Slippery fault unleashed destructive Tohoku-Oki earthquake and tsunami

UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. -  IODP
UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. – IODP

For the first time, scientists have measured the frictional heat produced by the fault slip during an earthquake. Their results, published December 5 in Science, show that friction on the fault was remarkably low during the magnitude 9.0 Tohoku-Oki earthquake that struck off the coast of Japan in March 2011 and triggered a devastating tsunami.

“The Tohoku fault is more slippery than anyone expected,” said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and coauthor of three papers on the Tohoku-Oki earthquake published together in Science. All three papers are based on results from the international Japan Trench Fast Drilling Project (JFAST), which Brodsky helped organize.

Because friction generates heat (like rubbing your hands together), taking the temperature of a fault after an earthquake can provide a measure of the fault’s frictional resistance to slip. But that hasn’t been easy to do. “It’s been difficult to get this measurement because the signal is weak and it dissipates over time, so we needed a big earthquake and a rapid response,” said Brodsky, a professor of Earth and planetary sciences at UCSC.

The JFAST expedition drilled across the Tohoku fault in 2012 and installed a temperature observatory in one of three boreholes nearly 7 kilometers below the ocean surface. The logistically and technically challenging operation successfully recovered temperature measurements and other data as well as core samples from across the fault.

The low resistance to slip on the fault may help explain the large amount of slip–an unprecedented 50 meters of displacement–that occurred during the earthquake, according to UC Santa Cruz researcher Patrick Fulton, who is first author of the paper focusing on the temperature measurements. An abundance of weak, slippery clay material in the fault zone–described in the two companion papers–may account for the low friction during the earthquake, he said.

The Tohoku-Oki earthquake occurred in a “subduction zone,” a boundary between two tectonic plates where one plate is diving beneath another–in this case, the Pacific plate dives beneath the Eurasian plate just east of Japan. Fulton explained that the epicenter, where the earthquake started, was much deeper than the shallow portion of the fault examined by JFAST. One of the surprising things about the earthquake, in addition to the 50 meters of slip, was that the fault ruptured all the way to the surface of the seafloor.

“The large slip at shallow depths contributed to the tsumani that caused so much damage in Japan. Usually, these earthquakes don’t rupture all the way to the surface,” Fulton said.

The strain that is released in a subduction zone earthquake is thought to build up in the deep portion of the fault where the two plates are “locked.” The shallow portion of the fault was not expected to accumulate a large amount of stress and was considered unlikely to produce a large amount of slip. The JFAST results show that the frictional stress on the shallow portion of the fault was very low during the earthquake, which means that either the stress was low to begin with or all of the stress was released during the earthquake.

“It’s probably a combination of both–the fault was pretty slippery to begin with, and whatever stress was on the fault at that shallow depth was all released during the earthquake,” Fulton said.

An earlier paper by JFAST researchers, published in Science in February 2013 (Lin et al.), also suggested a nearly total stress drop during the earthquake based on an analysis of geophysical data collected during drilling.

“We now have four lines of evidence that frictional stress was low during the earthquake,” Brodsky said. “The key measure is temperature, but those results are totally consistent with the other papers.”

One of the new papers (Ujiie et al.) presents the results of laboratory experiments on the material recovered from the fault zone. Tests showed very low shear stress (resistance to slip) attributable to the abundance of weak, slippery clay material. The other paper (Chester et al.) focuses on the geology and structure of the fault zone. In addition to the high clay content, the researchers found that the fault zone was surprisingly thin (less than 5 meters thick).

J. Casey Moore, a research professor of Earth sciences at UCSC and coauthor of the Chester et al. paper, said he suspects the clay layer observed in the Tohoku fault zone may play an important role in other fault zones. “Looking for something like that clay may give us a tool to understand the locations of earthquakes that cause tsunamis. It’s potentially a predictive tool,” Moore said.

According to Brodsky, measuring the frictional forces on the fault is the key to a fundamental understanding of earthquake mechanics. “We’ve been hamstrung without in situ measurements of frictional stress, and we now have that from the temperature data,” she said. “It’s hard to say how generalizable these results are until we look at other faults, but this lays the foundation for a better understanding of earthquakes and, ultimately, a better ability to identify earthquake hazards.”

Seismic gap outside of Istanbul

Earthquake researchers have now identified a 30 kilometers long and ten kilometers deep area along the North Anatolian fault zone just south of Istanbul that could be the starting point for a strong earthquake. The group of seismologists including Professor Marco Bohnhoff of the GFZ German Research Centre for Geosciences reported in the current online issue of the scientific journal Nature (Nature Communications, DOI: 10.1038/ncomms2999) that this potential earthquake source is only 15 to 20 kilometers from the historic city center of Istanbul.

The Istanbul-Marmara region of northwestern Turkey with a population of more than 15 million faces a high probability of being exposed to an earthquake of magnitude 7 or more. To better understand the processes taking place before a strong earthquake at a critically pressurized fault zone, a seismic monitoring network was built on the Princes Islands in the Sea of Marmara off Istanbul under the auspices of the Potsdam Helmholtz Centre GFZ together with the Kandilli Earthquake Observatory in Istanbul. The Princes Islands offer the only opportunity to monitor the seismic zone running below the seafloor from a distance of few kilometers.

