Lecture 14 - Landslide case studies

Question 1

On what did the 2014 Oso, Washington landslide occur?

Answer 1

This landslide occurred mid morning on 22nd March 2014 (Saturday).

Question 2

How did the 2014 Oso, Washington landslide start?

Answer 2

It started when Whitman Bench, a moderate elevation (∼200 m) but steep slope overlooking the Stillaguamish valley, collapsed catastrophically.

Question 3

What type of landslide was the 2014 Oso, Washington landslide?

Answer 3

It initiated as a highly-mobile debris slide, sourced from low down on the slope. The second stage is thought to have been a rotational rock slide higher up on Whitman Bench, triggered by the loss of buttressing lower down.

Question 4

What did the debris slide do in the 2014 Oso, Washington landslide?

Answer 4

It crossed the Stillaguamish river and inundated the small community of Steelhead, engulfing 35 family residences and killing 43 people.

Question 5

What do seismic recordings indicate about the 2014 Oso, Washington landslide?

Answer 5

Seismic recordings indicate two distinct episodes of landsliding separated by a ∼2 minute period of relative quiescence.

Question 6

What the rotational rock slide do in the 2014 Oso, Washington landslide?

Answer 6

The second stage is responsible for the clear head scarp visible on Whitman Bench today.

Question 7

What was the immediate trigger of the 2014 Oso, Washington landslide?

Answer 7

The immediate trigger was an intense period of rainfall (double the average monthly precipitation) immediately preceding the landslide, which saturated the ground.

Question 8

What were the preconditions of the 2014 Oso, Washington landslide?

Answer 8

The steep local terrain and long-term incision of the river banks by the Stillaguamish river made the hillside unstable, exacerbated by the clay-rich glacial till that blankets this formerly glaciated landscape. Analysis of pre-2014 lidar imagery revealed that an earlier, smaller landslide in 2006 may also have destabilised the slope.

Question 9

Was logging a precondition of the 2014 Oso, Washington landslide?

Answer 9

There is conjecture that logging in the area may have been a contributing factor. However, lidar imagery reveals numerous scars and deposits from Holocene landslides along the Stillaguamish river valley that precede industrial logging.

Question 10

What was the main precondition for the 2018 Montecito, California mud flow?

Answer 10

In December 2017 the Thomas wildfire — the largest in California’s history — raged across the Santa Ynez mountains west of Los Angeles, fuelled by a lengthy drought and seasonal dry, easterly Santa Ana winds. It destroyed vegetation whose roots had previously stabilized the topsoil, and blanketed the ground with a layer of ash with low permeability.

Question 11

What was the trigger for the 2018 Montecito, California mud flow?

Answer 11

On 8th January 2018, an intense rain storm in the central Santa Ynez mountains triggered several major mud and debris flows in catchments draining the southern slopes of the Santa Ynez range.

Question 12

How fast were the flows in Montecito, California in 2018?

Answer 12

up to 30 km/h

Question 13

How high were the flows in Montecito, California in 2018?

Answer 13

of up to 5 m

Question 14

How many homes were destroyed by the 2018 Montecito, California mud flows?

Answer 14

> 100 homes

Question 15

How many people were killed by the 2018 Montecito, California mud flows?

Answer 15

at least 21 people

Question 16

Why were people killed by the 2018 Montecito, California mud flows?

Answer 16

There is conjecture that residents were ‘evacuation fatigued’, having been forced out of their homes less than a month previously by the approaching Thomas fire.

Question 17

What was a precondition for the 2010 Mt Meager, BC composite landslide?

Answer 17

This landslide, in the early hours of 6th August 2010, was sourced high up Mt Meager, a steep volcano in the Garibaldi Volcanic Belt composed of rocks altered and weakened through hydrothermal activity.

Question 18

What were the two parts of the 2010 Mt Meager, BC composite landslide?

Answer 18

An initial 50,000,000 m³ rock fall plunged 500 m onto the mountain’s southern flank, where it destabilized even more material to form a giant rock avalanche.

Question 19

How far did the 2010 Mt Meager, BC composite landslide travel?

Answer 19

The rock avalanche travelled down Capricorn and Meager Creeks and into the Lillooet River, 13 km from the landslide source (but only 2 km below it).

Question 20

What did analysis of regional seismograms reveal about the 2010 Mt Meager, BC composite landslide?

Answer 20

Analysis of regional seismograms revealed the timing of the initial impact, the flow of debris through bends in Capricorn Creek, and its impact with the far side of Meager Creek.

Question 21

How many fatalities were there in the 2010 Mt Meager, BC composite landslide?

Answer 21

There were no fatalities due to the remote location, though four campers made a lucky escape.

Question 22

What did analysis of deposits 30–50 km downstream Lillooet River reveal about Mt Meager?

Answer 22

It revealed three much larger debris flows, each sourced from Mt Meager, in the past 12,000 years. The most recent of these reached the location of Pemberton (population ∼3,000) and is likely associated with the last eruption of Mt Meager ∼2,400 years ago.

Question 23

How long did it take the 2010 Mt Meager, BC composite landslide to reach the Lillooet river?

Answer 23

325 seconds