California Landslide Assessment Reveals More Trouble Spots

Satellite-based monitoring pinpoints landslide stress-indicators ahead of accidents.

October 25, 2023

Some areas of California’s coastline have a history of landslides. Earlier this year, a landslide occurred in San Clemente, a beach town north of San Diego. Value.Space’s satellite-based monitoring would have revealed warning signs that are typical for developing landslides well ahead of the accident and detected additional at-risk areas. The landslide impacted property and critical infrastructure, which is similar to a recent landslide in Sweden that Value.Space also assessed.

San Clemente landslide in California, April 2023

A landslide occurred on a coastal slope next to the Casa Romantica Cultural Centre in San Clemente on 27 April 2023. It interrupted service on a railroad corridor and raised concerns about the safety of adjacent buildings.

The cost of rail service interruption was later estimated to be between $5.5 to $6 million. After the accident, San Clemente City Council earmarked $8.5 million to secure the site to protect life and property ahead of the winter rain season.

For background and photos see local media reports here and here.

Figure 1 – Value.Space assessment of San Clemente landslide area

Ground movement at San Clemente landslide area. Red indicates downward, blue upward movement and green stable areas.

  • Value.Space conducted a risk assessment of the landslide area for a period between 9 October 2021 to 26 April 2023 to establish whether warning signs were present at the site before the landslide.

  • The assessment revealed that ground movement in the marked area stood out compared to neighboring slopes, it was moving in a distinctly different direction up to 46.64 mm per year. The sides of the marked area were moving in opposite directions. Additionally, the mid-section of the marked area stood out for the lack of stable satellite-measured reading. These signs combined indicate possible strong stress on the slope.

  • The assessment showed that the area started exhibiting warning signs about six months before the accident, in November 2022. Development of stress indicators on the coastal slope correlated with increased precipitation in California after a period of drought that media reports have identified as a possible factor behind the decreased ground stability.

Figure 2 – Additional at-risk areas identified in San Clemente

Additional at-risk areas with ground movement indications in San Clemente.  

  • While assessing the landslide at Casa Romantica (Find 01), Value.Space looked at a larger segment of San Clemente coastline, also between 9 October 2021 to 26 April 2023. The assessment revealed three more at-risk areas.

  • Finds 02 and 03 are to the south along the coastline from the Casa Romantica landslide area. They exhibit similar stress signs to the landslide area before the accident. The finds are also on the coastal slope between the railroad line and below built-up areas above.

  • Find 04 is slightly inland, on a slope next to properties. There is a movement cluster indicating ground movement on the top and mid-section of the slope reaching up to 32.69 mm per year.

Stenungsund landslide in Sweden, Spetember 2023

On 23 September 2023, a landslide occurred sear Stenungsund, Sweden, damaging a section of a highway and properties nearby. Value.Space conducted a risk assessment of the area for a two-year period before the accident to establish whether warning signs were present before.

For background and photos see a local media report here. 

Figure 3 - Value.Space assessment of Stenungsund landlside area

Ground movement at Stenungsund landslide area. Red indicates downward, blue upward movement and green stable areas.

  • The assessment revealed an area of notable bidirectional movement, where the top of the highway exerted pressure on the toe of the embankment. This indicated the accumulation of tensions typically associated with the development of landslide risks. The eventual landslide occurred precisely in the area identified as at-risk by Value.Space's assessment.

Had these areas been under regular monitoring by Value.Space, all the finds above would have led to warnings issued for clients.

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