The now available data allow the scientists around GFZ researcher Marco Bohnhoff to come to the conclusion that the area is locked in depth in front of the historic city of Istanbul: “The block we identified reaches ten kilometers deep along the fault zone and has displayed no seismic activity since measurements began over four years ago. This could be an indication that the expected Marmara earthquake could originate there”, says Bohnhoff.

This is also supported by the fact that the fracture zone of the last strong earthquake in the region, in 1999, ended precisely in this area – probably at the same structure, which has been impeding the progressive shift of the Anatolian plate in the south against the Eurasian plate in the north since 1766 and building up pressure. The results are also being compared with findings from other fault zones, such as the San Andreas Fault in California, to better understand the physical processes before an earthquake.

Currently, the GFZ is intensifying its activity to monitor the earthquake zone in front of Istanbul. Together with the Disaster and Emergency Management Presidency of Turkey AFAD, several 300 meter deep holes are currently being drilled around the eastern Marmara Sea, into which highly sensitive borehole seismometers will be placed. With this Geophysical borehole Observatory at the North Anatolian Fault GONAF, measurement accuracy and detection threshold for microearthquakes are improved many times over. In addition, the new data also provide insights on the expected ground motion in the event of an earthquake in the region. Bohnhoff: “Earthquake prediction is scientifically impossible. But studies such as this provide a way to better characterize earthquakes in advance in terms of location, magnitude and rupture progression, and therefore allow a better assessment of damage risk.”

Journey to the Center of the Earth: Discovery Sheds Light on Mantle Formation

Geologist unearths ancient rocks from ocean floor dating back two billion years

Uncovering a rare, two-billion-year-old window into the Earth’s mantle, a University of Houston professor and his team have found our planet’s geological history is more complex than previously thought.

Jonathan Snow, assistant professor of geosciences at UH, led a team of researchers in a North Pole expedition, resulting in a discovery that could shed new light on the mantle, the vast layer that lies beneath the planet’s outer crust. These findings are described in a paper titled “Ancient, highly heterogeneous mantle beneath Gakkel Ridge, Arctic Ocean,” appearing recently in Nature, the weekly scientific journal for biological and physical sciences research.

These two-billion-year-old rocks that time forgot were found along the bottom of the Arctic Ocean floor, unearthed during research voyages in 2001 and 2004 to the Gakkel Ridge, an approximately 1,000-mile-long underwater mountain range between Greenland and Siberia. This massive underwater mountain range forms the border between the North American and Eurasian plates beneath the Arctic Ocean, where the two plates diverge.

These were the first major expeditions ever undertaken to the Gakkel Ridge, and these latest published findings are the fruit of several years of research and millions of dollars spent to retrieve and analyze these rocks.

The mantle, the rock layer that comprises about 70 percent of the Earth’s mass, sits several miles below the planet’s surface. Mid-ocean ridges like Gakkel, where mantle rock is slowly pushing upward to form new volcanic crust as the tectonic plates slowly move apart, is one place geologists look for clues about the mantle. Gakkel Ridge is unique because it features – at some locations – the least volcanic activity and most mantle exposure ever discovered on a mid-ocean ridge, allowing Snow and his colleagues to recover many mantle samples.

“I just about fell off my chair,” Snow said. “We can’t exaggerate how important these rocks are – they’re a window into that deep part of the Earth.”

Venturing out aboard a 400-foot-long research icebreaker, Snow and his team sifted through thousands of pounds of rocks scooped up from the ocean floor by the ship’s dredging device. The samples were labeled and cataloged and then cut into slices thinner than a human hair to be examined under a microscope. That is when Snow realized he found something that, for many geologists, is as rare and fascinating as moon rocks – mantle rocks devoid of sea floor alteration. Analysis of the isotopes of osmium, a noble metal rarer than platinum within the mantle rocks, indicated they were two billion years old. The use of osmium isotopes underscores the significance of the results, because using them for this type of analysis is still a new, innovative and difficult technique.

Since the mantle is slowly moving and churning within the Earth, geologists believe the mantle is a layer of well-mixed rock. Fresh mantle rock wells up at mid-ocean ridges to create new crust. As the tectonic plates move, this crust slowly makes its way to a subduction zone, a plate boundary where one plate slides underneath another and the crust is pushed back into the mantle from which it came.

Because this process takes about 200 million years, it was surprising to find rocks that had not been remixed inside the mantle for two billion years. The discovery of the rocks suggests the mantle is not as well-mixed or homogenous as geologists previously believed, revealing that the Earth’s mantle preserves an older and more complex geologic history than previously thought. This opens the possibility of exploring early events on Earth through the study of ancient rocks preserved within the Earth’s mantle.

