Pernicious progressive coseismic rock-mass degradation and rock-avalanche volume
Abstract
Some effects of the 2010/11 Canterbury earthquake sequence provide new insights into processes leading to rock slides and rock avalanches. These processes are, as yet, part way through defining a mass that will fall. They are causing deep-seated gravitational slope deformation, but they have much relevance to predicting the reach of rock slides and rock avalanches because they are defining a future landslide volume. The phenomenon can be called pernicious, progressive, coseismic rock-mass degradation: pernicious because it will eventually destroy an affected portion of a mountain edifice, given time and enough shaking. In a series of severe aftershocks in 2011, small co-seismic ground movements opened cracks for 300 m along a hillside, some 50 m back from an 80-m-high former coastal cliff in an extinct (Miocene) basalt volcano. Investigations to determine the significance of the movement involved measuring displacement, crack widths and orientations, terrestrial laser scans of the cliff face, a geological log of the cliff, drilling with core recovery into the hill top to below the cliff base, and downhole logging of seismic velocities. The core revealed a scoriaceous basalt flow at depth containing non-cooling related fractures, some of which were fresh, but unrelated to drilling. The flow corresponded to a prominent down-hole logged seismic velocity contrast. We do not discuss details of this site, but discuss the insight it gave us into the wider problem of rock-mass degradation to become a rock slide or rock avalanche. A key to the process is the presence in a brittle rock mass of a large seismic velocity contrast to slow input elastic body waves, and thereby amplify seismic stress fluctuations, sometimes out of the elastic response range. With time and mountain erosion, the position of the velocity contrast is advected higher into the edifice. There, the rock mass is subject to high static shear stresses induced by the mass and the edifice shape. During some strong earthquakes, some dynamic excursions in rock stress exceed the Mohr-Coulomb threshold for brittle failure of the rock around the velocity contrast, and small cracks enlarge. The cracks induce small permanent downslope displacements of the rock mass, and also enhance the velocity contrast, so that smaller input transient elastic stresses are amplified to cause cracking. In time, repeated strong shaking creates an extensive low-velocity fractured zone, but this fractured zone has also been shaped by the distributions of static shear stress and tension in the edifice. Eventually, the mountain contains an extensive, weak, fractured zone sub-parallel to the static shear stress field in the edifice. A single strong earthquake can further weaken the upper edifice to trigger its catastrophic collapse. 'Fully developed' fractured zones are destroyed in the edifice collapse, but partially developed zones are easily located before collapse by geophysical means or direct drilling.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNH31C..08M
- Keywords:
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- 4302 NATURAL HAZARDS Geological;
- 7203 SEISMOLOGY Body waves