Incorporating Source Rupture Characteristics into Ground-Motion Hazard Analysis Models

Abstract

Source rupture characteristics have significant effects on the level and the distribution of near-source earthquake ground motions. The present state of knowledge concerning the impacts of source characteristics on ground motions makes a quantitative analysis of the effect of fault rupture on the near-source ground motions possible. To simplify incorporating source effects into seismic hazard analysis models, they may be classified into two groups: rupture directivity and rupture heterogeneity. Rupture directivity reflects the effects of the direction of rupture on the fault as viewed from the site. This characteristic is important even in the case of homogeneous rupture, which corresponds to uniform seismic energy release over the rupturing surface. The state of understanding of rupture directivity, particularly that derived from recordings of many recent earthquakes, warrants incorporation of homogeneous fault rupture effects into ground-motion hazard analysis methodologies. However, fault rupture, especially in large earthquakes, is often far from homogeneous. This is due to irregularities in the geometry of the fault, non-uniformities in the mechanical properties of the materials involved, and variability in the regional and local stresses. A major contributor to rupture heterogeneity is the existence of asperities, that is, zones with strength and deformation characteristics significantly different from the rest of the fault. The existence of asperities contributes to heterogeneous rupture of the fault, characterized by non-uniformities in slip, slip-rate, stress-drop, rupture velocity, post-earthquake residual stress, and after-shock activities. The state of understanding of source heterogeneities is not adequately developed to allow incorporation of this factor into seismic hazard analysis methodologies for design and decision-making. However, information on the nature of asperities, slip distributions, and other characteristics accumulated by the study of recent earthquakes can be tested to determine their effects on ground-motion hazards. One way to include source rupture characteristics—homogeneous and heterogeneous—into seismic hazard analysis models is through …