Global Urbanization and Increased Seismic Risk

By the year 2025 more than 5500 million people will live in cities - more than our entire 1990 combined rural and urban population. The growth of these giant urban agglomerations is a new experiment for life on Earth. Tragically, a significant fraction of the largest of these agglomerations (supercities and megacites) are located close to regions of known seismic hazard. With few exceptions (Tokyo 1923; Tangshan, 1976), recent large earthquakes (M>7.5) have spared the world's major urban centers, but this will not persist indefinitely. In the next millennium several megacities will be damaged by significant earthquakes. We are most certain of the fate of those cities near plate boundaries, however, mid-continent earthquakes also occur, albeit infrequently (c.f. M>8 events in the eastern US and India in the early 18th century), and these events will wreak great havoc in mid-continent cities where earthquake resistant construction is not mandated.

For the above reasons it is certain that the annual fatality rate from earthquakes will rise in the next 30 years, attributable partly to moderate earthquakes near large cities, but principally from a few catastrophic earthquakes near supercities (populations 2-28 million). The figure illustrates the predictable fatality rates from low mortality earthquakes, and the erratically growing death toll from high fatality earthquakes in the past 125 years.

Given this grim setting it is clear that today's generation of seismologists have a responsibility to educate the urban planners of today, who are responsible for the seismic safety of the next generation of city dwellers. Several courses of action are open to us, from careful studies of historically damaging earthquakes, to characterizing the details of future slip, and the driving kinematics of plate boundaries.

I am involved in some of these activities. In India where populations are large and where historical studies have been neglected we have initiated searches in colonial Indian Records that have resulted in improved understanding of earthquakes in 1737, 1819, 1833 and 1934, and more are planned. These studies been complemented by GPS studies of the stability of the Indian plate, the deformation and growth of Tibet, and the behavior of the edges of the Tibetan Plateau in the Altyn Tagh Mountains and the Himalaya. A recent result, for example, is the direct measurement of the rate of convergence across the Himalaya (21±2 mm/year), suggesting a recurrence interval of 300 years for 6-m-slip M=8 earthquakes.

A distracting but important role for seismologists at present is to talk not so much to ourselves, as to those members of the global community who are responsible for implementing earthquake resistant design in current and future buildings. It is thus essential that we speak at length, and in depth, to the news media, and participate in public outreach activities (lectures, films, interviews, etc.). These can be vexing and repetitive encounters, yet they are essential if we are to make the next millennium a safer place to live.

References

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Bilham, R. Death toll from earthquakes, Geotimes, 7(4), July 1998.