Longmanshen fault zone still hazardous, suggest new reports

The 60-kilometer segment of the fault northeast of the 2013 Lushan rupture is the place in the region to watch for the next major earthquake, according to research published in Seismological Research Letters (SRL). Research papers published in this special section of SRL suggest the 2008 Wenchuan earthquake triggered the magnitude 6.6 Lushan quake.

Guest edited by Huajian Yao, professor of geophysics at the University of Science and Technology of China, the special section includes eight articles that present current data, description and preliminary analysis of the Lushan event and discuss the potential of future earthquakes in the region.

More than 87,000 people were killed or went missing as a result of the 2008 magnitude 7.9 Wenchuan earthquake in China’s Sichuan province, the largest quake to hit China since 1950. In 2013, the Lushan quake occurred ~90 km to the south and caused 203 deaths, injured 11,492 and affected more than 1.5 million people.

“After the 2008 magnitude 7.9 Wenchuan earthquake along the Longmenshan fault zone in western Sichuan of China, researchers in China and elsewhere have paid particular attention to this region, seeking to understand how the seismic hazard potential changed in the southern segment of the fault and nearby faults,” said Yao. “Yet the occurrence of this magnitude 6.6 Lushan event surprised many. The challenge of understanding where and when the next big quake will occur after a devastating seismic event continues after this Lushan event, although we now have gained much more information about this area.”

Preliminary rupture details

The southern part of the Longmenshan fault zone is complex and still only moderately understood. Similar to the central segment where the 2008 Wenchuan event occurred, the southern segment, which generated the Lushan rupture, includes the Wenchuan-Maoxian fault, Beichuan-Yingxiu fault, the Pengxian-Guanxian fault and Dayi faults, a series of sub-parallel secondary faults.

Although the Lushan earthquake’s mainshock did not break to the surface, the strong shaking still caused significant damage and casualties in the epicentral region. Three papers detail the rupture process of the Lushan quake. Libo Han from the China Earthquake Administration and colleagues provide a preliminary analysis of the Lushan mainshock and two large aftershocks, which appear to have occurred in the upper crust and terminated at a depth of approximately 8 km. While the Lushan earthquake cannot be associated with any identified surface faults, Han and colleagues suggest the quake may have occurred on a blind thrust fault subparallel to the Dayi fault, which lies at and partly defines the edge of the Chengdu basin. Based on observations from extensive trenching and mapping of fault activity after both the Wenchuan and Lushan earthquakes, Chen Lichun and colleagues from the China Earthquake Administration suggest the Lushan quake spread in a “piggyback fashion” toward the Sichuan basin, but with weaker activity and lower seismogenic potential than the Wenchuan quake. And Junju Xie, from the China Earthquake Administration and Beijing University of Technology, and colleagues examined the vertical and horizontal near-source strong motion from the Mw 6.8 Lushan earthquake. The vertical ground motion is relatively weak for this event, likely due to the fact that seismic energy dissipated at the depth of 12-25 km and the rupture did not break through the ground surface.

Possible link between Lushan and Wenchuan earthquakes

Were the Lushan and Wenchuan earthquakes related? And if so, what is the relationship? Some researchers consider the Lushan quake to be a strong aftershock of the Wenchuan quake, while others see them as independent events. In this special section, researchers tackled the question from various perspectives.

To discover whether the Lushan earthquake was truly independent from the Wenchuan quake, researchers need to have an accurate picture of where the Lushan quake originated. Yong Zhang from the GFZ German Research Centre for Geosciences and the China Earthquake Administration and colleagues begin this process by confirming a new hypocenter for Lushan. To find this place where the fault first began to rupture, the researchers analyze near-fault strong-motion data (movements that took place at a distance of up to a few tens of kilometers away from the fault) as well as long distance (thousands of kilometers ) teleseismic data.

Using their newly calculated location for the hypocenter, Zhang and colleagues now agree with earlier studies that suggest the initial Lushan rupture was a circular rupture event with no predominant direction. But they note that their calculations place the major slip area in the Lushan quake about 40 to 50 kilometers apart from the southwest end of the Wenchuan quake fault. This “gap” between the two faults may hold increased seismic hazards, caution Zhang and colleagues.

Ke Jia of Beijing University and colleagues explore the relationship of the two quakes with a statistical analysis of aftershocks in the region as well as the evolution of shear stress in the lower crust and upper mantle in the broader quake region. Their analyses suggest that the Wenchuan quake did affect the Lushan quake in an immediate sense by changing the overall background seismicity in the region. If these changes in background seismicity are taken into account, the researchers calculate a 62 percent probability that Lushan is a strong aftershock of Wenchuan.

