Scientists in California have found that earthquakes can occur much deeper below the Earth's surface than originally believed, a discovery that alters their understanding of seismic behavior and potential risks.
Seismologists have long believed that earthquakes occur less than 12 to 15 miles underground in the planet's brittle, rocky crust. But new research has found evidence of quakes deeper than 15 miles under the surface, in the upper mantle, an area where the rock is so hot that it is no longer brittle but creeps, moving around like an extremely hard honey.
Three scientists at
Caltech seismology professor Jean Paul Ampuero, one of three authors of the study that was published Thursday in the journal Science, said the research raised the possibility that the Newport-Inglewood fault and others, such as the San Andreas, could see even more powerful earthquakes than expected. The earthquakes he and his colleagues studied were so deep that they were not felt at the surface by conventional seismic sensors.
The new report indicates that it's possible a quake much closer to the surface could travel much deeper into the Earth, producing a stronger, more damaging rupture than previously believed was possible.
"That got us thinking — that if earthquakes want to get big, one way of achieving that is by penetrating deep," Ampuero said. "The big question is: If the next, larger earthquake happens, if it manages to penetrate deeper than we think, it may be bigger than we expect."
It's an idea that was first raised in 2012, also by Ampuero and several colleagues in the journal Science, when a magnitude 8.6 earthquake struck the Indian Ocean.
That was the largest quake of its kind "that has ever happened," Ampuero said. It happened on a fault known as a "strike-slip," the same kind of fault as Newport-Inglewood and California's mighty San Andreas, the state's longest fault.
But that Indian Ocean earthquake was so large, it was impossible to explain how it happened with existing science.
So answering the question of how an 8.6 earthquake occurred required a new explanation — that the quake occurred on a fault that not only ruptured the crust, but went deeper into the mantle.
If deep earthquakes can occur, researchers say, then it's possible that the Newport-Inglewood fault — which runs under a densely populated swath of Southern California — could produce a larger earthquake that experts had believed possible. Scientists have long thought that the Newport-Inglewood fault could produce a temblor of up to magnitude 7.4.
But a lot more study needs to be done.
The deep quakes that the Caltech scientists detected were only microquakes — topping out at about a magnitude 2.
Therefore, it is also possible that these deep earthquakes remain small and don't help a larger earthquake closer to the surface become stronger. With this theory, earthquakes in this deep zone occur in small pockets far away from one another and don't link in a way that allows a big earthquake to get stronger.
"This could be good news, in a way, because if they never break together, that means they can break in tiny earthquakes, but they cannot break in large ones," Ampuero said. "So several questions are still open. I wouldn't say that this is cause for alarm at this point. These are very interesting questions that we need to pursue."
Scientists not involved with the study said further research is essential to understand the implications of deeper earthquakes. For instance, the discovery of magnitude 3 or magnitude 4 earthquakes in the mantle would raise additional concern, said U.S. Geological Survey research geophysicist Brad Aagaard.
"It's worth further investigation, on multiple fronts, to look at what the physics of the Newport-Inglewood really are," Aagaard said.
USGS research geologist Kate Scharer said the study has implications for "all of the faults here." It has long been thought that, for the most part, the mantle was so malleable it did not store seismic strain. "So this presents an interesting possibility that it might be," she said.
Still, it's plausible that the deep earthquakes are merely "chatter" that happens at depths where faults don't build up seismic strains powerful enough to generate a big quake, said USGS seismologist Susan Hough.
"It's a very cool study," Hough said. "Whether or not deep fault extensions store significant strain, understanding their behavior gives us a better understanding of fault and plate boundary systems."
Another thing to consider: The deep earthquakes were found in a 9-square-mile area underneath Long Beach, recorded over six months. When researchers looked farther northwest — over a shorter period, only four weeks — they did not find deep earthquakes there, Ampuero said.
So it's possible that deep earthquakes don't exist everywhere on the Newport-Inglewood fault. But it also could be true that scientists will eventually detect those deep earthquakes elsewhere on the fault if they keep on monitoring it for a longer period of time.
There might also be something particular about Long Beach that produces these deep quakes. Scientists found evidence that there are some liquids flowing from the mantle up to the surface below the city — an observation that was not found elsewhere on the Newport-Inglewood fault.
Long Beach was devastated by the 1933 quake, which killed 120 people and flattened many buildings in the city and beyond.
The scientists obtained the data from a group who installed sensors to better understand the area's oil fields. Once they collected the massive amounts of information, the scientists had to design a program to process it to understand what was going on miles underground, invisible to conventional seismic sensing equipment.
In addition to Ampuero, the study's other authors are Asaf Inbal and Robert Clayton.
Oct, 7, 10:35 a.m.: This article was updated with comments from USGS seismologist Susan Hough.
4:25 p.m.: This article was updated with comments from USGS scientists not affiliated with the study.
3 p.m.: This article was updated throughout.