A Guide to Differences between Stochastic Point Source and Stochastic Finite Fault Simulation Methods

Why don’t stochastic point-source and finite-fault simulation models agree on the predicted ground motions for moderate earthquakes at large distances? This question was posed by Ken Campbell, who attempted to reproduce the Atkinson and Boore (2006) ground-motion prediction equations for eastern North America using the stochastic point-source program SMSIM (Boore, 2005), in place of the finite-source stochastic program EXSIM (Motazedian and Atkinson, 2005) that was used by Atkinson and Boore (AB06) in their model. His comparisons suggested that a higher stress drop is needed in the context of SMSIM to produce an average match, at larger distances, with the model predictions of AB06 based on EXSIM – this is so even for moderate magnitudes, that should be well-represented by a point-source model. Why? The answer to this question is rooted in significant differences between pointsource and finite-source stochastic simulation methodologies, specifically as implemented in SMSIM (Boore, 2005) and EXSIM (Motazedian and Atkinson, 2005) to date. Point-source and finite-fault methodologies differ in general in several important ways: (i) the geometry of the source; (ii) the definition and application of duration; and (iii) the normalization of finite-source sub-source summations. Furthermore, the specific implementation of the methods may differ in their details. The purpose of this note is to provide a brief overview of

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