Similarly, Yanzhao Wang from the China Earthquake Administration and colleagues quantified the stress loading of area faults due to the Wenchuan quake and suggest the change in stress may have caused the Lushan quake to rupture approximately 28.4 to 59.3 years earlier than expected. They conclude that the Lushan earthquake is at least 85 percent of a delayed aftershock of the Wenchuan earthquake, rather than due solely to long-term tectonic loading.

After the Wenchuan quake, researchers immediately began calculating stress changes on the major faults surrounding the rupture zone, in part to identify where dangerous aftershocks might occur and to test how well these stress change calculations might work to predict new earthquakes. As part of these analyses, Tom Parsons of the U.S. Geological Survey and Margarita Segou of GeoAzur compared data collected from the Wenchuan and Lushan quakes with data on aftershocks and stress change in four other major earthquakes, including the M 7.4 Landers and Izmit quakes in California and Turkey, respectively, and the M 7.9 Denali quake in Alaska and the M 7.1 Canterbury quake in New Zealand.

Their comparisons reveal that strong aftershocks similar to Lushan are likely to occur where there is highest overall aftershock activity, where stress change is the greatest and on well-developed fault zones. But they also note that by these criteria, the Lushan quake would only have been predicted by stress changes, and not the clustering of aftershocks following the 2008 Wenchuan event.

Future earthquakes in this region

After Wenchuan and Lushan, where should seismologists and other look for the next big quake in the region? After the 2008 Wenchuan quake, seismologists were primed with data to help predict where and when the next rupture might be in the region. The data suggested that the Wenchuan event would increase seismic stress in the southern Longmenshan fault that was the site of the 2013 Lushan quake. But that information alone could not predict that the southern Longmenshan fault would be the next to rupture after Wenchuan, say Mian Liu of the University of Missouri and colleagues, because the Wenchuan earthquake also increased the stress on numerous others faults in the region

Additional insights can be gained from seismic moment studies, according to Liu and colleagues. Moment balancing compares how much seismic strain energy is accumulated along a fault over a certain period with the amount of strain energy released over the same period. In the case of the Longmenshan fault, there had been a slow accumulation of strain energy without release by a major seismic event for more than a millennium. After the Wenchuan quake, the southern part of the Longmenshan fault became the fault with the greatest potential for a quake. And now, after Lushan, Liu and colleagues say that the 60 kilometer-long segment of the fault northeast of the Lushan rupture is the place in the region to watch for the next major earthquake.

Volcano discovered smoldering under a kilometer of ice in West Antarctica

Mount Sidley, at the leading edge of the Executive Committee Range in Marie Byrd Land is the last volcano in the chain that rises above the surface of the ice. But a group of seismologists has detected new volcanic activity under the ice about 30 miles ahead of Mount Sidley in the direction of the range's migration. The new finding suggests that the source of magma is moving beyond the chain beneath the crust and the Antarctic Ice Sheet. -  Doug Wiens
Mount Sidley, at the leading edge of the Executive Committee Range in Marie Byrd Land is the last volcano in the chain that rises above the surface of the ice. But a group of seismologists has detected new volcanic activity under the ice about 30 miles ahead of Mount Sidley in the direction of the range’s migration. The new finding suggests that the source of magma is moving beyond the chain beneath the crust and the Antarctic Ice Sheet. – Doug Wiens

It wasn’t what they were looking for but that only made the discovery all the more exciting.

In January 2010 a team of scientists had set up two crossing lines of seismographs across Marie Byrd Land in West Antarctica. It was the first time the scientists had deployed many instruments in the interior of the continent that could operate year-round even in the coldest parts of Antarctica.

Like a giant CT machine, the seismograph array used disturbances created by distant earthquakes to make images of the ice and rock deep within West Antarctica.

There were big questions to be asked and answered. The goal, says Doug Wiens, professor of earth and planetary science at Washington University in St. Louis and one of the project’s principle investigators, was essentially to weigh the ice sheet to help reconstruct Antarctica’s climate history. But to do this accurately the scientists had to know how the earth’s mantle would respond to an ice burden, and that depended on whether it was hot and fluid or cool and viscous. The seismic data would allow them to map the mantle’s properties.

In the meantime, automated-event-detection software was put to work to comb the data for anything unusual.

When it found two bursts of seismic events between January 2010 and March 2011, Wiens’ PhD student Amanda Lough looked more closely to see what was rattling the continent’s bones.

Was it rock grinding on rock, ice groaning over ice, or, perhaps, hot gases and liquid rock forcing their way through cracks in a volcanic complex?

