For Under-Earth Exploration, Engineers Deepen Understanding Of Rock Stress

Measuring unobservable forces of nature is not an easy feat, but it can make the difference between life and death in the context of an earthquake, or the collapse of a coal mine or tunnel.To manage the risk of such events, researchers often rely on estimating a quantity called rock stress.

“Rock stress—the amount of pressure experienced by underground layers of rock—can only be measured indirectly because you can’t see the forces that cause it,” explains Hiroki Sone, an assistant professor of civil and environmental engineering and geological engineering at the University of Wisconsin-Madison. “But instruments for estimating rock stress are difficult to use at great depths, where the temperature and pressure increase tremendously.”

Addressing this challenge, Sone and his colleagues in China and Japan have now pushed the limits of rock stress measurements that don’t require temperature-sensitive instruments to new depths, from a previous maximum of 4.5 kilometers (2.8 miles) to a whopping 7 kilometers (4.3 miles).

In a study published in July 2017 in Scientific Reports, the researchers used rocks sampled from a well bore of that depth to show that stress estimates obtained by the so-called anelastic strain recovery method were consistent with a visual analysis of borehole wall images, a reliable but often infeasible approach that requires a specialized scanner.

The scientists conducted their proof-of-principle study in the Tarim Basin in northwest China, an area almost two-thirds the size of Alaska that is surrounded by K2, the world’s second highest mountain after Mount Everest, and several other mountain ranges. The region is well known to historians because of its association with the Silk Road, an ancient trade route between China and the Mediterranean.

Today, in addition to historians and mountain climbers, petroleum companies have taken an interest in Tarim Basin, as it contains some of the largest oil and gas resources in Central Asia. These companies want to understand the region’s geology to assess whether drilling may trigger seismic activity, given that many smaller earthquakes have occurred in the surrounding mountains.

For Sone and his colleagues, this presented a unique opportunity to advance the methodology for measuring rock stress.

“We wanted to test the reliability of the anelastic strain recovery method at up to 7 kilometers depth because its main advantage is that you only need to sample and analyze the rock itself,” Sone says. “It estimates stress indirectly by measuring how much the rock sample expands in different directions after it has been recovered.”

With that kind of depth, the recovery process—pulling a large enough rock sample out of a borehole—can take a few days, which is why the researchers were excited to prove that the method still worked.

For the first time, they measured rock stress even when sensors weren’t attached to the sample until 65 hours after coring and found that the results matched a conventional image analysis of the borehole wall, obtained with a resistivity scanner. While the visual method also worked in this case, it can be infeasible at such great depths because of the scanner’s temperature limitations.

In addition to proving the easier method’s validity at greatly increased depth, the study resolved a longstanding geological puzzle in the Tarim Basin: The rock stress in Earth’s outer shell—which consists of many large pieces of cooler rock (tectonic plates) floating on a very thick layer of hot magma—differs between the Basin’s periphery and its interior.
Other scientists had found evidence for this difference before, but the current study confirmed it.

In the interior of Tarim Basin, tectonic plates are relatively stable, even though they crash and fold up against each other in the periphery, explaining the observed seismic activity. This translates to a lower risk of earthquakes in the interior and informs a petroleum company’s decisions about the depth at which boreholes should be stabilized to minimize the risk of structural collapse.

For earth scientists, the new study is an important validation of a more practical method for estimating rock stress. “These new results give us confidence that we can use the anelastic strain recovery method at greater depths than we thought possible,” Sone says. “As long as the rock deforms the same amount in vertical and horizontal directions, this method is much easier to apply when very high temperatures and pressures in the Earth’s crust challenge the other options in our toolbox.”

IMPORTANT UPDATE: New Research Shows Quake-Causing Cracks on Pacific Sea Floor

New research published in the journal ‘Science Advances’, has focused their study off the west coast of North America giving seismologists a better understanding of what one scientist describes as “the single greatest geophysical hazard to the continental United States”.

Zach Eilon, a geophysicist at the University of California Santa Barbara, has developed a new method that uses an array of scientific instruments spread across the sea floor to measure shock waves that travel through the planet’s crust. “Because we think this particular phenomenon is strongly related to temperature and to molten rock beneath the Earth, this is a technique that can be applied to volcanoes to get a better sense of their plumbing system,” says Eilon.

Eilon’s research targets the Juan de Fuca plate, which runs several hundred kilometers off the coast between southern British Columbia and northern California and is the youngest and smallest of the planet’s 13 major tectonic plates. The collision zone in this region has the potential to generate massive quakes and destructive tsunamis, which occur when the plates overcome friction and slip past one another, quickly displacing huge amounts of water.

His data suggest the interior of the Juan de Fuca plate is cooler than previously believed, meaning the edge that is being pushed westward below the North American plate is able to bring with it more water. The water acts as a lubricant and increases the likelihood of the slipping that leads to a quake.

