Aerial photo showing the aftermath of the landslide that buried the town of Oso in WA, USA.
Credit: U.S. Geological Survey
Department of the Interior/USGS
U.S. Geological Survey/photo by Air Support Unit , King County Sheriff’s Office
This week, the world has been shocked by images of devastation after a huge landslide buried the town of Oso, north of Seattle, in Washington state, USA. At 11:00 PDT on Saturday 22nd March 2014, a 500m-wide section of mud and rock became detached from a hillside above the 180 population town, and hurtled down the slope at high speed. Deposits from the landslip are up to 6m deep and cover over a square mile. At the time of writing, there are 25 confirmed fatalities and 90 people remain unaccounted for.
The USGS has confirmed that there was no seismic trigger for the landslip. Instead, it is thought that exceptionally heavy rain caused a section of hillside to form a rotational slide complex. In such cases, material detaches along a pre-existing plane of weakness and falls ‘top first’, with the basal section moving upwards relative to the ground surface. The problem was exacerbated by the fact that the hillside above Oso comprises loosely consolidated glacial till; some of this material mixed with water from a nearby river to form a devastating debris flow.
Although there is geologic evidence for other large post-glacial landslides occurring in the western foothills of the northern Cascades, making precise predictions about where landslides will take place remains almost impossible. The extent of the current slide is being mapped using LiDAR and aerial photographs, in the hope that future hazards related to the newly deposited mud can be alleviated.
Declan Valters
The rainfall experienced in February and March in Washington state was significantly higher than average for the time of year. January was by contrast much drier than average, and I would suggest that this might have been a trigger to this massive landslide.
On the day of the landslide, the conditions were actually quite dry – the heavy rainfall had occurred in the week previously, which adds to the difficulty of predicting these types of events. After the critical rainfall event, (which could have been days or even weeks previously) it’s difficult to predict how long a slope will hold up before failure.