The effect of glaciation on the intensity of seismic ground motion

摘要

Seismicity is known to contribute to landscape denudation through its role in earthquake-triggered slope failure; but little is known about how the intensity of seismic ground motions, and therefore triggering of slope failures, may change through time. Topography influences the intensity of seismic shaking - generally steep slopes amplify shaking more than flatter slopes - and because glacial erosion typically steepens and enlarges slopes, glaciation may increase the intensity of seismic shaking of some landforms. However, the effect of this may be limited until after glaciers retreat because valley ice or ice-caps may damp seismic ground motions. Two-dimensional numerical models (FLAC 6.0) were used to explore how edifice shape, rock stiffness and various levels of ice inundation affect edifice shaking intensity. The modelling confirmed that earthquake shaking is enhanced with steeper topography and at ridge crests but it showed for the first time that total inundation by ice may reduce shaking intensity at hill crests to about 20-50% of that experienced when no ice is present. The effect is diminished to about 80-95% if glacier ice level reduces to half of the mountain slope height. In general, ice cover reduced shaking most for the steepest-sided edifices, for wave frequencies higher than 3Hz, and when ice was thickest and the rock had shear stiffness well in excess of the stiffness of ice. If rock stiffness is low and shear-wave velocity is similar to that of ice, the presence of ice may amplify the shaking of rock protruding above the ice surface. The modelling supports the idea that topographic amplification of earthquake shaking increases as a result of glacial erosion and deglaciation. It is possible that the effect of this is sufficient to have influenced the distribution of post-glacial slope failures in glaciated seismically active areas.