Exciting news! We're transitioning to the Statewide California Earthquake Center. Our new website is under construction, but we'll continue using this website for SCEC business in the meantime. We're also archiving the Southern Center site to preserve its rich history. A new and improved platform is coming soon!

Stability of Earthquake Clustering Models: Criticality and Branching Ratios

Jiancang Zhuang, Maximilian J. Werner, & David Harte

Published December 2013, SCEC Contribution #1809

We study the stability conditions of a class of branching processes prominent in the analysis and modeling of seismicity. This class includes the ETAS (epidemic-type aftershock sequence) model as a special case, but more generally comprises models in which the magnitude distribution of direct offspring depends on the magnitude of the progenitor, such as the BASS (branching aftershock sequence) model and another recently proposed branching model based on a dynamic scaling hypothesis. These stability conditions are closely related to the concepts of the criticality parameter and the branching ratio. The criticality parameter summarizes the asymptotic behavior of the population after sufficiently many generations, determined by the maximum eigenvalue of the transition equations. The branching ratio is defined by the proportion of triggered events in all the events. Based on the results for the generalized case, we show that the branching ratio of the ETAS model is identical to its criticality parameter because its magnitude density is separable from the full intensity. More generally, however, these two values differ and thus place separate conditions on model stability. As an illustration of the difference and of the importance of the stability conditions, we employ a version of the BASS model, reformulated to ensure the possibility of stationarity. In addition, we analyze the magnitude distributions of successive generations of the BASS model via analytical and numerical methods, and find that the compound density differs substantially from a Gutenberg-Richter distribution, unless the process is essentially subcritical (branching ratio less than 1) or the magnitude dependence between the parent event and the direct offspring is weak.

Zhuang, J., Werner, M. J., & Harte, D. (2013). Stability of Earthquake Clustering Models: Criticality and Branching Ratios. Physical Review E, 88(6), 062109. doi: 10.1103/PhysRevE.88.062109.