Terra firma. It’s what fishermen, astronauts and exhausted long-haul passengers alike, long to set foot on, literally meaning ‘solid earth’ in Latin. Compared to the uncertainty of the sea or sky, the solidity of the ground beneath us inspires a feeling of safety and security, which is why, perhaps, earthquakes and the havoc they wreak are so horrifying.
Humans have recorded earthquakes for close to 4,000 years. The Chinese astronomer, mathematician and engineer Zhang Heng even invented the first earthquake detector in AD132. However, accurately predicting these natural phenomena remains elusive to this day.
What causes an earthquake?
The scientific theory of plate tectonics explains how the Earth’s rigid, outermost shell comprises enormous slabs of solid rock, known as tectonic plates, that have been slowly moving on top of the planet’s more fluid mantle for billions of years.
“Small earthquakes can occur anywhere,” says Professor of Seismology and Rock Physics, Ian Main. “But ‘great’ earthquakes [8-8.9 magnitude] happen on the cracks or fault lines, where two plates meet. When the immense pressure of the plates grinding together becomes so great that they suddenly move, they release violent and powerful vibrations, known as seismic waves.”
Now generally accepted, the theory was still relatively new when it inspired the young physics graduate to pursue a career in geophysics in the early 1980s. “It seems funny now, particularly as plate tectonics is part of the curriculum for primary school students, as I found out when asked to give a talk to my daughter’s class, but back then, this science felt new and an exciting thing to get into,” he recalls.
Why can’t we see them coming?
Seismologists can calculate the long-term probability of earthquakes in a given area, by averaging data on seismic activity from seismometers (an instrument that responds to ground noises and shaking) and historical archives. They can also dig directly into faults to look for evidence of past tremors and use satellites to measure Earth deformations.
Since the early 20th century, governments and city planners in earthquake-prone areas have used such calculations to design buildings. These techniques remain the first line of defence against earthquakes. However, Professor Main says they only tell part of the story: “Unfortunately, earthquake mainshocks are close to random. For example, trenches in the San Andreas fault in California show the time between large earthquakes varies from 50 to 350 years. However, much like dominoes toppling into each other, we can see a sequence of seismic activity on shorter timescales around these mainshocks. Analysing these shorter-term sequences enables us to forecast the daily chance of a large event happening in the near future.”
The quest to predict the unpredictable
During the second half of the 20th century, the scientific community turned its focus to making meaningful predictions from precursory signals prior to rock failure, in laboratory tests. “We were searching for the holy grail, measuring everything from radioactive emissions in groundwater, deformations in the Earth surrounding seismic activity, to the nearest nanometres per second, and even strange animal behaviour,” Professor Main recalls. “Towards the end of the 90s, there was a huge debate around the significance of these reported precursors and whether deterministic, reliable prediction of individual earthquakes was even possible. When the scientific community is arguing so much about a topic, it usually means we don’t know something, or that things are about to change.”
At the height of this scientific struggle, the Edinburgh seismologist began approaching the question differently. “While accurately predicting earthquakes was a noble, long-term objective, I believed it was so far beyond the capabilities of the science of the time, that it mustn’t distract us from the immediate imperative to reduce the risk to life,” he reflects.
In 2009, the Italian government invited Professor Main to share his scientific expertise as a member of the International Commission on Earthquake Forecasting, exploring ways to improve civil protection in the wake of the devastating L’Aquila earthquake earlier the same year.
The commission’s work laid the groundwork for a range of Operational Earthquake Forecasting (OEF) systems to assess and communicate the likelihood of damaging earthquakes. OEF combines scientific insights on the background potential for earthquakes and their frequency, with seismic sequence analysis, to create forecasts of the probability of an earthquake happening tomorrow, in a few weeks or a few years. It also allows scientists to test these forecasts over many lab trials to determine their accuracy.
Local authorities, civil protection agencies and emergency services in Italy, New Zealand, the United States, Japan, the Netherlands and worldwide now employ the OEF approach outlined in the report to communicate and manage the risk to society of both natural and human-caused seismic activity.
At the heart of Main and his colleagues’ work on OEF lies an attitude to uncertainty and risk that Professor Main first encountered growing up in East Lothian’s mining communities. “My geography teacher explained when the coal industry started to replace the traditional wooden pit props that supported the roofs of mines with solid steel ones, miners felt it could make their working conditions more dangerous,” he remembers. “Steel props may be stronger, but they didn’t audibly groan like the wood when they start to fail – a vital early warning that the mine would collapse – they just snapped.”
Setting the global standard
Main is the first to point out that these forecasts are not as accurate as weather forecasts. There are also issues around how authorities communicate risk. “If scientists work out there is a 1 in 1,000 chance of a damaging earthquake tomorrow, then 999 times out of 1,000 they will not occur, which people could interpret as false alarms,” he says. However, he believes making people aware that earthquakes are possible in the immediate future, however small the probability, could help communities balance the risks they may face.
Levelling the playing field for future development
The risk of earthquakes, along with floods, landslides, volcanoes and fires, is more acute for the two billion people living in cities in low- and middle-income countries, where planning regulation to mitigate hazards is often less developed. Moreover, as urban areas expand at unprecedented rates, estimates suggest that people living in these areas will double within the next thirty years.
Professor Main is currently applying his broader expertise in natural hazards research, to address this challenge as part of a global team of interdisciplinary researchers led by his colleague John McCloskey, also at Edinburgh, collaborating to reduce disaster risk for the poor in rapidly developing cities, who often feel marginalised in development decisions.
“Science has demonstrated to motorcyclists that wearing a helmet could save their lives in the unlikely, but possible, event of a crash,” he states. “In the same way, OEF gives people an opportunity to minimise their risk of being harmed, or worse, in the unlikely, but possible, event of an earthquake.”