Some of the most fascinating images to come out of the Alaska earthquake are the photos that show what geologists refer to as “slope failure,” in which portions of formerly solid hills caved in, tore or let loose and succumbed to gravity. And if there happened to be a road or other structure on top of the slope that failed, the result is dramatic.
Slopes can fail without earthquakes, as the result of torrential rain or weathering. But the shaking caused by a quake — particularly a large one like what occurred near Anchorage on Friday — can either speed up the process or create entirely new failures that wouldn’t have otherwise happened.
Several slope failures were documented in the Anchorage area in the hours after the earthquake Friday, the most interesting (and photogenic) of which was on Vine Road southwest of Wasilla. The photographer, Marc Lester, captured the image from a charter plane as he surveyed damage for the Anchorage Daily News.
“The roadway looked like pieces of a puzzle,” Lester wrote in a piece about how he got the photo. “The light snow cover helped the cracks in the Earth stand out and tell the story.”
It’s as if someone came along with a giant wrecking ball and dropped it on Vine Road, which crumpled like a smacked egg.
Geologists seemed surprised at how many slope failures occurred as a result of the 7.0-magnitude quake. The epicenter was deep, at 27 miles below the surface — a key reason the damage in Anchorage was relatively light. “Such an earthquake would be expected to generate shaking over a large area,” Dave Petley wrote on the American Geophysical Union blog, “but probably with reasonably modest peak ground accelerations.” Translation: Friday’s earthquake would certainly be felt, but the surface of Earth wouldn’t be moving dramatically enough to produce widespread landslides.
Photos from Alaska show a lot of lateral spread, in which the ground tears open, causing cracks and fissures. Whatever is on top of the ground at the place it tears in half is also subsequently torn in half, like highways or buildings. The buildings surrounding the perimeter of the spread are also at risk of damage because of the force of the ground spreading out away from the tear. There’s also documentation of block spreading, in which a portion of ground breaks off a hill and slides away mostly intact.