Professor Iain Stewart FRSGS, Royal Scientific Society, Jordan; Mike Robinson, RSGS Chief Executive

Earthquakes are the constant hum of our dynamic planet, controlled by the motion of the Earth’s tectonic plates, driven by heat generation from the decay of radioactive elements deep inside our planet. Most of the hundreds of tremors that are recorded by sensitive monitoring equipment every day go entirely unnoticed by us going about our daily lives. They are just too small to be ‘felt’, or simply too remote, such as those occurring deep on the ocean seabed. But the ones that we remember are those few that are big enough to wreak havoc on people and places.

But what is ‘big enough’? There are several different ways to think about earthquake size. Probably the most familiar is the ‘Richter magnitude’, which is a physical measure of the amplitude of the seismic waves recorded on a local seismometer. The Richter (or local) magnitude scale (ML) ranges from 0 to 10 and is logarithmic, meaning that a magnitude 7 earthquake is ten times stronger than a magnitude 6 earthquake, and around a hundred times stronger than a magnitude 5 earthquake. The trouble with the Richter scale, however, is that above magnitude 8 it blurs out (‘saturates’) and so fails to accurately capture the size of the very biggest quakes.

For that reason, we instead prefer to measure the total energy generated by an earthquake: its ‘seismic moment’. This ‘moment magnitude’ (M or MW) is also scaled in increments from 0 to 10, with each whole number representing an increase of 32 times the amount of energy released. So, a magnitude 7 earthquake releases 32 times more energy than a magnitude 6 earthquake, but over 1,000 (32 x 32) times more energy than a magnitude 5 earthquake. Indeed, for each increase of 0.1 in moment magnitude, the energy release increases by 1.5 times, meaning that whether an earthquake is a magnitude 7.0 or a 7.1 might seem like seismological hair-splitting to some, but it may represent a life or death difference for those on the receiving end.

What earthquake magnitude doesn’t tell us much about is how all that seismic energy actually affects people or places. Around one in ten of all earthquakes are of magnitude 5 or greater, and only around 20 of these annually are of a magnitude of 7 or greater. But when a more powerful earthquake hits areas with high populations, especially those with poor infrastructure, it can be a recipe for disaster.

To measure the physical effects of seismic energy release we shift attention to earthquake ‘intensity’, which determines the degree of seismic shaking at a given place and broadly decreases with distance from the earthquake epicentre, and depth of the hypocentre directly below the ground-level epicentre. In the United States and many other parts of the world, this has long been described by the Modified Mercalli Intensity (MMI) scale, but in the 1990s, seismologists in Europe developed the European Macroseismic Scale (EMS). Whilst the MMI is a subjective scale based on inferences about the observed effects on people, structures and environment, the EMS also incorporates geological data and measurements of ground motion. Both these scales ranges from I (Not felt) to XII (Extreme (MMI) or Completely devastating (EMS)). Because the scales measure slightly different things, they are difficult to compare, but each is instructive in its own way.

Much of our knowledge about earthquake impacts comes from our observations of their past incidence. In that sense earthquakes are rarely ‘bolts from the blue’. There is a persistent and consistent geography to their lethal occurrence. Long before the 1960s when the pattern of global seismicity first revealed the edges of our planet’s moving tectonic plates, the main earthquake belts were well known. Along the western edge of the Americas and its continuation around the Pacific’s volcanic ‘Ring of Fire’, across Southeast Asia and along the foothills of the Alpine-Himalayan mountain chain, and slicing down the Great Rift from Turkey through the Middle East and deep into East Africa; this is earthquake country.

Here, over millennia, human cultures have co-evolved with seismicity. Many ancient societies developed ways of countering this endemic threat in the manner that they built or the materials they used, and settlements were smaller and more loosely scattered. But with the rapid industrialisation and economic growth of the late 20th century, more and more people have crowded into urban sprawls dominated by concrete, steel and glass. Nature’s tectonic fault lines are now within striking distance of many of the planet’s most populous cities, raising the spectre of seismic death tolls that are far in excess of anything witnessed previously in human history. We know where these human targets are and, thanks to geological science, we broadly know the likely culpable seismic fault lines, so the critical challenge becomes how best to prepare both people and places. Nations with advanced economies can now engineer against all but the very largest of earthquake dangers, but much of the chronic seismic threat lurks in some of the poorest quarters of the planet, where wealth, health and infrastructure are at a minimum. Earthquakes might routinely emerge from Nature, but the disasters that repeatedly arise from them are made in Society.

The ten deadliest earthquakes of this century.

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