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The magintude 7.9 Denali Fault earthquake was the largest earthquake on earth in 2002. Releasing energy that had been building along the fault for hundreds- and in some sections perhaps even thousands-of years, it was a global event.
Always in mothion the earth's crust is fractured into irregular plates like those of a cracked eggshell and driven by convection from deep within.
Erupting magma continually creates new oceanic crust, which moves like a great conveyor belt until it collides with other crustal plates- forming geological faults-and is forced down and re-melted. The lighter continental plates are pushed around like rafts by the spreading sea floors an fuse together or grind past one another when they collide.
Bending and deforming on a time scale of millions of years, the motion is, for us an imperceptible creep punctuated by violent jerks.
The Denali Fault system sweeps from the Bering sea across central Alaska and the western Yukon, down the Lynn Canal to the Pacific Ocean. It is the boundary between the North American Plate ( essentially the entire continent) and the smaller Wrangell sub-plate whech broke free of the North American plate millions of years ago, that is now being pushed northwest and mashed into the rest of Alaska by the conveyor action of the Pacific Plate.
The Alaska Range, which is pushed constantly upward by this grinding collision, follows the arc of the fault across central Alaska.
Rocks do not slide smoothly; they stick and they slip. A section of the Denali Fault might have moved in `1912 but evidence suggest much of the central Denali Fault had been struck for many centuries. All the while, the Wrangell Sub-plate was being pushed west and north by the Pacific Plate at roughly on third inch per year, deforming the rock and increasing stress along the fault. On Oct.23,2002 a magnitude 6.7 earthquake on the Denali Fault just east of Denali Park released a short section of the fault. Eleven days later at 1:12 pm. on Nov.3, a previously unknown fault near the Susitna Glacier failed. A 24 mile section thrust upward as much as 13 feet in places and a near propagating rupture that ran east along the fault line, roughly six miles deep. As the earth ripped, two additional subevents major earthquakes in their own right was triggered. The speed of sound.
In a little more than 90 seconds the three successive hammer blows broke 209 miles of the earth's crust, sending seismic waves through the planet, ringing it like a church bell.
As the rupture passed under the susitana and Black Rapids glaciers, rock on the south side of the fault lurched west while th other side rebounded east, shearing ice 1,000 feet thick and splitting glaciers down the middle. Roughly 15 seconds after the earthquake began, the second focus of the great stress triggered and the second hammer blow slammed the mountains. Rock and avalanches swept across Black Rapids Glacier. On McGinnis peak the blow launched rocks an dice from high on the walls of the east flank, sending avalanches down both branches of McGinnis Glacier, covering the entire glacier in debris. The largest avalanche swept six miles and contained more than 50 million cubic yards of rock an ice.
Twenty-five seconds after the earthquake began, the leading edge of the rupture ripped through Augustana Pass, cut across Isabelle Pass, and was about to hit the trans Alaska oil pipeline,
Buried in places and supported on steel pilings in others, the 800 miles , 48-inch -diameter pipeline carries 17 percent of America's oil supply. Where the pipeline crossed the Denali Fault, the engineers and scientists had planned for a major earthquake. The pipe lay in large zigzags atop long skids that sat directly on the ground. The pipe's Teflon shoes are not attached to the skids, allowing maximum shift and flex as the earth moves. Trees split, and the pipeline whipped back and forth like a great silver snake as the rupture shot past. The adjacent Richardson Gighway sheared an was offset 10 feet, but the pipeline held.
Summit Lake, at the divide between the copper an Yukon river drainages south of Isabelle Pass, is one of the many lakes left by the glaciers of the last ice age. Waters Edge cottages, a cluster of little cabins and a small, white two story house, sits just off the Richardson Highway at the south end of the lake. East of Isabelle Pass, the canwell, Gakona and Christochina glaciers sheared lenghwise in the wake of the rupture. Converging shock waves blew house-sized blocked from the mountainside above the Gakona, leaving them strwn on the glacer. At 43 seconds, the third and strongest sub-event began and the earth was offset almost 29 feet. The Ahtna Native village of Mentasta, where few residents knew the fault existed, was directly in its path- less than 20 seconds away.
When the first shock hit Mentasta, Eva John scooped up her two grandchildren and ran, but with her arms full she couldn't open the door. Her new enterainment center lurched forward and crashed to the floor where the little ones had been playing moments before. She squeezed them tight to her chest and prayed.
Marvin Sanford felt the shudder of the on coming quake an grabbed two of his kids, yelling to his wife, Rachel to get out. She grabbed their on yer old but hesitated. By the times she got to the door the house was rocking like it would fly apart. Just outside the door, their truck was sliding back and forth on the ice, The only escape route was a path between the trucks and a snowmachine. Rachel Sanford ran with her child helt tight with both arms, but the wet ices was like greased glass. Her feet flew out from under her and they both slid under the truck, The tires came at them one way, the the other, Clutching her baby with one arm she dragged herself from under the truck toward the snowmachine. With no solid footing , she grabbed the handlebar of the snowmachine with one arm and pulled herself forward, Then she saw the truck hurtling toward her an knew she had no time to escape. She tucked her little one close and turned away.
"Lord-Jesus!" her husband cried. Amazingly, the tround suddenly jerked the other way, the truck halted and Marvin Sanford pulled his family free.
A minute after the initial rupture, a tractor trailer was passing Mentasta Lodge on the Tok cutoff headed towrd Anchorage, Drivers billy Carey and Bill Miller were on a run from SEattle. The road was thick with ice from freezing rain the previous night, and a deep slough of the slana River lay at teh bottom of a steep bank on the right side of the road. Carey was at the wheel when the truck lurched an began to shake. As the bouncing increased, he struggled to keep the rig on the heaving surface of icy blacktop then the highway ruptured in front of them, As teh fissures widened, Carey fought to straddle the biggest one until his right front wheel dropped into a crack, Grounding on the axle, the semi lurched toward teh water, stopping on the shoulder's edge.Several miles of the Tok Cutoff were impassable and the road into Mentasta was destroyed. The community was isolated from useful imformation and rumors spread, shortly after the quake on Sunday afternood, as residents of Mentasta were still locating frends and loved ones, word got around that a bigger quake was coming later that afternoon- a magnitude nine or 10.
The magnitude 9.2 Good Friday quaked taht devastated Southcentral Alaska in 1964 released almost 100 times the energy of the quake they had just experienced.