Surface breakage of the 1992 Landers earthquake and its effects on structures

The Landers, California, earthquake (Mw = 7.3) provides an exceptional opportunity to study surface rupture of an earthquake fault. Detailed maps of the lateral distribution of fracturing adjacent to main traces show that rupture patterns are much more complex than documented in past studies of surface ruptures. The rupture occurs in tabular zones, up to hundreds of meters wide. A main trace within each rupture zone accommodates much of the shear deformation, but considerable fracturing occurs throughout the tabular zone. The en-echelon pattern of fracturing in step-over zones between main traces is typically even more complex than those along major fault zones. Inspection of several on-grade concrete slabs indicates that unreinforced concrete foundations generally crack when subjected to distinct ground ruptures beneath them or when they are twisted because of differential ground movements across broad zones. Methods of mitigating the potential hazards associated with earthquake fault rupture are presented.