A volume V is assumed to contain both distributed and discrete dislocation sources that contribute to the instantaneous velocity field. Discrete dislocation sources are divided into two classes: (1) Γ_n on which the past slip rate , time of previous earthquake t_n, and interevent time t_n are considered known (or estimated) from geologic information, (2) Γ_m on which the long-term slip rate is possibly known but slip history is poorly constrained.