Geoff Abers, an earth-sciences professor at Cornell University who co-authored the paper with Eilon, said improvements in sea-floor technology and the sheer number of sensors that were deployed make this project the first time researchers have been able to study an entire tectonic plate in the ocean. “We’re not directly looking at the just earthquake cycles, but we’re looking at the broader, theoretical framework for how the Earth works and getting a much better handle on that,” Abers said.

Thank you for your continued support. We’re now about half way there.


Harvesting Earthquake Fault Slip f­rom Laser Images of Napa’s Vineyards

A new U.S. Geological Survey-led study suggests that earthquake-related deformation just below the Earth’s surface can be quite different from how it is expressed at the surface. Scientists using laser images of grapevine rows deformed by the 2014 South Napa earthquake have found that the amount of surface displacement caused by the earthquake could be significantly less than estimates of the actual slip across the fault plane. The laser images show the amount that the portion of a vine row on one side of the fault was shifted horizontally with respect to the portion on the other side.

The findings are important because they provide unprecedented details of the process of earthquake-related fault slip reaching near the Earth’s surface, the place where structures are built and where people reside.

The team used computer models to relate the measured surface distortion to fault slip at depth and verified their results with trenches cut across sections of the West Napa Fault that produced surface disruption associated with the 2014 earthquake.

“If fault slip at a couple of meters’ depth could be different than slip right at the surface, and we can infer that using these types of high-resolution laser images and slip models, then we can use this information to make better estimates of the rates at which faults slip over multiple earthquake cycles,” said lead author USGS geophysicist Ben Brooks.

This, in turn, would lead to more accurate seismic hazard assessments and improved methodologies for mitigating and monitoring the possible interruption of underground infrastructures such as pipelines by near-surface faulting.

The multi-institutional team, including researchers from the California Geological Survey and geological consultants, used the same type of LIDAR (Light Detection and Ranging) laser technology that is mounted on the roofs of self-driving vehicles for navigation.

“We’re using the same imaging and navigation technology mounted on the roofs of the test robotic vehicles that people see driving around these days,” said co-author professor Craig Glennie of the University of Houston’s National Center for Airborne Laser Mapping. “Only we are processing the data to higher precision and accuracy so that we can detect ground motions on the order of centimeters.”

Before the ground-based laser scanning technology employed by the authors became available, geologists had to rely on satellite-based measurements of ground deformation that could only be made with much less spatial resolution and less frequently — measurements which wouldn’t have revealed the detail necessary to make the study’s breakthrough.

“What’s so exciting about these new imaging technologies is that we can now learn how much earthquake slip happens very close to the surface, which is where all the people and infrastructure are located,” said USGS geophysicist and co-author Sarah Minson.

‘Strong’ 6.1-Magnitude Earthquake Strikes Off Japan Coast

An earthquake of magnitude 6.1 struck in the Pacific Ocean, off the coast of Japan, this morning.

The shaker – which is classed as “strong” – was near the island of Okinawa, which has a population of more than 1.4million.

Almost 16,000 people were killed by an earthquake off Japan’s coast in 2011.

The under-sea tremor – which has a similar depth to today’s quake – caused a tsunami, which led to the Fukushima Daiichi nuclear disaster.

The epicentre was in the Pacific Ocean – between Japan, China, South Korea and the Philippines, which has a population of 100million.

The island of Taiwan – where 24million people live – is very near.

There were no immediate reports of damage or injuries in the quake, which hit at a depth of 21 miles, about 166 miles east of Okinawa.

The US Geological Survey – which monitors earthquakes and volcanoes worldwide – said there is a “low likelihood” of casualties and damage.
There is a one in three chance up to 10 people could be killed – and only a 4% chance of more deaths.

The 1.4million people who leave nearby will have felt “weak” effects.

A USGS spokesman said: “Overall, the population in this region resides in structures that are resistant to earthquake shaking – though vulnerable structures exist.

“The predominant vulnerable building types are low-rise concrete wall and light wood frame construction.”

Japan lies on the notorious “Ring of Fire” – land around the Pacific Ocean regularly rocked by earthquakes and volcanoes.

A powerful 6.6-magnitude earthquake struck Indonesia – also in the region – back in May.

Two people were killed and more than 120 injured when an earthquake hit the Mediterranean last week.

A series of earthquakes in Wyoming has sparked fears a giant supervolcano in Yellowstone National Park could blow.

Strength Of Tectonic Plates May Explain Shape Of The Tibetan Plateau

Geoscientists have long puzzled over the mechanism that created the Tibetan Plateau, but a new study finds that the landform’s history may be controlled primarily by the strength of the tectonic plates whose collision prompted its uplift. Given that the region is one of the most seismically active areas in the world, understanding the plateau’s geologic history could give scientists insight to modern day earthquake activity.

The new findings are published in the journal Nature Communications.

Even from space, the Tibetan Plateau appears huge. The massive highland, formed by the convergence of two continental plates, India and Asia, dwarfs other mountain ranges in height and breadth. Most other mountain ranges appear like narrow scars of raised flesh, while the Himalaya Plateau looks like a broad, asymmetrical scab surrounded by craggy peaks.

