11 September 2018
Drone footage of landslides from the 2018 Hokkaido Eastern Iburi earthquake
The Mw=6.6 earthquake last week in Japan is now known as the 2018 Hokkaido Eastern Iburi earthquake. The official death toll stands at 44 people, the vast majority of whom were lost in the large number of landslides that were triggered around the town of Atsuma. On Facebook, Yasuykui Shimizu, a professor at the Hokkiado Universiy Hydraulic Lab, has uploaded three drone videos that show these landslides in detail – they can be accessed from here. These provide a brilliant overview of the nature and scale of these slides. You should be able to view the three videos below (but apologies if the WordPress embed function does not work so well):-
These videos provide an interesting insight into the landslides. A few things to note – first, the landslides do indeed start from the ridge tops, or from significant breaks in slope, as I thought would be the case. This is a signature of the effects of topographic amplification of the earthquake shaking – I wonder if we will find that this area was subjected to very high peak ground accelerations?
Second, the landslides appear to primarily involve heavily weathered materials. Take a look at the still below:-
Note that pumice appears to outcrop at the crown and on the margins of the landslides, but the exposed shear surface appears to be in weathered materials. I am not quite sure how this fits with the theory that these landslides were the result of collapse of pumice layers, but I am no expert in these events (and others certainly are).
Third, the landslides appear to have been quite mobile. In the image above the degree to which they have flowed and spread across the flatlands is clear. This does suggest a fluid state. Note that in the image above there is pooled water on the landslide deposit.
And fourth, the landslides appear to be very shallow, for the most part. The image below is fascinating:-
This landslide shows all the features noted above (and look at the runout distance across the fields for the landslide in the background on the left hand side). But the really interesting aspect is the silo in the middle of the shot. The landslide has flowed around it, leaving it intact and standing, even though it is at the foot of the slope. This is shown more clearly in the image below – you can even see the “slipstream” of the silo in the deposit downslope of the silo:-
I suspect that this can only happen if the landslide is both shallow and very fluid. The silo may also have benefited from the unfailed portion a little way upslope (note the trees on the spur), although that might also ask some interesting questions about cause and effect.
Thanks to Sanjay Giri for highlighting this imagery in a comment.