A Geodesy- and Seismicity-Based Local Earthquake Likelihood Model for Central Los Angeles

We estimate time-independent earthquake likelihoods in central Los Angeles using a model of interseismic strain accumulation and the 1932-2017 seismic catalog. We assume that on the long-term average, earthquakes and aseismic deformation collectively release seismic moment at a rate balancing interseismic loading, mainshocks obey the Gutenberg-Richter law (a log linear magnitude-frequency distribution [MFD]) up to a maximum magnitude and a Poisson process, and aftershock sequences obey the Gutenberg-Richter and Bath laws. We model a comprehensive suite of these long-term systems, assess how likely each system would be to have produced the MFD of the instrumental catalog, and use these likelihoods to probabilistically estimate the long-term MFD. We estimate M-max = 6.8 + 1.05/-0.4 (every similar to 300 years) or M-max = 7.05 + 0.95/-0.4 assuming a truncated or tapered Gutenberg-Richter MFD, respectively. Our results imply that, for example, the (median) likelihood of one or more M-w >= 6.5 mainshocks is 0.2% in 1 year, 2% in 10 years, and 18-21% in 100 years. Plain Language Summary We develop a method to estimate the long-term-average earthquake hazard in a region and apply it to central Los Angeles. We start from an estimate of how quickly faults are being loaded by the gradual bending of the crust and assume that on the long-term average, they should release strain in earthquakes at this same total loading rate. We then use a well-established rule that for every M-w > 7 earthquake, there are about ten M-w > 6 earthquakes, a hundred M-w > 5 earthquakes, and so on (with some variability from an exact 1-10-100 slope), and we assume that there is a maximum magnitude that earthquakes do not exceed. We use these constraints to build long-term earthquake rate models for central LA and then evaluate each model by assessing whether an earthquake system obeying it would have produced the relative rates of small, moderate, and large earthquakes in the 1932-2017 earthquake catalog. We estimate a maximum magnitude of M-w = 6.8 + 1.05/-0.4 (every similar to 300 years) or M-w = 7.05 + 0.95/-0.4 in central LA depending on specific assumptions. Our results imply that, for example, median likelihood of one or more M-w = 6.5 mainshocks in central LA is 0.2% in 1 year, 2% in 10 years, and 18-21% in 100 years.