Small and even moderate-scale earthquakes are on the rise in North America, and not at a modest pace. The US Geological Survey (USGS) and other government agencies are scrambling to evaluate this sudden new reality, and with good reason: a few small earthquakes might not be much to worry about, but a future full of more frequent seismic activity could have disastrous effects.
Between 1973 and 2008, the USGS reported that the central and eastern United States experience an average of 21 earthquakes each year of magnitude three or greater (M3+). From 2009 to 2013, the M3+ average for the same area was 99 per year—and in 2014 alone, there have been over 650.
The past several years of USGS research have proven that this increase is not a natural anomaly; rather, the increase is largely attributed to "induced" earthquakes, or ground tremors caused by human activity.
And although activity of this magnitude is usually too small to be detected without specialized equipment, "the more earthquakes you have, the more likely you are to have bigger earthquakes," USGS seismologist Justin Rubinstein told Motherboard. "No matter the method you use to assess the hazard, the hazard in many of these areas where we're seeing induced earthquakes has dramatically increased."
In its newest earthquake hazard models the USGS now evaluates induced earthquake data as a major consideration
Human activities that add or remove a large amount of pressure to the geography of an area can cause that area to shift—sometimes resulting in surface tremors, or induced earthquakes. These activities include the construction of artificial lakes and the drilling of boreholes for geothermal energy, and the subterranean injection of wastewater by oil and gas industry plants.
The largest induced earthquake on record in America was an M5.6 earthquake that hit Prague, Oklahoma in 2011. This was the largest earthquake of any kind in Oklahoma's history, and it was likely triggered by a cascade of smaller earthquakes arising from wastewater injection. The Prague quake, and a number of others like it, have dispelled the idea that small induced earthquakes can't cause damage to human life and infrastructure.
The biggest issue is the model that played out in Prague, where it's believed that small induced earthquakes released a larger, pre-existing seismic tension. This induced-natural earthquake mechanism has also been theorized to be behind a massive M8.0 earthquake in China's Sichuan province in 2008, which killed almost 70,000 people and created a major humanitarian crisis.
As a result, in its newest earthquake hazard models the USGS now evaluates induced earthquake data as a major consideration. Yet even as the USGS begins to build models to predict and plan for induced earthquakes, not everybody is interested in the results.
Rubinstein said that though their work reveals "dramatically increased" earthquake hazards in some areas, from the perspective of a land developer, "any change to the engineering increases the cost of the building. If there's a [man made] earthquake hazard that could be turned off tomorrow, you don't want to design for that."
"Engineers themselves are often reticent to use [our models]," he said. "We expect regulators will be primary consumers of these products."
Part of the reason, Stanford University civil engineer Jack Baker told Motherboard, is that unless induced earthquakes trigger larger quakes, they likely pose little direct danger to modern cities.
"We certainly do think about the effects of repeated shaking, of building up damage over time," he said, "[but] repeated shaking is really a second-order concern relative to the primary concern: buildings being exposed to [violent] shaking when they weren't designed for it."
University of British Columbia earthquake engineering expert Perry Adebar agreed that small induced earthquakes cause little damage to buildings: "From my perspective, it's not a very big issue… most of our infrastructure can tolerate small earthquakes quite well."
One danger, however, is when the frequency ofinduced earthquakes increases in regions where natural earthquakes are already known to occur. In places like Vancouver and Salt Lake City, the majority of the local population lives within easy reach of a major earthquake fault. Both these faults have been described as "overdue" for a major slip—but an induced earthquake could cause such a slip sooner, and perhaps even make it more violent in the process.
As small and moderate earthquakes become the new normal in many areas, though, impacts could stretch beyond pure economics. "If you're feeling earthquakes every day, every other day… I think it's very stressful for some people," Rubinstein said. "One instance or two instances of shaking might not do anything, but the cumulative effect of being shaken 100 times, 200 times, 600 times, could."