A reassuring rule of thumb about earthquakes is breaking down. For decades seismologists had assumed that individual faults—as well as isolated segments of longer faults—rupture independently of one another. That limits the maximum size of potential quakes that a fault zone can generate. But the magnitude-7.8 earthquake that struck New Zealand just after midnight on 14 November 2016—among the largest in the islands' modern history—has reduced that thinking to rubble. According to a new study, the heavy shaking in the so-called Kaikōura quake was amassed by ruptures on at least 12 different faults, in some cases so far apart that they were thought to be immune to each other's influence.
The quake suggests that scientists may be misjudging seismic hazard around the world by underestimating the possibility that slip on seemingly isolated faults can add up to something far bigger. "I think it's a wake-up call," says Ned Field, a seismologist at the U.S. Geological Survey in Golden, Colorado, who leads California's seismic hazard modeling team and recently upgraded the likelihood of a large quake in the state to account for the phenomenon.
"There's this long-held view that gaps between faults of around 5 kilometers will stop a rupture from continuing," says Ian Hamling, a geodesist at government agency GNS Science in Lower Hutt, New Zealand, and lead author of the report on the Kaikōura quake, which appears online this week in Science. But evidence against that view was already growing. For example, a magnitude-7.2 earthquake that struck Mexico just north of the Gulf of California in 2010 included a 10-kilometer jump between faults. The following year, the deadly magnitude-9.0 Tōhoku earthquake in Japan was also larger than expected because it ruptured several fault segments previously believed to be independent. The Kaikōura quake is the most dramatic evidence yet that a new approach to assessing earthquake risk is needed, Field says.
Based on field observations, seismic shaking data, GPS measurements, and radar imagery from satellites, Hamling and his colleagues found that surface ruptures in the New Zealand quake were widely separated—in one case by more than 15 kilometers. Because the magnitude of an earthquake is directly related to the length of the fault it ruptures, the New Zealand quake was much larger than it would have been had it not jumped the gaps.
The phenomenon doesn't just increase the maximum size of a potential quake. It also changes the odds: With more faults potentially acting together, there are more ways to assemble big quakes, increasing their likelihood. And that means higher risk for long bridges and skyscrapers, which are more vulnerable to the long-period seismic waves released by very large quakes, Field says.
California is leading the way in accounting for these big, complex quakes. For the state's most recent earthquake forecast in 2015, Field and his colleagues relaxed the model's strict fault segmentation rules and for the first time included the possibility of multiple individual faults rupturing simultaneously. Though the California model still uses a 5-kilometer cutoff for faults to rupture together, Field says, the new model connects the vast majority of faults in the state.
The changes raised the estimated likelihood of a magnitude-8 or larger quake in California over the next 30 years from 4.7% to 7%. But because a fault system can only release as much energy as is built up by grinding tectonic plates, increasing the frequency of large events means there will be less energy to fuel smaller quakes. For California, this means the expected number of quakes around magnitude-6.7 dropped by about 30%, which more closely approximates the number in the historic record than previous models. "It's a significant step toward being a more realistic representation of the interconnectedness of the faults," Field says. The new model has already been used to update the state's seismic hazard maps, which in turn will inform the engineering of buildings and other important infrastructure.
Scientists are still working on exactly how seemingly unconnected faults separated by 15 kilometers or more ruptured together in the Kaikōura earthquake. Hamling's team concluded that previously unmapped faults near the surface helped bridge the gap, which suggests that hidden faults could be a source of unrecognized risk. But unseen deeper connections could be at work as well. Many of the faults involved in the Kaikōura quake may join up lower in the crust, Hamling says—perhaps at the tectonic boundary deep beneath New Zealand where the Pacific plate is being dragged beneath the Australian plate, which could act as a sort of master structure aiding connectivity.
But faults may not even need a physical connection in order to rupture together, says Jean-Philippe Avouac, a geologist at the California Institute of Technology in Pasadena. It's possible that seismic waves from a rupture on one fault can propagate through the ground with enough energy to cause a distant fault to slip, a process called dynamic triggering. "I'm not sure that we need these links to exist actually," Avouac says.
The New Zealand quake is not only impacting the modeling of future quakes, but is also changing the way scientists think about past ones, says earthquake geologist Kate Clark of GNS Science, a co-author on the Science paper. Clark looks for signs in the geologic record of coastal uplift caused by past earthquakes, and usually attributes movement to earthquakes rupturing one fault at a time. "We've probably misinterpreted some past records of coastal uplift and probably oversimplified past scenarios of earthquakes."