Civilization exists by geologic consent, subject to change without notice.
For my post this week, I originally planned to write about the 50th anniversary of the Great Alaska earthquake, March 27, 1964. It was the largest measured quake in U.S. history, measuring 9.2 on the moment magnitude scale, and caused huge amounts of destruction in southern Alaska. It also profoundly changed the science of geology as well, since it proved that subduction zones were real, and showed us how the land changed after subduction zone earthquakes. And then as I finished this post on March 28, the earthquakes along the Puente Hills fault started to roll in. It seemed that earthquakes were the theme of the week!
But then another disaster grabbed the headlines, and pointed to some important issues that seemed even more timely. On Monday, March 24, we heard the news of the gigantic landslide that buried the tiny town of Oso, Washington, about an hour north and east of Seattle. When I originally wrote this (March 28), the official death toll was 25, but over 90 were missing and feared dead. At last count it is 28 dead, 22 still missing. Most of the news coverage of the story focuses on the horrors of thousands of tons of mud and rocks roaring down on people in a matter of seconds, burying them alive or crushing them under the enormous weight of wet mud, ripping houses and cars to shreds, and even shredding the clothes off the bodies of the victims. In the weeks before the slide, over twice the amount of the normal rainfall fell, saturating the ground and increasing the pore pressure so the sediment was like quicksand or wet concrete. The slide material itself was made of ancient glacial lake and valley sediments, mostly porous sand that can absorb a lot of moisture, and is loose and crumbly, so when it is saturated, it will flow. Witnesses described the debris flow as a “fast-moving wall of mud” containing trees and other debris cutting through homes directly beneath the hill. A firefighter stated, “When the slide hit the river, it was like a tsunami”. In numerical terms, the size of the mudflow is staggering. It’s estimated at 15 million cubic yards moved all at once. A Washington state geologist said the slide was one of the largest landslides he’d seen. The mud, soil and rock debris left from the mudslide is 1,500 ft (460 m) long, 4,400 ft (1,300 m) wide and deposited debris 30 to 40 ft (9.1 to 12.2 m) deep.comments (21)