Uncertain at first, the more Lough and her colleagues looked, the more convinced they became that a new volcano was forming a kilometer beneath the ice.

The discovery of the new as yet unnamed volcano is announced in the Nov. 17 advanced online issue of Nature Geoscience.

Following the trail of clues


The teams that install seismographs in Antarctica are given first crack at the data. Lough had done her bit as part of the WUSTL team, traveling to East Antarctica three times to install or remove stations in East Antarctica.

In 2010 many of the instruments were moved to West Antarctica and Wiens asked Lough to look at the seismic data coming in, the first large-scale dataset from this part of the continent.

“I started seeing events that kept occurring at the same location, which was odd, “Lough said. “Then I realized they were close to some mountains-but not right on top of them.”

“My first thought was, ‘Okay, maybe its just coincidence.’ But then I looked more closely and realized that the mountains were actually volcanoes and there was an age progression to the range. The volcanoes closest to the seismic events were the youngest ones.”

The events were weak and very low frequency, which strongly suggested they weren’t tectonic in origin. While low-magnitude seismic events of tectonic origin typically have frequencies of 10 to 20 cycles per second, this shaking was dominated by frequencies of 2 to 4 cycles per second.

Ruling out ice


But glacial processes can generate low-frequency events. If the events weren’t tectonic could they be glacial?

To probe farther, Lough used a global computer model of seismic velocities to “relocate” the hypocenters of the events to account for the known seismic velocities along different paths through the Earth. This procedure collapsed the swarm clusters to a third their original size.

It also showed that almost all of the events had occurred at depths of 25 to 40 kilometers (15 to 25 miles below the surface). This is extraordinarily deep-deep enough to be near the boundary between the earth’s crust and mantle, called the Moho, and more or less rules out a glacial origin.

It also casts doubt on a tectonic one. “A tectonic event might have a hypocenter 10 to 15 kilometers (6 to 9 miles) deep, but at 25 to 40 kilometers, these were way too deep,” Lough says.

A colleague suggested that the event waveforms looked like Deep Long Period earthquakes, or DPLs, which occur in volcanic areas, have the same frequency characteristics and are as deep. “Everything matches up,” Lough says.

An ash layer encased in ice


The seismologists also talked to Duncan Young and Don Blankenship of the University of Texas who fly airborne radar over Antarctica to produce topographic maps of the bedrock. “In these maps, you can see that there’s elevation in the bed topography at the same location as the seismic events,” Lough says.

The radar images also showed a layer of ash buried under the ice. “They see this layer all around our group of earthquakes and only in this area,” Lough says.

“Their best guess is that it came from Mount Waesche, an existing volcano near Mt Sidley. But that is also interesting because scientists had no idea when Mount Waesche was last active, and the ash layer is sets the age of the eruption at 8,000 years ago. “

What’s up down there?


The case for volcanic origin has been made. But what exactly is causing the seismic activity?

“Most mountains in Antarctica are not volcanic,” Wiens says, “but most in this area are. Is it because East and West Antarctica are slowly rifting apart? We don’t know exactly. But we think there is probably a hot spot in the mantle here producing magma far beneath the surface.”

“People aren’t really sure what causes DPLs,” Lough says. “It seems to vary by volcanic complex, but most people think it’s the movement of magma and other fluids that leads to pressure-induced vibrations in cracks within volcanic and hydrothermal systems.”

Will the new volcano erupt?


“Definitely,” Lough says. “In fact because of the radar shows a mountain beneath the ice I think it has erupted in the past, before the rumblings we recorded.

Will the eruptions punch through a kilometer or more of ice above it?


The scientists calculated that an enormous eruption, one that released a thousand times more energy than the typical eruption, would be necessary to breach the ice above the volcano.

On the other hand a subglacial eruption and the accompanying heat flow will melt a lot of ice. “The volcano will create millions of gallons of water beneath the ice-many lakes full,” says Wiens. This water will rush beneath the ice towards the sea and feed into the hydrological catchment of the MacAyeal Ice Stream, one of several major ice streams draining ice from Marie Byrd Land into the Ross Ice Shelf.

By lubricating the bedrock, it will speed the flow of the overlying ice, perhaps increasing the rate of ice-mass loss in West Antarctica.

