YOSEMITE NATIONAL PARK — As the giant granite El Capitan briefly slumbers, scientists are racing to identify any areas of dangerous new instability on a popular but restless climbing route that last week released enough rubble to fill 750 dump trucks.

It’s likely that Mother Nature — not climbers — caused the two massive rockfalls that killed Welsh athlete Andrew Foster, 32, severely injured his 28-year-old wife, Lucy, and smashed through the sunroof of an SUV driven by Florida tourist Jim Evans, who is now hospitalized with head injuries. Related Articles Yosemite National Park to close due to heavy smoke

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What triggered the tragedy? Were recent temperature swings to blame? Can our scientific tools ever predict the day, even the hour, when an ancient mountain suddenly fractures? Scientists are inching closer to clues, but the research is as challenging as an ascent up the sheer monolith.

While climbers can cause rockfalls, accidentally loosening a piece of rock, those falls tend to be much smaller in size than natural events, said Menlo Park-based U.S. Geological Survey civil engineer Brian Collins, who is now in Yosemite with U.S. Park Service geologist Greg Stock to study the rockfalls.

The team has scrutinized more than a century of unexplained rockfalls in Yosemite, seeking patterns of rock shedding.

“Larger rockfalls are most often a natural occurrence,” Collins said, “the reason being that natural forces such as water seepage, thermal expansion and earthquakes have much larger forces involved that can destabilize and trigger larger areas of rock.”

Early on Wednesday, the day of the first El Capitan rockfall, climbers Ryan Sheridan and Peter Zabrok had ascended the same route, and later told the Associated Press that they noticed the rock was loose. In June 2014, a large block fell from the same spot, fragmenting on impact and creating a large dust cloud that lingered for more than an hour. But because it happened at 4:30 a.m., no one was hurt.

No new movement was detected this weekend by National Park Service experts who are monitoring the mountain with high-powered spotting scopes and binoculars.

By helicopter, NPS geologist Stock quickly took hundreds of photos of the new scar. The giant flake collapsed over two days split seconds in geologic time. A lower section released during seven separate rockfalls over four hours on Wednesday; the much larger upper section dropped on Thursday.

Then Collins and Stock deployed their computer model to compare the new rock face to the old images of the cliffs, obtained using lidar and laser techniques. The computer modeling, which calculated the volumes and dimensions of the rockfall, was done in collaboration with geologists at the University of Lausanne in Switzerland.

“This allows us to view where the rockfall came from in three dimensions,” Collins said, “and gives us a much better idea of how the rocks became dislodged and fell. We can then look for areas of potential new instability based on these models,” which helps experts monitor any perilous movement from the ground.

In addition to computer analyses, Collins and Stock have measured the influence of temperature swings by roping up, climbing a slab of unstable rock and installing a long thin wire that measures strain. The device records data every five minutes — tracking rock movement as tiny as one-thousandth of a centimeter.

While it remains impossible to predict exactly when a rock will release, the new tools “help us understand trigger mechanisms — why they happen — and the shapes and sizes,” said Yosemite spokesman Scott Gediman. “It is amazing science.”

In just one day, the scientific team precisely measured the size of the fracture: 394 feet tall, 148 feet wide and 8 to 26 feet thick. They calculated the collapse released nearly 362,000 cubic feet of rock, weighing about 27,675 metric tons.

It created a white scar that looks like a raised middle finger, evoking historian Will Durant’s famous aphorism: “Civilization exists by geologic consent, subject to change without notice.”

Above the fracture is a huge slab, now unsupported, that’s poised for the next release.

“We cheated death on this one,” Zabrok, who has climbed El Capitan 58 times, wrote on his Facebook page. His team “completed our ascent safely, only to watch the first two-thirds of the route fall off beneath us in this spectacular and terrifying rockfall. … We literally felt the granite of El Capitan shaking beneath our feet like an earthquake!”

While dramatic, rockfalls rarely kill. Sixteen people have died from rocks in Yosemite over the past 150 years. Many more people have died in the park from drownings, car accidents and natural causes, according to Yosemite data.

On average, rockfalls this size in Yosemite Valley occur every six years, according to Stock. The 2009 Ahwiyah Point rockfall, near Half Dome, was roughly four times larger than all of last week’s events, knocking down hundreds of trees and burying hundreds of feet of the Mirror Lake Loop Trail, he said.

In October 2008, rocks peeled off the cliff below Glacier Point, thundering down on Curry Village and forcing the permanent closure of 233 tent cabins, about a third of the village’s overnight capacity.

Rocks don’t have to be big to be fatal. In 2013, London climber Felix Joseph Kiernan, 28, died on the East Buttress of El Capitan when his partner, climbing above him, stepped on the loose rock. The block — measuring one-by-two feet — fell about 150 feet before striking Kiernan.

According to The Times of London, Welsh climber Foster died while trying to shield his wife, jumping to cover her from cascading rocks. The Florida man was injured when his car’s sunroof was shattered.

The new fracture was stunning not just for its fatality, but also its high visibility. It was witnessed by hundreds of tourists along the busy Yosemite loop, on a blissfully warm autumn day.

It occurred on the southeast flank of El Capitan, across from a popular meadow, rather than the long, aesthetic and immediately visible “nose” of the monolith.

It’s a climbing area called the Waterfall Route, where the seasonal Horsetail Falls flows, between the popular routes Zodiac and East Buttress. Several famed pitches have been wiped off the face of El Capitan, and now lie on the valley floor and in the South Fork of the Merced River.

While geologists have not conducted specific studies in the area of the Waterfall Route, they have documented several rockfalls from that area in the past, Collins said.

Elsewhere in the park, hikers said it’s no reason to stay away from one of the most beautiful places on Earth.

“I’ve been all over national parks and things happen all the time,” said Taylor Moore, 23, of San Francisco, on her first-ever hike on Thursday to the dramatic top of Yosemite Falls, 2,500 feet above the valley floor. “It is gorgeous here.”

Phillip Chayka, 22, of Sacramento, gazed up at granite walls near Yosemite Falls and said, “If that comes down, I don’t want to be here.”

He quickly added that he wasn’t scared. “But my mom is,” he joked.

But climbing carries higher risks because it follows seams and cracks — lines of weakness in rock.

Rocks look static, but they’re not, Collins said. They expand and contract with changing temperatures, according to his research conducted with NPS’s Stock.

They looked at the pattern of 228 past rockfalls in Yosemite with no known cause and found that about 15 percent occurred in the hottest hours of the day, from noon to 6 p.m, between July and September. That’s more than double the expected rate.

In the heat of a summer afternoon, an unstable slab in Yosemite bulges and moves off a cliff about eight millimeters, almost one-third of an inch, they found. On cool nights, the slab reverses itself, shifting back about seven millimeters.

This summer, the Sierra Nevada had one of the hottest summers on record — then drastically cooled, with snow dusting the peaks of Yosemite National Park and briefly closing Tioga Pass Road on Sept. 21. Last week, temperatures soared to the high 90s.

While rock movement is most profound in heat, it happens all year long, Collins said. And other things — such as winter rain and snow, even tree roots or lightning strikes — may actually trigger the final collapse.