The rocks were found during two expeditions Snow and his team made to the Arctic, each lasting about two months. The voyages were undertaken while Snow was a research scientist at the Max Planck Institute in Germany, and the laboratory study was done by his research team that now stretches from Hawaii to Houston to Beijing.

Since coming to UH in 2005, Snow’s work stemming from the Gakkel Ridge samples has continued, with more research needed to determine exactly why these rocks remained unmixed for so long. Further study using a laser microprobe technique for osmium analysis available only in Australia is planned for next year.

Newly Discovered Active Fault Building New Islands Off Croatian Coast

This old fortress in Dubrovnik sits on top of an ancient thrust fault, visible in the photo. The newly discovered active thrust fault lies not far offshore. (Richard A. Bennett)
This old fortress in Dubrovnik sits on top of an ancient thrust fault, visible in the photo. The newly discovered active thrust fault lies not far offshore. (Richard A. Bennett)

A newly identified fault that runs under the Adriatic Sea is actively building more of the famously beautiful Dalmatian Islands and Dinaride Mountains of Croatia, according to a University of Arizona researcher and colleagues.

Geologists had previously believed that the Dalmatian Islands and the Dinaride Mountains had stopped growing 20 to 30 million years ago.

From a region northwest of Dubrovnik, the new fault runs northwest at least 200 km (124 miles) under the sea floor.

The Croatian coast and the 1,185 Dalmatian Islands are an increasing popular tourist destination. Dubrovnik, known as “the Pearl of the Adriatic,” is a UNESCO-designated World Heritage site.

At the fault, the leading edge of the Eurasian plate is scraping and sliding its way over a former piece of the African plate called the South Adria microplate, said lead researcher Richard A. Bennett, an assistant professor of geosciences.

“It’s a collision zone,” Bennett said. “Two continents are colliding and building mountains.”

Bennett and his colleagues found that Italy’s boot heel is moving toward the Croatian coast at the rate of about 4 mm (0.16 inches) per year. By contrast, movement along parts of California’s San Andreas fault can be 10 times greater.

The region along the undersea fault has no evidence of large-magnitude earthquakes occurring in the last 2,000 years. However, if the fault is the type that could move abruptly and cause earthquakes, tsunami calculations for the region need to be redone, he said.

“It has implications for southern Italy, Croatia, Montenegro and Albania.”

At its southern end, the newly identified fault connects to a seismically active fault zone further south that caused a large-magnitude earthquake in Dubrovnik in 1667 and a magnitude 7.1 earthquake in Montenegro in 1979.

Bennett and his colleagues published their article, “Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides,” in the January issue of the journal Geology. His co-authors are UA geoscientists Sigrún Hreinsdóttir and Goran Buble; Tomislav BaÅ¡ic’ of the University of Zagreb and the Croatian Geodetic Institute; Željko Bac(ic’ and Marijan Marjanovic’ of the Croatian State Geodetic Administration in Zagreb; and Gabe Casale, Andrew Gendaszek and Darrel Cowan of the University of Washington in Seattle.

The research was funded by the Croatian Geodetic Administration and the U.S. National Science Foundation.

Geologists have been trying to figure out how the collision between the African and Eurasian continents is being played out in the Mediterranean.

Bennett was studying the geology of Italy’s Alps and Apennine Mountains and realized he needed to know more about the mountains on the other side of the Adriatic.

The Croatian mountains and coasts are relatively understudied, in part because of years of political turmoil in the region, he said. So he teamed up with Croatian geologists.

Bennett is an expert in a technique called geodesy that works much like the Global Positioning System in a car.

“We put GPS units on rocks and watch them move around,” he said. “We leave an antennae fixed to a rock and record its movement all the time. We basically just watch it move.”

Just as the GPS in a rental car uses global positioning satellites to tell where the car is relative to a desired destination, the geodesy network can tell where one antenna and its rock are relative to another antenna.

Recent improvements in the technology make it possible to see very small movements of the Earth. “In Croatia we can resolve motions at the level of about one millimeter year,” he said.

The researchers found that the motion between Italy’s boot heel and Eurasia is absorbed at the Dinaride Mountains and Dalmatian Islands.

Combining geodetic data with other geological information revealed that the movement is accommodated by a previously unknown fault under the Adriatic.

Bennett likens movement of the Eurasian plate to a snowplow blade piling up snow in front of it. The snow represents the sea floor being pushed up to form the Dalmatian Islands and the Dinaride Mountains.

“You can see hints of new islands out there,” he said.

But those islands may not provide seaside vacations forever. Bennett said the Adriatic Sea is closing up at the rate of 4.5 km (2.8 miles) per million years. If things continue as they are now, he calculates the eastern and western shores of the Adriatic Sea will meet in about 50 to 70 million years.

“This new finding is an important piece in the puzzle to understanding Mediterranean tectonics,” he said.

He plans to set out additional antennas to learn more about current movement of the region and to figure out what the fault has been doing for the past 40 million years.

The additional information will also help gauge the region’s earthquake potential.

“We want to see if the fault is freely slipping or is accumulating strain and therefore may produce a large earthquake in the future,” Bennett said.