No, it was not an anomaly – the massive landslide that buried the majority of Oso, Washington, in March last year likely happens every 500 years, not thousands of years past as previously thought.
University of Washington researchers analyzed earlier slides’ debris and used radiocarbon dating to study the deadliest landslide in the country’s history. They found that in the last 2,000 years, the slopes in Oso’s surrounding areas collapse about once every 500 years on average and about once every 140 years at a higher rate.
Study author Sean LaHusen, an Earth and space sciences doctoral student at the university, said their team found that landslides occurred continually in modern history.
“The soil in this area is all glacial material, so one hypothesis is the material could have fallen apart in a series of large landslides soon after the ice retreated, thousands of years ago. We found that that’s not the case,” said LaHusen.
Rather than thousands of years old, many of the larger slides were hundreds of years in age, noted co-author and assistant professor Alison Duvall.
The team used lidar, a laser scanning technique clearing away vegetation in the area, to produce maps for analyzing previous landslide events. They also dated previous landslides using debris samples from slides that happened up and down the valley.
According to results, the Rowan slide, which is about five times the Oso slide’s size, occurred between 300 and 694 years ago, while the Headache Creek slide took place about 6,000 years ago. This discovery, according to Duvall, suggested that the Oso slide is “not so much of an anomaly.”
The radiocarbon dates offered a roughness curve, which determines the age of two slides through the amount of surface erosion, to date previous events along the Stillaguamish River's north fork.
LaHusen said this is the first time such calibrated surface dating technique was used to know the chronology of slides. He added that the details into how often the slides recur can be the first step to conducting regional risk analysis.
However, to apply the novel method to other areas requires gathering samples, as each landslide site maintains its own soil composition and erosion qualities. The Oso landslide history findings, for instance, may not apply to other sections of the Stillaguamish River or Washington state.
Both authors are currently part of the M-9 Project, studying risks from magnitude-9 quakes along the Cascadia subduction zone. They are seeking to learn if slides across the state coincided with previous quakes and to simulate future shakes to predict which locations are most prone to quake-incited landslides.
The fatal Oso slide occurred on Mar. 22 last year, its debris covering around 40 homes, other buildings and almost a mile of State Route 530. It killed 43 in Steelhead Haven near Oso.
The mudslide, according to the U.S. Geological Survey, moved at around 40 miles per hour with higher maximum speeds, and transported about 18 million tons of sand, clay and till. It dammed the river at a 25-feet depth, forming a temporary 2.5-mile-long lake that flooded residences and communities.
It is crucial to know how to stay safe before, during, and after landslide events, particularly in high risk areas. Here are some landslide safety guidelines provided by the American Red Cross.
The team published its findings on Dec. 22 in the journal Geology.
