Sudden and gradual compaction of shallow brittle porous rocks

Abstract

Porous brittle rocks fail in shear and compaction. Rate and state friction describes such failure on a planar surface, while end-cap failure describes the failure of a continuum. Both formalisms include Coulomb failure in shear at low normal tractions. Rate and state friction includes slow compaction of gouge by normal traction when shearing is not occurring. Rate and state friction also represents slow compaction of porous sandstone. End-cap failure includes both grain crushing and shear. Micromechanically, all these mechanisms involve exponential creep at stresses of a few gigapascals. Rate and state friction is modeled by tabular real contacts where shear and extrusion occur. The observed compaction rate depends on the normal traction to a modest power of ∼27 for gouge and ∼10 for sandstone. End-cap failure involves grain cracking in the neighborhood of the grain-grain contact. Its rate should depend on the normal traction raised to a power of ∼100. This strong rate dependence is not measurable in practice as a failure cascade commences once some grains crack. Materials with pointy contacts including unwelded tuff and "regolith" in the uppermost tens of meters that has repeatedly been damaged by strong seismic waves compact in this manner at dynamic stresses modestly above their ambient lithostatic stress. This failure produces significant nonlinear attenuation of compressional P waves with sustained dynamic accelerations that are a modest fraction of the ambient acceleration of gravity. Copyright 2010 by the American Geophysical Union.