Human-triggered earthquakes – how do we know?

Nature is following the geologists who are tracking, for the first time, the impressive number of – and factors involved in –earthquakes believed to be caused by human activity:

Research now highlights how big these quakes can get — and how little scientists agree on which ones are caused by people.

The Human-Induced Earthquake Database, or HiQuake, contains 728 examples of earthquakes (or sequences of earthquakes) that may have been set off by humans over the past 149 years. Most of them were small, between magnitudes 3 and 4. But the list also includes several large, destructive earthquakes, such as the magnitude-7.8 quake in Nepal in April 2015, which one paper linked to groundwater pumping.

Miles Wilson, a hydrogeologist at Durham University, UK, and his colleagues describe the database in a paper set to be published on October 4 in Seismological Research Letters. The scientists say that HiQuake is the biggest, most up-to-date public listing of human-caused quakes ever made. By bringing the data together in this way, they hope to highlight how diverse induced quakes can be — and help society to understand and manage the future risk.

The result is a database in which the earliest entry dates to 1868, with a quake triggered by an Australian coal-mining operation. Of the 728 events, 271 (37%) are linked to mining — often from tunnel collapses. About 23% are linked to water piling up behind a dam and 15% to conventional oil and gas development. Just 4% are linked to hydraulic fracturing, or fracking, for oil and gas. Some of the more unusual cases involve quakes triggered by the building of heavy skyscrapers or by an underground nuclear-bomb test.

The largest event in the database is the magnitude-7.9 earthquake that struck in Sichuan, China, in 2008, which some have linked with the filling of a nearby reservoir. Wilson says his team was initially startled to see quakes that large proposed as human-induced. But in retrospect, he says, “we probably shouldn’t be surprised by any anthropogenic cause”. All the projects linked to earthquakes — whether blasting a mining tunnel, injecting wastewater or pumping groundwater — involve moving mass around on Earth’s surface in ways that can nudge already-stressed faults.

The scientists found a relationship between the volume of material moved — such as the size of the reservoir filled before the Chinese quake — and the magnitude of the largest linked earthquake that followed.

But that could mislead people about the real hazard from induced quakes, says Raphaël Grandin, a geophysicist at the Institute of Earth Physics in Paris. “When you put a dot in the database, and a scientific reference behind it, then you may lead the non-expert to think that the earthquake was caused by humans,” he says. Such a listing might hide scientific uncertainty, as with the Chinese quake: despite the paper linking it to reservoir filling, many seismologists do not believe it was triggered by human activity.