Research May Explain Mysterious Deep Earthquakes In Subduction Zones

Geologists from Brown University may have finally explained what triggers certain earthquakes that occur deep beneath the Earth’s surface in subduction zones, regions where one tectonic plate slides beneath another.

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Subduction zones are some of the most seismically active areas on earth. Earthquakes in these spots that occur close to the surface can be devastating, like the one that struck Japan in 2011 triggering the Fukushima nuclear disaster. But quakes also occur commonly in the subducting crust as it pushes deep below the surface — at depths between 70 and 300 kilometers. These quakes, known as intermediate depth earthquakes, tend to be less damaging, but can still rattle buildings.

Intermediate depth quakes have long been something of a mystery to geologists.

“They’re enigmatic because the pressures are so high at that depth that the normal process of frictional sliding associated with earthquakes is inhibited,” said Greg Hirth, professor of earth, environmental, and planetary sciences at Brown. “The forces required to get things to slip just aren’t there.”

But through a series of lab experiments, Hirth and postdoctoral researcher Keishi Okazaki have shown that as water escapes from a mineral called lawsonite at high temperatures and pressures, the mineral becomes prone to the kind of brittle failure required to trigger an earthquake.

“Keishi’s experiments were basically the first tests at conditions appropriate for where these earthquakes actually happen in the earth,” Hirth said. “They’re really the first to show strong evidence for this dehydration embrittlement.”

The work will be published on February 4, 2016 in the journal Nature.

The experiments were done in what’s known as a Grigg’s apparatus. Okazaki placed samples of lawsonite in a cylinder and heated it up through the range of temperatures where water becomes unstable in lawsonite at high pressures. A piston then increased the pressure until the mineral began to deform. A tiny seismometer fixed to the apparatus detected sudden cracking in the lawsonite, a signal consistent with brittle failure.

Okazaki performed similar experiments using a different mineral, antigorite, which had been previously implicated as contributing to intermediate depth seismicity. In contrast to lawsonite, the antigorite failed more gradually — squishing rather than cracking — suggesting that antigorite does not play a role in these quakes.

“That’s one of the cool things about this,” Hirth said. “For 50 years everyone has assumed this is a process related to antigorite, despite the fact that there wasn’t much evidence for it. Now we have good experimental evidence of this dehydration process involving lawsonite.”

If lawsonite is indeed responsible for intermediate depth earthquakes, it would explain why such quakes are common in some subduction zones and not others. The formation of lawsonite requires high pressures and low temperatures. It is found in so-called “cold” subduction zones in which the suducting crust is older and therefore cooler in temperature. One such cold zone is found in northwest Japan. But conditions in “hot” subduction zones, like the Cascadia subduction zone off the coast of Washington state, aren’t conducive to the formation of lawsonite.

“In hot subduction zones, we have very few earthquakes in the subducting crust because we have no lawsonite,” Okazaki said. “But in cold subduction zones, we have lawsonite and we get these earthquakes.”

Ultimately, Hirth says research like this might help scientists to better understand why earthquakes happen at different places under different conditions.

“Trying to put into the context of all earthquakes how these processes are working might be important not just for understanding these strange types of earthquakes, but all earthquakes,” he said. “We don’t really understand a lot of the earthquake cycle. Predictability is the ultimate goal, but we’re still at the stage of thinking about what’s the recipe for different kinds of earthquakes. This appears to be one of those recipes.”

UPDATE :Can Slow Creep Along Thrust Faults Help Forecast Megaquakes?

In Japan and areas like the Pacific Northwest where megathrust earthquakes are common, scientists may be able to better forecast large quakes based on periodic increases and decreases in the rate of slow, quiet slipping along the fault.

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This hope comes from a new study by Japanese and UC Berkeley seismologists, looking at the more than 1,000-kilimeter-long fault off northeast Japan where the devastating 2011 Tohoku-oki earthquake originated, generating a tsunami that killed thousands. There, the Pacific Plate is trundling under the Japan plate, not only causing megaquakes like the magnitude 9 in 2011, but giving rise to a chain of Japanese volcanoes.

The scientists studied 28 years of earthquake measurements, looking at quakes of magnitude 2.5 or greater between 1984 and 2011. They discovered 1,515 locations off the coast of Japan where small repeating earthquakes happen — 6,126 quakes in all.

According to co-author Robert Nadeau, a UC Berkeley seismologist and a fellow with the Berkeley Institute for Data Science (BIDS), an analysis of these quakes found that larger, more destructive earthquakes — those of magnitude 5 or greater — occurred much more frequently when the periodic slow-slip was fastest. This included the great Tohoku-oki earthquake, which also devastated a nuclear power plant and led to widespread radioactive contamination.

“The persistence of the periodic pattern over time may help us refine earthquake probabilities in the future by taking into account the times of expected slow-slip pulses,” he said. “Right now, seismologists gives forecasts on a 30-year time frame and assume nothing is changing on a shorter time scale. Our study points out that things are changing, and in a periodic way. So it may be possible for scientists to give shorter time ranges of greater and lower probability for larger events to happen.”

The research was led by Naoki Uchida, a seismologist at Tohoku University, and included UC Berkeley seismologist Roland Burgmann, professor of earth and planetary science. They published their findings in the Jan. 29 issue of Science.

