Modeling of vertical deformation associated with the 1931 Mach earthquake, Pakistan
Academic Affiliation: University of Texas at El Paso
Science Research Mentors: Walter Szeliga & Roger Bilham, University of Colorado, Boulder
Miriam Garcia was a first-year protégé and had just finished some undergraduate research during her junior year at the University of Texas at El Paso. Miriam joined the research team of Roger Bilham in the Department of Geological Sciences at the University of Colorado. PhD student Walter Szeliga was her direct supervisor and taught Miriam the necessary software package to conduct some computer modeling of an area of Pakistan that has experienced several significant earthquakes in the 1900's. The modeling attempted to decipher whether the 1931 Mach earthquake was caused primarily by compression of the Indian Plate into the Eurasian Plate or by a west-dipping thrust fault which created the Bolan Pass fold.
The Kirthar Range in western Pakistan is the result of east-west compression caused by the indentation of the Indian Plate into the Eurasian Plate. The 1931 Mw 7.3 Mach earthquake resulted in 65 cm of local uplift on a leveling line through the Bolan Pass in the northern Kirthar Range. Previous studies modeled the fault as an east-dipping blind thrust with a top depth of 4 km and a bottom depth of 35 km, yet geologic cross-sections illustrated a blind wedge thrust system verging to the east with a horizontal dÃ©collement at 8 km. Extensive simulations of slip on this inferred structure suggested that this subsurface geometry could not be responsible for slip in the Mach earthquake. A west-dipping thrust was also considered a viable fault, as it was geologically capable of producing the anticlinal fold seen at the Bolan Pass. Forward elastic-modeling methods applied to the west-dipping thrust showed that the earthquake could not have occurred on a simple fault of this form either. A new approach, merging the wedge and west-dipping geometries may ultimately explain what happened in the 1931 earthquake sequence. Understanding fault constraints in Bolan Pass will give insight into correlations between the Mach earthquake and other seismic events during the 1930s.