Deformation, Fragmentation and Seismic Hazards on the Western Edge of India

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

The 2005 catastrophic Mw7.6 earthquake in Kashmir (death toll 85,000) was one of several earthquakes that were anticipated by seismologists because of the known seismic hazard of the Himalaya where similar damaging earthquakes have occurred throughout the past 1000 years. The earthquakes there are caused by the northward movement of India towards Asia at a rate of 2 inches/year. For the same reasons earthquakes also occur along the western and eastern edges of the Indian plate. The largest in the east was the Mw9.1 Sumatra/Andaman earthquake of 2004 (death toll 300,000). No great earthquakes have occurred recently on the western edge of the Indian plate in Pakistan, yet there are good reasons to suppose that they should. An earthquake in SW Pakistan could cripple Karachi, Pakistan's largest city with >12 million people, just as Quetta was destroyed in 1935 by an Mw7.7 earthquake (death toll 35,000). The destruction of Karachi would be catastrophic to the economy of Pakistan. Like Los Angeles, Karachi is a city that was a fishing village 250 years ago and has only in the past 50 years achieved megacity status. Unlike Los Angeles no earthquake resistance is in place, and no history of damaging earthquakes has survived. One aim of the project is to find out whether the historical absence of earthquakes is because no earthquakes occur, or whether, as we suspect, it is because they recur at intervals too long to be recorded by its 200 year colonial history. The edge of the Indian plate, which passes from the coast west of Karachi in the south, to the mountains that separate Afghanistan from Pakistan east of Kabul in the north, could be locked, preparing for a future earthquake, it could be sliding aseismically (like parts of central California, or it could be a wide region of rotating geological blocks, that act like ball-bearings permitting the plates to slide past each other. We plan a series of Global Positioning System (GPS) measurements that will uniquely distinguish between these three possibilities. The measurements will tell us the relative velocities at the edge of the plate, and the width of the zone of deformation, and the depth at which the two plates are locked - information that will be used to quantify the seismic productivity of the region, and identify the most probable future location of future damaging earthquakes. This information is of fundamental importance to earthquake engineers in the region, and will dictate building policies for the next 30 years. The measurements will also provide important constraints on the physics and nature of the stresses that permit continental transform plate boundaries, like the San Andreas fault, to slip and evolve with time. The field investigations are being undertaken in close collaboration with scientists and engineers from three Pakistan, and two Indian Universities, and as such forge international ties that are of great benefit to the image of the United States in these developing nations. Because of the international nature of the project, this work is jointly funded by the Geophysics Program and the Office of International Science and Engineering.