21 December 2021
The movement of the San Isidro landslide in Peru, and a possible relationship with the 28 November 2021 Northern Peru earthquake?
Posted by Dave Petley
The movement of the San Isidro landslide in Peru, and a possible relationship with the 28 November 2021 Northern Peru earthquake?
Yesterday I wrote about the failure of the San Isidro Landslide in La Peca, Peru on 12 December 2021, which has led to the relocation of about 250 people. Later in the day, Sotiris Valkaniotis tweeted satellite images of the site, derived from the European Space Agency Sentinel instruments, that both confirmed the location of the landslide and provided an indication of the movement of the slope. The Tweet should be visible below:
Copernicus #Sentinel2 satellite imagery captures the motion from an activated #landslide of 800+ meters length in San Isidro, Peru (Dec 12 2021). Left; total horizontal displacement as measured by optical image correlation (before & after imagery). 1/2 https://t.co/691BgrfwE2 pic.twitter.com/4NrHKpL6hW
— Sotiris Valkaniotis (@SotisValkan) December 20, 2021
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This provisional analysis indicates that the landslide was about 800 m in length, with displacements exceeding 20 metres in places. The devastating impact on the village is clear, despite the low resolution of the imagery.
There is an interesting other dimension to this landslide. On 28 November 2021 the Amazonas region suffered an M=7.5 earthquake. The epicentre was about 215 km from the San Isidro landslide. Images posted to the La Peca Facebook page indicate that significant landsliding was triggered in the area by the earthquake.
https://www.facebook.com/Miqueridalapeca/posts/1876581976063557
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Thus, it seems quite possible that the slope at San Isidro was weakened by the earthquake, and then failed in the first heavy rainfall event after the seismic event. This is a pattern that we have seen elsewhere.
Of course this is only speculation at this point, but it is an interesting angle to explore.
As an aside, the images in the Facebook page indicate that in at least one location the valley is blocked by a landslide, possibly in the area of Bagua Grande. I hope that this has been (or is being) addressed.
Given quakes often derange water-table and sub-surface flows, such may have ‘lit the fuse’ on subsequent slope failure…
Sounds reasonable. Similar disturbances of glacial drainage systems occur annually, and periodically for surging glaciers, where clogged or closed conduits, lead to build up of water pressure, and formation of a linked cavity – conduit system at the glacier bed and/or sliding interface. The more the water system is disturbed and clogged the higher the water pressure that builds up and the larger the cavities in the linked cavity conduit system. In a surging glacier that ends up leading to a failure and an increase in motion 100 fold or more downvalley. For a landslide it makes sense that subsurface deposits jiggled by an earthquake could start the process going and pore pressure increase, if the groundwater drainage network was broken, and water could access more areas along a potential sliding surface to lubricate sliding surface and to fluidize supporting sediment structures. The first heavy rain event after a large earthquake could have a greater impact in that case. Data should be available to prove that hypothesis. The landslide that ran over the ancient Quinault area indigenous village is an example from the 1700 PNW Cascadia megaquake earthquake I believe. I expect rain and snow melt would compound earthquake impacts leading to more landslides. fascinating ..and relevant.