Examples of past landslides that are known to have been triggered by earthquakes are rarities. But Hervé Philip and Jean-François Ritz believe they have identified one, and it was huge.
From satellite and aerial images of the Gobi-Altay mountain range in Mongolia, and data from ground surveys, Philip and Ritz conclude that the structure shown here, running from the ridge at the bottom of the picture downslope into a valley to the north, is a ‘palaeolandslide’— one of uncertain age, which was triggered by ground-shaking and involved the slippage of 50 km3 of earth. The authors calculate that some 100 billion tons of material moved which, put another way, is enough to cover The Netherlands waist deep in mud and rock.
The account appears in Geology (27, 211-214; 1999), and the authors surmise that there is an earthquake connection for various reasons, most notably the landslide's occurrence close to the currently active Bogd fault system. The fault ruptured most recently in 1957, in an earthquake of magnitude 8.3. But the most curious aspect of the story is that the average slope from ridge to valley in the landslide area is a very gentle 3°. That means that, once the ground shaking had stopped, there must have been some unusual conditions for such a mass of material to continue in motion for several kilometres; this picture shows an extent of ground about 34 km from bottom to top, 18 km of which was affected by the slump.
Philip and Ritz suggest that part of the explanation is that there was little or no frictional or cohesive strength at the landslide's shear plane, a condition that might have pertained when the water table was much higher than it is now.
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Lincoln, T. A big slip. Nature 398, 667 (1999). https://doi.org/10.1038/19428
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DOI: https://doi.org/10.1038/19428
