Riding the mobile Karachi arc, Pakistan: Understanding tectonic threats

Abstract

This paper presents the latest seismicity data from the Karachi Arc and illuminates recent activity within the greater Karachi metropolitan area. Since the epicenter locations cover the entire Karachi Arc, the whole structure seems to be currently active with hypocenters located in a depth range of (0-500) kilometers. Most of these, however, are shallow (10-50 kilometers). Many of the epicenters are located within or near the Karachi neighborhoods plagued by recurrent earthquake activity. That explains why the people are so tormented. Coastal areas in southern Karachi are also exposed to tsunami threats from earthquakes along the shorelines or from within the Arabian Sea. 

This study also provides the first outcrop evidence of wrench nature of the southern boundary of Karachi Arc. The photographed fault, named Makli Fault here, is located 2 kilometers south of Makli (about 6 kilometers southwest of Thatta) in the Sindh province and appears to be part of a system that extends westwards through Karachi, where it is hidden under the concrete and urban sprawl. Geomorphic observations suggest that faults belonging to this system might have locally deflected the Malir and Lyari River channels and also created accommodation space for deposition of such coarse clastics as the Korangi Conglomerate. 

Progressive folding and shear-related drag - along the southern wrench boundary of Karachi Arc - as it moved eastward during the Neogene - probably resulted in the formation of large SW plunging folds - such as the Monghopir anticline and the adjacent syncline. The limbs of both of these folds are also torn by prominent NW oriented Riedel shears with enhanced drag-related wrench offsets. More fieldwork is urgently needed to map active faults in Karachi and develop a better understanding of the tectonic threats.