The 2014 Working Group on California Earthquake Probabilities (WGCEP14) present the time-independent component of the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), which provides authoritative estimates of the magnitude, location, and time-averaged frequency of potentially damaging earthquakes in California. The primary achievements have been to relax fault segmentation and include multifault ruptures, both limitations of UCERF2. The rates of all earthquakes are solved for simultaneously and from a broader range of data, using a system-level inversion that is both conceptually simple and extensible. The inverse problem is large and underdetermined, so a range of models is sampled using an efficient simulated annealing algorithm. The approach is more derivative than prescriptive (e.g., magnitude-frequency distributions are no longer assumed), so new analysis tools were developed for exploring solutions. Epistemic uncertainties were also accounted for using 1440 alternative logic-tree branches, necessitating access to supercomputers. The most influential uncertainties include alternative deformation models (fault slip rates), a new smoothed seismicity algorithm, alternative values for the total rate of M w ≥ 5 events, and different scaling relationships, virtually all of which are new. As a notable first, three deformation models are based on kinematically consistent inversions of geodetic and geologic data, also providing slip-rate constraints on faults previously excluded due to lack of geologic data. The grand inversion constitutes a system-level framework for testing hypotheses and balancing the influence of different experts. For example, we demonstrate serious challenges with the Gutenberg-Richter hypothesis for individual faults. UCERF3 is still an approximation of the system, however, and the range of models is limited (e.g., constrained to stay close to UCERF2). Nevertheless, UCERF3 removes the apparent UCERF2 overprediction of M 6.5-7 earthquake rates and also includes types of multifault ruptures seen in nature. Although UCERF3 fits the data better than UCERF2 overall, there may be areas that warrant further site-specific investigation. Supporting products may be of general interest, and we list key assumptions and avenues for future model improvements. Manuscript OrganizationBecause of manuscript length and model complexity, we begin with an outline of this report to help readers navigate the various sections:

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GPS time series in northeast Japan exhibit nonlinear trends from 1996 to 2011 before the Mw 9.0, 2011 Tohoku‐oki earthquake. After removing reference frame noise, we model time series as linear trends plus constant acceleration, correcting for coseismic and postseismic effects from the numerous Mw ∼ 6.5+ earthquakes during this period. We find spatially coherent and statistically significant accelerations throughout northern Honshu. Large areas of Japan outside the Tohoku region show insignificant accelerations, demonstrating that the observation is not due to network‐wide artifacts. While the accelerations in northern Tohoku (Sanriku area) can be explained by decaying postseismic deformation from pre‐1996 earthquakes, the accelerations in south‐central Tohoku appear unrelated to postseismic effects. The latter accelerations are associated with a decrease in average trench‐normal strain rate and can be explained by increasing slip rate on the Japan trench plate interface and/or updip migration of deep aseismic slip in the decades before the 2011 Tohoku‐oki earthquake.

The abundance of geodetic and seismic data recording postseismic deformation following the 2004 Parkfield earthquake provides an unprecedented opportunity to resolve frictional properties on the Parkfield section of the San Andreas fault. The Parkfield segment is a transition between the locked section to the southeast that last ruptured in the 1857 Fort Tejon earthquake and the creeping section to the northwest. We develop three-dimensional rate-and state-dependent friction models of afterslip following the 2004 earthquake to investigate the frictional behavior of the fault. It is assumed that the coseismic rupture occurred on an area of the fault surrounded by aseismic creep that accelerated after the earthquake. We estimate the distribution of coseismic slip, afterslip, and rate-state frictional parameters by inverting a two-step slip model. In the model we (1) estimate the coseismic slip distribution from 1-Hz Global Positioning System (GPS) data and (2) use the corresponding coseismic shear stress change on the fault as input into a numerical afterslip model governed by rate-state friction. We find the rate-state frictional parameter A-B, an indicator of frictional stability, is in the range 10 ‫4מ‬-10 ‫3מ‬ at 50 MPa normal stress, which is near the transition from potentially unstable (negative A-B) to nominally stable (positive A-B) friction. The estimate of A-B values falls within a wide range of experimental values reported for serpentinite, which crops out along the San Andreas fault zone. The critical slip distance, d c , which characterizes the distance over which strength breaks down during a slip event, is in the range 0.01-0.1 m, consistent with seismic estimates and a fault gouge thickness of 1-10 m. The afterslip model reproduces most features observed in the GPS time-series data including high surface velocities in the first few months after the earthquake and lower rates at later times, as well as the cumulative postseismic displacement. The model tends to underpredict the displacement data at later times, suggesting that perhaps the modeled afterslip period ends too quickly or an unmodeled deformation process dominates the signal at later times.

We develop a two‐dimensional boundary element earthquake cycle model including deep interseismic creep on vertical strike‐slip faults in an elastic lithosphere coupled to a viscoelastic asthenosphere. Uniform slip on the upper part of the fault is prescribed periodically to represent great strike‐slip earthquakes. Below the coseismic rupture the fault creeps in response to lithospheric shear stresses within a narrow linear viscous fault zone. The model is applied to the GPS contemporary velocity field across the Carrizo Plain and northern San Francisco Bay segments of the San Andreas fault, as well as triangulation measurements of postseismic strain following the 1906 San Francisco earthquake. Previous analysis of these data, using conventional viscoelastic coupling models without stress‐driven creep [Segall, 2002], shows that it is necessary to invoke different lithosphere‐asthenosphere rheology in northern and southern California in order to explain the data. We show that with deep stress‐driven interseismic creep on the San Andreas fault, the data can be explained with the same rheology for northern and southern California. We estimate elastic thickness in the range 44–100 km (95% confidence level), fault zone viscosity per unit width of 0.5–8.2 × 1017 Pa s/m, and asthenosphere relaxation time of 24–622 years (0.1–2.9 × 1020 Pa s) for northern and southern California. We estimate a slip rate of 21–27 mm/yr and recurrence time of 188–315 years for the northern San Francisco Bay San Andreas fault and slip rate of 32–42 mm/yr with recurrence time of 247–536 years for the Carrizo Plain.

Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-independent model published previously, renewal models are utilized to represent elasticrebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new methodology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ≥ 6:7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative importance of logic-tree branches, vary throughout the region and depend on the evaluation metric of interest. For example, M ≥ 6:7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis.