Two decades ago, John McCloskey drew a red line on a map of southeastern Turkey to pinpoint where a large earthquake would probably strike. The only question was when. The answer came last month, when a magnitude-7.8 shock hit the precise location that McCloskey and his team had identified. It struck at 4.17 a.m. local time on 6 February, when most people were asleep, and killed more than 50,000 residents in Turkey and neighboring Syria.
McCloskey’s work shows both the promise and limitations of the science of earthquake forecasting. Although geologists have long attempted to provide warnings of the location, magnitude and exact time of future quakes, decades of research have shown that it’s probably impossible to predict when a geological fault will start to shake. “When you try to winnow it down to know what’s going to happen next, it tends to be a lesson in humility,” says Susan Hough, a geophysicist in the Earthquake Hazards Program at the United States Geological Survey (USGS). “The real focus in most of the world is not on prediction, but on assessing the hazard and the long-term rates of earthquakes.”
Today, researchers work on forecasting: identifying which fault segments are most dangerous and what size earthquakes they are expected to produce. Armed with that knowledge, policymakers can take steps to reduce death and destruction by, for example, requiring better building practices or urging local residents to prepare. Some regions of Japan, the United States and Turkey have developed early-warning systems that alert residents when an earthquake has started nearby. “In principle, you can get rid of seismic risk,” McCloskey says.
Danger zone
Turkey is a seismically active junction at which several pieces of Earth’s crust meet and grind against each other. In southeast Turkey and northern Syria, the Arabian plate is pushing north against the Anatolian plate, squeezing it to the west. But the shift isn’t one smooth movement. Instead, friction holds the plates in place, sometimes for centuries. When the stress overcomes the friction, the plates on either side of the fault line will shudder past each other, releasing tremendous energy in the form of an earthquake.
This has happened time and time again in Turkey a history that allowed McCloskey and his colleagues to map the stresses along one of its major quake sources, the East Anatolian fault. Like other faults, it is divided into segments that slip at different times. When one segment shifts and shakes, it alters the stress on neighbouring sections of the same fault and other faults nearby. That increases the stress in some places, bringing them closer to failure, but relaxes stress on others making them safer for the time being.
“They are not just randomly occurring earthquakes,” says Ross Stein, chief executive of Temblor, a company specializing in seismic hazard and risk assessment. “They are in a conversation. And that conversation is carried out through stress transfer.”
In 2002, McCloskey (now a geophysicist at the University of Edinburgh, UK) and his colleagues used this technique to diagnose regions on the East Anatolian fault that were highly stressed. With the help of historical records, the team incorporated the stress changes caused by ten earthquakes since 1822 into a model of ongoing plate movement. The modelling suggested that a region of the fault line south of Kahramanmaraş the precise location and length of the fault that ruptured on 6 February was at a heightened risk of giving way at some point in the future1. The team even knew that it would be devastating, forecasting a quake of magnitude 7.3 or higher. “The correspondence is remarkable,” McCloskey says.
It isn’t the first time that this method, technically known as Coulomb stress transfer, has accurately pinpointed an upcoming trembler. In 1997, Stein and his colleagues analysed the earthquakes that had already struck Turkey’s North Anatolian fault to estimate that the next might occur near the city of Izmit2. Two years later, that quake arrived killing more than 17,000 people. In 2005, McCloskey and his colleagues calculated that the shift in stress after the 2004 Sumatra Andaman quake in Indonesia might cause one in the Sunda trench west of Sumatra3. It came 12 days after the study was published. And in 2008, Shinji Toda from the Geological Survey of Japan in Tsukuba and his colleagues projected that the Wenchuan earthquake earlier that year in China would increase the stress of three adjacent faults4. In the following decade, two of those faults unleashed powerful earthquakes.
