6 September 2018
Landslides triggered by the 6th September 2018 Hokkaido earthquake
There is little doubt now that the major impact of the 6th September 2018 Hokkaido earthquake is in the form of geotechnical failures. Whilst there is abundant evidence of liquefaction, it is the dramatic landslides (some reports suggest about 800 individual events) that have caught the eye and, sadly, caused most of the casualties in all probability. The USGS Shakemap data is providing some information about the distribution of ground shaking, although at this stage it is likely to be provisional. I have added to the map below the locations of the two areas of landslides that have gained most of the attention:-
The two areas of landslides shown above are dramatic. The ridge landslides are shown in the image below – here we seem to see failure from the ridge crest (which is typical for earthquakes) along the vast majority of the topographic feature. Sadly, at the toe of the slope were located a number of houses. The level of destruction seems to be very high; at 3 am the likelihood of escape for the occupants would have been low. Fortunately the integrity of at least some of the houses may have protected the occupants:-
In some cases the runout distance seems to be high. Note that the exposed slopes look to be both deeply weathered and wet, although there is not much evidence of water flowing.
The second area shown in the maps above is the zone of extremely dense landslides shown in the image below:-
Again note that most of the landslides, which are again in deeply weathered materials, originate from the ridge crest. The runout distances look quite long again, and in some cases they have coalesced in the drainage lines (channels) to form larger flows.
Given the earthquake magnitude (Mw=6.6 or 6.7), and depth (USGS reports 33.4 km), this level of landsliding is unusual. Indeed the USGS PAGER tool continues to indicate a low level of landslide activity. It is is likely that this high level of landslides is the result of the effects of Typhoon Jebi (Typhoon 21 in Japan), which passed over Japan in the few days before the earthquake. This might have saturated the slopes. Alternative factors may include some aspect of the shaking – perhaps the frequency was exactly right to create dramatic topographic amplification in this topography. Or perhaps the materials – these slopes may well be formed from young volcanic deposits I suspect – were particularly susceptible to seismic shaking. I wonder if there might be an element of liquefaction in some of these failures?
I am sure a great deal more detail will emerge in the days ahead. Comments and observations are very welcome.