Seismicity-based earthquake forecasting techniques: Ten years of progress

Earthquake fault systems interact over a broad spectrum of spatial and temporal scales and, in recent years, studies of the regional seismicity in a variety of regions have produced a number of new techniques for seismicity-based earthquake forecasting. While a wide variety of physical assumptions and statistical approaches are incorporated into the various methodologies, they all endeavor to accurately replicate the statistics and properties of both the historic and instrumental seismic records. As a result, the last ten years have seen significant progress in the field of intermediate- and short-term seismicity-based earthquake forecasting. These include general agreement on the need for prospective testing and successful attempts to standardize both evaluation methods and the appropriate null hypotheses. Here we differentiate the predominant approaches into models based upon techniques for identifying particular physical processes and those that filter, or smooth, the seismicity. Comparison of the methods suggests that while smoothed seismicity models provide improved forecast capability over longer time periods, higher probability gain over shorter time periods is achieved with methods that integrate statistical techniques with our knowledge of the physical process, such as the epidemic-type aftershock sequence (ETAS) model or those related to changes in the b-value, for example. In general, while both classes of seismicity-based forecasts are limited by the relatively short time period available for the instrumental catalog, significant advances have been made in our understanding of both the limitations and potential of seismicity-based earthquake forecasting. There is general agreement that both short-term forecasting, on the order of days to weeks, and longer-term forecasting over five-to-ten year periods, is within reach. This recent progress serves to illuminate both the critical nature of the different temporal scales intrinsic to the earthquake process and the importance of high quality seismic data for the accurate quantification of time-dependent earthquake hazard.