“The asymmetric shape and complex subsurface structure of the Tibetan Plateau make its formation one of the most significant outstanding questions in the study of plate tectonics today,” said University of Illinois geology professor and study co-author Lijun Liu.

In the classic model of Tibetan Plateau formation, a fast-moving Indian continental plate collides head-on with the relatively stationary Asian plate about 50 million years ago. The convergence is likely to have caused the Earth’s crust to bunch up into the massive pile known as the Himalaya Mountains and Tibetan Plateau seen today, but this does not explain why the plateau is asymmetrical, Liu Said.

“The Tibetan Plateau is not uniformly wide,” said Lin Chen, the lead author from the Chinese Academy of Sciences. “The western side is very narrow and the eastern side is very broad — something that many past models have failed to explain.”Many of those past models have focused on the surface geology of the actual plateau region, Liu said, but the real story might be found further down, where the Asian and Indian plates meet.

“There is a huge change in topography on the plateau, or the Asian plate, while the landform and moving speed of the Indian plate along the collision zone are essentially the same from west to east,” Liu said. “Why does the Asian plate vary so much?”

To address this question, Liu and his co-authors looked at what happens when tectonic plates made from rocks of different strengths collide. A series of 3-D computational continental collision models were used to test this idea.

“We looked at two scenarios — a weak Asian plate and a strong Asian plate,” said Liu. “We kept the incoming Indian plate strong in both models.”

When the researchers let the models run, they found that a strong Asian plate scenario resulted in a narrow plateau. The weak Asian plate model produced a broad plateau, like what is seen today.

“We then ran a third scenario which is a composite of the strong and weak Asian plate models,” said Liu. “An Asian plate with a strong western side and weak eastern side results in an orientation very similar to what we see today.”

This model, besides predicting the surface topography, also helps explain some of the complex subsurface structure seen using seismic observation techniques.

“It is exciting to see that such a simple model leads to something close to what we observe today,” Liu said. “The location of modern earthquake activity and land movement corresponds to what we predict with the model, as well.”

Magnitude 7.2 Quake Hits Near Fiji; Tsunami Alert Issued

A powerful earthquake struck off the coast of Fiji on Wednesday, prompting a brief tsunami warning for the Pacific island nation. There were no immediate reports of damage or injuries.

The magnitude 7.2 quake, which hit at 9:52 a.m. local time, struck about 220 kilometers (135 miles) southwest of the tourist hub of Nadi, the U.S. Geological Survey said. The quake was a relatively shallow 15 kilometers (9 miles) deep. Shallower quakes generally cause more damage than ones that strike deeper.

The Pacific Tsunami Warning Center issued a tsunami warning for coastlines within 300 kilometers (190 miles) of the epicenter, then lifted the warning about an hour later. A tsunami of just 1 centimeter (less than an inch) was observed in the capital of Suva, the center said.

Fiji’s Principal Disaster Management Officer, Sunia Ratulevu, said there had been no reports of damage or injuries from the quake, and no unusual wave activity had been reported. The quake struck far offshore and was not felt in Suva or Nadi, he said.

There have been several big aftershocks in the same area. The strongest two had a magnitude of 5.2 and 5.4.

When the tsunami alert was issued, people in Suva fled their offices and headed inland, Ratulevu said. But by early afternoon, authorities were telling people the threat had passed and it was safe to return to work.

Fiji is prone to earthquakes because of its location on the “Ring of Fire,” an arc of seismic faults around the Pacific Ocean.

There was no threat to nearby Pacific island nations Vanuatu and New Caledonia, authorities said.

A 2004 quake and tsunami killed a total of 230,000 people in a dozen countries, most of them in Aceh, Indonesia.

Large 7.9 Quake Near Papua New Guinea Sets Off Tsunami

A powerful earthquake struck off the coast of Papua New Guinea, generating a small tsunami and knocking out power in parts of the Pacific island nation. There were no immediate reports of injuries or damage.


The magnitude-7.9 quake struck 46 kilometers (29 miles) east of Taron in Papua New Guinea, the U.S. Geological Survey said. The quake was deep, at 103 kilometers (61 miles). Deeper earthquakes tend to cause less damage than shallow ones.

The USGS initially said the quake’s magnitude was 8.0, but later downgraded the strength.

The Pacific Tsunami Warning Center said there was a threat of a tsunami in Papua New Guinea and nearby areas. It said tsunami waves reaching 1-3 meters (yards) high were possible along the coasts of Papua New Guinea, while waves in other areas, including the Solomon Islands, would likely be less than 0.3 meter (1 foot) high.

A tsunami measuring less than 1 meter (3 feet) hit the coast of the island of New Ireland shortly after the earthquake, said Felix Taranu, seismologist with the Geophysical Observatory in the capital, Port Moresby. There were no immediate reports of damage from the tsunami or the quake, though officials were still working to contact people on the island, he said.

The quake knocked items off shelves and caused a blackout in the town of Kokopo in northeastern Papua New Guinea, Taranu said. But there were no reports of widespread damage in the town.