Seismic Hazard in Karachi, Pakistan: Uncertain Past, Uncertain Future

Abstract

The city of Karachi, Pakistan (population 14 million), sits close to a plate boundary and within reach of earthquakes on numerous tectonically active structures surrounding the city. One can draw parallels—geologic as well as demographic—with another megacity for which seismic hazard is known to be high: Los Angeles, California (figure 1). Yet with a short historical record, limited instrumental seismic data, and little geological or geodetic constraint on slip rates, seismic hazard in Karachi is poorly characterized. In this report we present a critical review of the historical record as well as an overview of potential earthquake sources in and around Karachi. Prior to 1800, the history of Karachi, the current capital of Sindh (one of Pakistan’s four provinces), is indistinguishable from that of many fishing villages on the northern shores of the Arabian Sea. It was known to Arab and Portuguese traders who sometimes stopped at the village on the way to the Malabar coast. Colonial trade with Sindh was limited in the 18th century. Exploratory surveys starting in 1808 led to the annexation of Sindh to British rule in 1843. By 1901 Karachi had grown from a village to a town with a population of fewer than 140,000 people. The population grew to 500,000 by the time Pakistan became an independent nation in 1947, at which time the city grew dramatically with the influx of a million refugees from India. By 1960 its population exceeded 2 million, a figure that had doubled by 1980 and more than doubled again to 10 million by 2000. With an estimated annual growth rate of 3.75%, its population will exceed 30 million within two decades. Karachi lies approximately 150 km east of the triple junction between the Arabian, Indian, and Asian plates (figure 2). The western and north-trending arms of the triple junction sustain convergent and transcurrent rates of 28–33 mm/ yr respectively (Apel et al. 2006). A recently discovered active fault, the Sonne fault, indicates that the Arabian plate has been fragmented across the southwest corner of the triple junction defining a triangular plate, the Ormara plate (Kukowski et al. 2000) whose velocity relative to the Arabian plate increases subduction velocities by a few millimeters per year compared to the rate to the west. In addition to these clearly defined plate boundaries, two other active structural zones have produced damaging earthquakes that have been felt in the city in the past 200 years: a thrust-and-fold belt extending northward parallel to the transform fault separating India from Asia, and the Kachchh fault system trending westward toward the city. Although residents of Karachi felt shaking from the 1945 Makran and 2001 Bhuj earthquakes, and occasional shaking from M 4–5 earthquakes on faults north and northwest of the city, no earthquake has ever produced documented damage in Karachi. The question currently faced by earthquake engineers is whether Karachi truly enjoys an aseismic setting or whether the absence of damaging earthquakes is only due to Karachi’s short and incomplete history. A review of the known historical data on earthquakes within 500 km of the city shows that the historical record prior to 1800 is limited and unreliable.