Earthquakes: The Enemy Below

As the global population increases and is concentrated into megacities, there are widespread issues relating to pollution, overcrowding, poverty and health. But there’s another major threat to life and infrastructure that is often overlooked: earthquakes.

Many megacities in Asia have grown from smaller settlements that have been destroyed by earthquakes numerous times in the past. Take Tehran, current population around 10 million, which was badly damaged by earthquakes in 855, 958, 1177 and 1830. These events all happened when the city’s population was a small proportion of its present-day total. If an equivalent earthquake occurred today, the death-toll would probably be in the hundreds of thousands or millions. It’s a similar story in many major cities across the tectonically active parts of Asia, such as Delhi, Istanbul, Xi’an, Karachi, Almaty and Tbilisi.

This is not a coincidence. The original settlements from which these cities have grown were established on ancient trade routes running through Asia and the Middle East. These followed coastlines and the edges of the mountain ranges — which were formed by repeated earthquakes — because the interiors of the mountains and the intervening deserts were too hostile. This means that Asia’s megacities are concentrated in the areas of highest earthquake hazard, as the map opposite shows.

Nor is a long time interval between seismic events a cause for comfort — in fact, the opposite could be true. The frequency with which earthquakes happen in a given location depends upon the rate at which the motions of the Earth’s tectonic plates are bending the rocks. This bending happens until the rocks break, causing an earthquake. In the oceanic margins, such as Sumatra and Japan, the threat is from a single well-defined fault. But in the tectonically active areas of the continental interiors there is a large network of active faults, each accommodating a small proportion of the overall plate motion, and each breaking infrequently — but in large earthquakes. The time intervals between successive events on a given fault can therefore be large — from centuries to tens of thousands of years — and only a small proportion have ruptured during recorded human history. However, such long timescales mean there may be large population growth between earthquakes, and that awareness of earthquake hazard is not as prominent in the public and administrative psyche as in regions that experience regular shaking. Many of the world’s megacities are therefore in regions that are guaranteed to experience large earthquakes in the coming years to centuries, but which have been built with only minor consideration of this hazard.

A major research area in the Earth sciences involves identifying active faults before they rupture in earthquakes. The backbone of this work is the use of satellite data to identify signs of past events preserved in the landscape, and chemical techniques to date the age of offset deposits. We are able to identify the locations and likely magnitudes of future events, and this information can be used to regulate building standards, strengthen existing structures and put in place earthquake preparedness and response plans. Such measures have earthquake-related deaths in seismically active areas including Chile, Japan and California. However, these methods do not allow us to predict when an earthquake will occur — widely thought to be an intractable problem.

Identifying and studying active faults is not a complete solution to the problem of understanding earthquake hazard. Although rapid progress is being made, the large numbers of active faults in the continents means that only a small proportion have yet been studied in detail. In addition, local geological effects can make some faults invisible — for example, if their surface expression is buried in sediment deposited by a large river. Opinion remains divided on how to assess earthquake hazard in such areas. One option is to assume that the hazard level is similar to that in nearby regions with known faults. This is the cautious approach, and the cost of mitigating earthquake hazards in regions where it may not be necessary will inevitably place a heavy burden on resources.

On the other hand, ignoring a hazard in regions with no visible faults could result in devastating loss of life. Finding the optimal solution between these two extremes will require close collaboration between Earth scientists, policymakers and governments, and those involved in creating the built environment.

Alex Copley is a lecturer in the Department of Earth Sciences at the University of Cambridge. He is a member of Earthquakes without Frontiers and the Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), two research partnerships on earthquake hazard and risk