate boundary. Immediately after the Quetta earthquake, a first-order leveling line through the Bolan Pass revealed 650 mm maximum uplift. This was initially interpreted without structural control as caused by approximately 1 m of slip on an east-dipping reverse fault, and subseqently as slip on a wedge thrust structure [Garcia et al. 2006]. The recent availability of a detailed structural section controlled by seismic reflection and borehole data permits a more satisfactory solution based on slip on one or more listric faults dipping to the west. We find the most likely mechanism for the earthquake to consist of up to 3 m of slip on the flat portion of a listric ramp-flat-ramp structure, with minor slip on faults to the east and west. We examine a possible stress-triggering sequence that links all three earthquakes in the sequence.