Earthquake-Induced Spalling of Rock Slopes, Northridge Earthquake, January 17, 1994 — A Case Study

The 1994 Northridge earthquake (Mw=6.7, ML=6.4), southern California produced a 100-foot thick, 3,000-foot linear zone of fractures interpreted as a spalled slab near the crest of a ridge in the Simi Hills, 7 miles northwest of the epicenter. The strike and dip of the slab parallel the strike of the ridge and the dip of the topographic slope, respectively. The fractures cut across bedding and joint planes. An analytical model based on earthquake-induced stresses is used to describe the mechanism of spalling. Earthquake-induced tensile stresses are related to (I) propagation direction of the wavefront, (2) peak particle velocity, (3) reflection of the incident P-wave, and (4) converted P- and S-waves at a free surface. At the site, seismic raypaths were oriented perpendicular to the northwest-facing topographic slope. The calculated thickness of the spalled slab compared favorably with geologic observations. The calculated induced tensile stress at ground surface was -285 psi. The calculated induced stress equal to the tensile strength of the rock (-140 psi) occurs at a vertical depth of 1 10 feet and approximates the IOO-foot thickness of the spalled slab. The method may be used to assess the potential for spalling to occur at a site and to recommend set-back criteria for facilities.