“We weren’t expecting to find anything like this,” Wiens says

Fatal Mine Collapse Covered 50 Acres





This map shows the part of the Crandall Canyon Mine that was being mined in August, 2007, when a collapse occurred. A smaller collapse in March 2007 (red rectangle marked ‘March Damage’) forced coal miners out of the north barrier pillar. Mining later resumed in the south part of the mine. The solid red rectangle marked “Aug 6 Collapse” represents the Mine Safety and Health Administration’s original estimate of how much of the mine collapsed. A new study by University of Utah seismologists concludes the collapse covered an area four times larger, or 50 acres, represented by red-dashed rectangle. The new study also concludes the mine collapse’s ‘hypocenter’ — the underground point (red star) where a seismic event begins — was located very near where miners were working at the time of the collapse and near areas that had been mined during July and early August.

Seismology report: Disaster began near where miners worked



New calculations show that the deadly Crandall Canyon mine collapse – which registered as a magnitude-3.9 earthquake – began near where miners were excavating coal and quickly grew to a 50-acre cave-in, University of Utah seismologists say in a report on the tragedy.



The University of Utah Seismograph Stations estimated the size of the collapse is about four times larger than was thought shortly after the time of the Aug. 6, 2007, disaster that resulted in the deaths of six miners and, 10 days later, three rescuers.



The seismologists’ 53-page report has been submitted to the journal Seismological Research Letters and to federal Mine Safety and Health Administration (MSHA) investigators. Among the key findings:



  • Seismological and other data suggest the size of the area that collapsed in the nearly horizontal mine measured 920 meters (3,018 feet) from east to west – extending from about mine crosscut 143 to crosscut 120 – and measured 220 meters (722 feet) from north to south – a total of 50 acres. A crosscut is a north-south tunnel in this mine.
  • During the collapse, the space between the mine’s roof and floor decreased by an average of only 1 foot, but enough coal and rock exploded from the mine’s walls to fill much of the collapse area with rubble that likely prevented further collapse.
  • The collapse likely lasted only seconds – leaving no time for escape – and not for misery-prolonging minutes as some miners’ families have feared. The misconception arose from the fact seismic waves reverberate for much longer than the collapse or earthquake that generated them.
  • The mine collapse was followed in August by 37 measurable aftershocks, clustered near the east and inferred west ends of the collapse area, probably from post-collapse stress and from a vertical crack on the west end of the collapsed block of rock.
  • Seismologists recalculated the epicenter of the magnitude-3.9 mine collapse, and found it “was within the mine boundary and very close to where the miners were working,” says the study’s lead author, seismologist Jim Pechmann, a research associate professor of geology and geophysics at the University of Utah Seismograph Stations.


They did this “relocation” using new techniques, calibrated by data from five seismometers placed above and near the mine after the collapse and by the known location of the magnitude-1.6 coal “burst” on Aug. 16 that killed three rescuers and injured six others.



The epicenter is the point on the ground surface above a seismic event’s hypocenter, which is the underground point where the event begins.



The location of the preliminary epicenter calculated soon after the collapse was 0.4 miles outside the mine boundary and 0.6 miles to the west-southwest of the relocated epicenter. It was somewhat inaccurate partly because the nearest seismic station at the time of the collapse was 12 miles away, says seismologist Walter Arabasz, director of the University of Utah Seismograph Stations and a co-author of the new report. “That led some to conclude the seismic event was separate from the mine collapse.”



Arabasz and other seismologists insisted from the day of the collapse that available evidence indicated the magnitude-3.9 seismic event was the mine collapse itself. The mine’s owner argued it was a natural quake that triggered the collapse.



By showing the recalculated epicenter was within the mine boundary and near active mining, the new study adds strong evidence that the quake was the collapse itself.



“As seismologists, we’re as certain as we can be that the seismic event registered as a magnitude-3.9 shock was due to the collapse of the mine and not a naturally occurring earthquake,” Arabasz says.



Pechmann and Arabasz conducted the study with seismologists Kris Pankow and Relu Burlacu, both of the Seismograph Stations, and with Michael K. “Kim” McCarter, chair and professor of mining engineering at the University of Utah. The study was funded by the State of Utah and the U.S. Geological Survey.



Arabasz says all the information in the report has been given to MSHA investigators, but “we don’t have full access to their information, so we had to develop our interpretations, to a significant extent, independent of key information in the mine.”



Scientists usually don’t release studies until they are published in journals. But in this case, there have been numerous requests for the information, which is a matter of public interest, so researchers released the report now. They delayed the release a couple of weeks at MSHA’s request to give the agency time to inform disaster victims’ families.


Evidence of a Vertical Crack during the Mine Collapse



Soon after the Aug. 6 collapse, Utah seismologists gained support from University of California, Berkeley, and Lawrence Livermore National Laboratory seismologists, who said their analysis of the seismic recordings revealed implosive, downward movement – like a mine collapse and not like shearing motion along a fault.



