The increasing exposure of society to natural hazards has made the forecasting of extreme events a pressing scientific concern. No aspect of this problem has been more vexing than earthquake prediction. The century-long failure to identify observable precursory signals diagnostic of impending events has led to an alternative approach, pioneered by earthquake engineers, in which a variety of constraints on earthquake location, magnitude, and long-term frequency are synthesized into probabilistic seismic hazard models, such as those produced by the USGS National Seismic Hazard Mapping Project. This presentation will describe how recent progress in earthquake system science is improving hazard and risk forecasting. These system-level problems can be partitioned according to causal sequences described in terms of conditional probabilities. For example, the exceedance probabilities of shaking intensities at geographically distributed sites conditional on a particular fault rupture (a ground motion prediction model or GMPM) can be combined with the probabilities of different ruptures (an earthquake rupture forecast or ERF) to create a seismic hazard map. Deterministic simulations of ground motions from very large suites (millions) of ruptures, now feasible through high-performance computational platforms such as SCEC’s CyberShake, are allowing seismologists to replace empirical GMPMs with physics-based models that more accurately represent wave propagation through heterogeneous geologic structures, such as sedimentary basins that amplify seismic shaking. A notable advance is the development of ERFs conditioned on preceding seismic activity, such as the Uniform California Earthquake Rupture Forecasts produced by the Working Groups on California Earthquake Probabilities. These time-dependent probability models account for the stress-renewal processes of elastic rebound, and they are beginning to capture aftershock triggering. However, they have not fully reconciled the long-term phase modulation of stress renewal with the short-term clustering observed in aftershocks and extended seismic sequences. For this and other reasons, the stochastic ERFs are likely to be replaced by deterministic earthquake simulators that explicitly model rupture nucleation and stress evolution within fault systems. Several outstanding issues will be highlighted, such as the assimilation of historical seismicity data into simulator-based forecasting and the problem of forecasting rupture directivity, which can strongly influence ground motions.
Thomas H. Jordan is a University Professor and the W. M. Keck Foundation Professor of Earth Sciences at the University of Southern California. His current research is focused on system-level models of earthquake processes, earthquake forecasting, continental dynamics, and full-3D waveform tomography. As the director of the Southern California Earthquake Center (SCEC), Jordan coordinates an international research program in earthquake system science that involves over 600 scientists at more than 60 universities and research organizations. He is a member of the California Earthquake Prediction Evaluation Council and the Board of Directors of the Seismological Society of America, and he has served as an elected member of the Council of the National Academy of Sciences (2006-2009) and the Governing Board of the National Research Council (2008-2011). Jordan received his Ph.D. from the California Institute of Technology in 1972 and taught at Princeton University and the Scripps Institution of Oceanography before joining the Massachusetts Institute of Technology in 1984. He was head of MIT’s Department of Earth, Atmospheric and Planetary Sciences from 1988 to 1998. He was awarded the Macelwane and Lehmann Medals of the American Geophysical Union, the Woollard Award of the Geological Society of America, and the 2012 Award for Outstanding Contribution to Public Understanding of the Geosciences by the American Geosciences Institute. He has been elected to the National Academy of Sciences, the American Academy of Arts and Sciences, and the American Philosophical Society.