Slip, Nadeau said, is the relative motion between two sides of a fault, sometimes but not always resulting in ground shaking. So-called slow-slip or creep is what scientists call “fault slip,” which happens quietly, without generating shaking, not even microquakes or faint tremors.

Regions of a fault that slip quietly are considered to be weak or un-coupled. But within these un-coupled regions of rock underground there are variously-sized patches of fault that are much stronger, or coupled. These patches resist the quiet slip happening around them, only slipping when the pushing and pulling from the surrounding quiet slip stresses them to their breaking point and they “snap” in an earthquake.

“There is a relationship, which we showed here in California, between the time between ‘snaps’ on the small, strong patches where earthquakes happen and how much slip took place on the quiet fault surrounding them,” Nadeau said. “Using this relationship for thousands of repeating earthquakes in Japan, we were able to map out the evolution of slow-slip on the megathrust. Then, by studying the pattern of this evolution, we discovered the periodic nature of the megathrust slow-slip and its relationship to larger earthquakes.”

Nadeau and the late UC Berkeley seismologist Thomas McEvilly showed 12 years ago that periodic slow slip occurred all along the San Andreas Fault, from Parkfield to Loma Prieta, Calif. In 2009 the group also observed deeper, transient and periodic slow-slip on the San Andreas — this time associated with faint shaking called tremor — and that it was linked with two larger quakes occurring in 2003 and 2004 at San Simeon and Parkfield, Calif. respectively.

“The phenomenon we found in Japan may not be limited to megathrust zones,” he said. “Our 2004 study, more limited in scope than this one, showed a similar periodic slip process and an association between larger quakes — those of magnitude 3.5 or greater — and repeating earthquakes along the 170 km stretch of San Andreas Fault that we studied.”

Seismic Data Suggests Slow Slip Events May Presage Larger Earthquakes

A team of researchers, two from Tohoku University in Japan and two from the University of California in the U.S., has found evidence that suggests that a speedup in small underground deformations may occur prior to larger earthquakes, possibly providing a means for sounding a warning. In their paper published in the journal Science, the team describes how they pored over seismic data that spanned 28 years and which included approximately 6,000 seismic events, and what they found as a result—they also suggest that their findings might one day lead to a true earthquake early warning system.

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Scientists the world over have for years been searching for a way to predict when an earthquake will strike, with enough certainty to warn people in the area. To date such efforts have come up empty, though much has been learned in the process. In this new effort, the researchers report that they believe they may have found a possible indicator of an impending quake, and it is based on what are known as slips, small underground movement similar to earthquakes, but which happen so slowly that they don’t cause damage or even register on seismic monitors—the only way to detect them is to use GPS equipment.

To come to these conclusions, the researchers analyzed seismic data for Japan’s two largest islands, going back to 1984. Doing so led to the identification of 1,500 instances where there appeared to be a pattern of repetition—that allowed them to estimate the speed at which the tectonic plates below were moving. They then used statistics to correlate slippages with non-repeating measurable quakes with a magnitude of 5 or higher. Doing so revealed that there appeared to be a speedup in slippage just prior to major earthquakes. The team also looked at GPS data, which can actually be used to measure tectonic shifting, and report that it matched the rates they had calculated earlier.

The team acknowledges that much more work needs to be done before it can be confirmed that GPS monitoring devices could one day offer an early warning system, but suggest their research shows that there is the potential for such an outcome.

6.7 Magnitude Quake Hits India’s Northeast

A 6.7 magnitude earthquake hit India’s remote northeast region before dawn on Monday, killing at least four people, injuring more than 100 others and causing damage to several buildings.

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The death and injuries were caused by falling debris in and around Imphal, the capital of Manipur state, police said.

The powerful tremor left large cracks in walls and a portion of a popular market building collapsed in the state capital. The area is dotted with small houses. There are few tall buildings in the region, although a newly constructed six-story building collapsed in Imphal, the police control room said.

India’s Meteorological Department said the epicenter of the quake was in Tamenglong region of Manipur state. It struck before dawn on Monday at a depth of 17 kilometers (about 10 miles) in the India-Myanmar border region.

Police officer L. Ragui said dozens of homes were slightly damaged in Tamenglong.

No deaths had been reported so far, but four people suffered injuries when a wall collapsed on them, Ragui said by the telephone.

Shangthon Kamei, a teacher in Tamenglong, said the earthquake rattled buildings.

“It lasted for around one minute. We were sleeping and were woken up by the earthquake,” he said.

Telephone and electricity connections were disrupted in some areas.

The epicenter of the earthquake was 35 kilometers (20 miles) northwest of Imphal. The area is remote with poor cellphone and Internet connections, and information about conditions outside of major cities may take time to emerge.

Nearly 90 members of the National Disaster Response Force, a specialized federal force for natural disasters, have left to check on remote areas, police said.

People panicked and rushed out of their homes in Gauhati, the capital of neighboring Assam state, as they felt massive shaking at least twice within 60 seconds.

In Imphal, residents said furniture was knocked over and books fell off shelves.

“The ground swayed for almost a minute, jolting people awake in their homes,” said one resident, Apem Arthur.

The tremors were also felt in Kolkata, the capital of West Bengal state.