The California seismologists also have submitted a report of their work to Seismological Research Letters. Pechmann says the new Berkeley paper shows that while seismic waves from the Aug. 6 mine collapse are incompatible with a natural earthquake, about 20 percent of the seismic energy released came from vertical shearing motion.



“The mostly likely explanation is a vertical, north-south crack in the roof of the mine that developed along the western edge of the collapse,” with the ground on the east side of the crack dropping downward, Pechmann says.



He emphasizes that seismic records show the shearing motion “did not occur at the start of collapse. It cannot be interpreted as an earthquake that triggered the collapse.”

Size of the Collapse



MSHA initially estimated the collapse extended 680 meters (2,231 feet) east to west – or from crosscut 137 to crosscut 120 – and at least 80 meters (262 feet) north to south, the study says, quoting an MSHA official. That is about 13 acres.



Pechmann says his team’s “model,” which shows a 50-acre collapse beginning at crosscut 143, is “based on seismological data, available underground observations, and constraints on how much collapse could occur given the amount of coal left in the pillars.



“It’s not the only possible scenario to describe the collapsed area, but it fits all the available data we have,” he adds. “The epicenter was near the western end of our proposed collapse area, suggesting the collapse started at the western end, and propagated mostly eastward” toward the mine entrance.



How did the calculated 1-foot roof collapse have such deadly consequences?



Pechmann says that within the collapsed area, only 37 percent of coal had been removed, and the rest was left behind in support pillars. “If those pillars shatter and covert to rubble – and if the coal increases 40 percent in volume when it shatters – then the closure you can get between the roof and the floor averages 0.3 meters [1 foot].” The roof can only collapse that far “because it gets stopped by the rubble,” he adds.


Collapse Began Near Where Miners Worked



The seismology report notes that the MSHA-approved 2007 amended mining plan called for removing coal from east-west tunnels called “entries” and from north-south tunnels, or crosscuts, leaving behind pillars about 110 feet long and 60 feet wide.



“The next phase of the plan was to mine coal in some of these pillars, working from west to east and allowing the roof around these pillars to collapse,” the report says.



The study shows the collapse hypocenter “was right at the edge of where miners were removing pillars in July and early August,” Pechmann says. The last known working location of the six miners was just east of where those pillars were removed.


How Long the Collapse Lasted



The report says: “Some people have interpreted a four-minute seismic signal duration reported for the Crandall Canyon main shock … as indicative of an extraordinarily long collapse duration. In reality, the duration of the collapse was probably only a few seconds, at most, as evidenced by reports that the surface building at the Crandall Canyon mine portal vibrated for a few seconds at the time.”



Seismologists could not directly measure how long the Aug. 6 collapse lasted, but note that seismic waves reverberate many times longer than an actual quake or collapse.



“The collapse probably happened within just a few seconds and was not a long, drawn-out affair,” Pechmann says. “There would have been no time for anybody to get out of the way. It would have happened too fast for that.”



He notes the deadly Aug. 16 “bump” that ended rescue efforts lasted a minute on seismographs, “but underground observers said it was essentially instantaneous.”


A History of Mine-Related Seismicity



Mining-induced seismicity is common in Utah’s Wasatch Plateau-Book Cliffs coal mining region, where more than 17,000 seismic events (most weaker than magnitude 3) were attributed to underground mining from 1978 through August 2007. Less than 2 percent of the area’s seismicity originates from natural earthquakes.



The researchers noted that from Jan. 1, 2007, until the Aug. 6 collapse and within 1.9 miles of it, there were 28 seismic events large enough to be detected and located. Of those, eight (all magnitude 1.9 or weaker) happened within 2.5 weeks before the collapse, and 15 (all 1.8 or less) happened in late February and early March.



“These events occurred primarily in areas where there was concurrent or recent mining activity,” the report states.



A large “bounce” on March 10, 2007, forced miners to abandon the north side of the active part of the mine due to damage in the event, and shift to the south side, where thicker pillars were left during mining but where the August tragedy occurred.



“We didn’t see any indication of accelerating seismic activity in the hours before the [Aug. 6] collapse,” Pechmann says. “We specifically looked for that.”



Afterward, many aftershocks likely were “continued failures of pillars supporting the roof due to stresses induced by the original collapse,” Pechmann says. “And some aftershocks may have been due to adjustments within the roof.”



The “Seismological Report on the 6 Aug. 2007 Crandall Canyon Mine Collapse in Utah” is available at: http://www.seis.utah.edu/