Slope Stability of Continental Megalandslides
Continental megalandslides, although rare, are found in various morphological settings including mountain fronts and valley sides with steep (> 30°) to shallow (<3°) slopes, under climates varying from high-precipitation temperate to hyperarid, and are associated with poorly active tectonics as well as hyper-active tectonics with frequent and very high-magnitude (≥ Mw 8) earthquakes. All but one (Mayunmarca, 25 April 1974) are pre-historic with ages ranging from 3.5 Ma to 9 ky BP.
Static and dynamic slope stability analyses show that in all cases the slope will remain stable when using values of effective cohesion and angle of internal friction for hard rocks. It appears that the studied megalandslides could not have formed without the presence of accurately oriented pre-existing discontinuities, including bedding, tectonic fractures, foliation and intrusive contacts. With such discontinuities being present, steep-sloping megalandslides may have formed with no help of seismic shaking. In any case, three of the four studied shallow-sloping megalandslides require high to very high magnitude earthquakes.
The large discrepancies between the Arias intensities predicted by the different attenuations relationships established when passing from moderate/high magnitude domains to very high magnitude domains show that megalandslides could hardly be used as paleomagnitude indicators.
KeywordsMegalandslides stability analyses earthquakes paleo magnitude
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