Short-Term Earthquake Predictability in California

Maximilian J. Werner, Agnes Helmstetter, David D. Jackson, & Yan Y. Kagan

Published September 2013, SCEC Contribution #1896

To investigate the predictive skills of earthquake forecasting models, we developed a suite of event-based and short-term forecasting models for California. The event-based forecasts are specified by the conditional intensity function of the ETAS (Epidemic-Type AfterShock) class and evaluated using their exact likelihood functions, rather than the discrete, grid-based, Poisson likelihood function currently used in CSEP experiments. We evaluated the influence of eight popular spatial triggering kernels on the probability gain and found that power-law kernels with scale parameter that grows with mainshock rupture length work best. We also found that lowering the learning catalog threshold to m2+ improves forecasts of target earthquakes m3.95+, providing further evidence that small quakes improve the predictive skill of clustering models. To probe the importance of anisotropic spatial aftershock patterns, we compare three methods: early-aftershock smoothing, focal mechanism-based directionality, and elliptical spatial kernels.

Event-based forecasting solves several shortcomings in the current CSEP experiment design, but involve a potentially costly numerical integration. A simpler solution to increase probability gains is to reduce the forecast horizon from one day to one hour or less. To compete in such a class, and to understand the influence of forecast horizon on probability gain, we developed sub-24 hour earthquake forecasts based on the ETAS model [Werner et al., 2011] and two new models K2 and K3 [Helmstetter and Werner, 2013], which are based entirely on adaptive kernel smoothing of seismicity in time, space and magnitude. 1-hour forecasts reach gains of 200 over time-independent, spatially-varying forecasts – significantly larger than the gains of 55 of 24-hour forecasts.

Since installation of ETAS and K3 within CSEP in September 2012, 30 earthquakes m ≥ 3.95 have occurred within the testing region. Although it is too early to judge the practical significance of these results, all models thus far perform better than any other installed one-day model, including the STEP model by Gerstenberger et al. [2005] and the critical-branching model by Kagan and Jackson [2010].

Werner, M. J., Helmstetter, A., Jackson, D. D., & Kagan, Y. Y. (2013, 9). Short-Term Earthquake Predictability in California. Poster Presentation at SCEC Annual Meeting 2013.