Researchers picked up warning signs on Friday – days before a 6.2 magnitude quake struck Gansu – but were unable to say exactly where it would strike. The team in Shaanxi looks for abnormalities in gravitational waves, and are now trying to work out how they can calculate where a quake is likely to strike. — SCMP
Chinese researchers expected an earthquake to strike hours before the deadly 6.2 magnitude quake in the northwestern province of Gansu – but were unable to say where it would happen.
More than 120 people have died since the quake struck just before midnight on Monday, with the epicentre in a township located in the autonomous county of Jishishan.
Predicting earthquakes has almost appeared to be a mission impossible, with most equipment only able to give a few seconds’ warning once it has started.
But researchers in the Chinese province of Shaanxi have developed a groundbreaking technique that has allowed them to forecast successfully every earthquake with a magnitude of 7.0 or more that has struck around the globe in the past 10 years.
But to their dismay, there is still no way to predict the location of these earthquakes.
The team uses high-precision equipment to monitor the Earth’s gravitational field at low frequencies, looking for abnormalities in gravitational waves that could be a warning sign an earthquake is imminent.
The researchers first realised that the latest earthquake was on the way when they received a text alert about abnormal data readings from multiple sensors on Friday morning.
The team then began discussing where the earthquake might strike and, in the afternoon before it hit, Zhang Maosheng, a professor and dean at Xian Jiaotong University, was thinking: “We need to forecast a location as soon as possible.”
By this point, the team had already calculated that the earthquake had a high probability of hitting within three to five days of the abnormal data readings being recorded.
They were also able to predict that the magnitude of the earthquake would be around 6.27 based on the strength of the data peaks observed, Zhang said.
But they could not forecast that the earthquake was about to happen in a province bordering theirs. “[We] didn’t think it would be this close,” Zhang said.
The researchers have been able to detect early warning signs of multiple quakes, including the devastating Turkey-Syria earthquake that struck in February, and published details of their precursor monitoring method and forecasting results in the Chinese journal Northwestern Geology in June.
Four days before the Gansu earthquake hit, the team recorded abnormal gravitational wave readings at four different sensor locations around Shaanxi.
Although the sensors were located in different cities, the anomalous waves were all recorded at nearly the same time, something that indicated they were travelling at the speed of light, said Liu Huaqiang, a professor at Northwestern Polytechnical University in Xian.
While other gravitational disturbances from other sources could cause abnormal data readings, gravitational earthquakes waves characteristically travelled at the speed of light “so it wasn’t hard to understand whether this was an earthquake precursor”, Liu said.
Through studying nearly 500 earthquakes over the past two years, the team could now estimate the magnitude and a time frame for when the earthquake might occur, Zhang said.
But the final piece of the puzzle they now need to crack is where the quake will strike.
“The only aspect we cannot determine is the location,” Liu said.
One idea for how to solve this problem was to place sensors all around the world and use the difference in time it takes for the waves to hit different gravimeters to help pinpoint a location.
Zhang said that if the team was able to begin pinpointing the locations of earthquakes once they saw an abnormal data reading, they hoped to share their findings for global use. But he warned that because these waves travelled at the speed of light, the difference in timing was currently “far too small” to make such a calculation.
Liu said the method they were pursuing now was to “look for patterns related to the tectonic plates”, to determine unique signal waves originating from specific locations.
The China Earthquake Early Warning Network was able to send out text alerts within 30 seconds of the earthquake hitting areas around the Gansu quake’s epicentre. Those closest to it received a text alert only 12 seconds before they felt the effects of the earthquake.
These early warning systems provide alerts “when the earthquake has already started”, Liu said. Those who live further away from the epicentre – and who therefore are less likely to be affected – are given the most time to prepare. – South China Morning Post
