frequency of ruptures dependent (in part) on how the slippage is distributed across the various faults. Modeling Like its predecessor, UCERF3 determines this based on four layers of modeling: The fault models (FM 3.1 and 3.2) describe the physical geometry of the larger and more active faults. Deformation models determine the slip rates and related factors for each fault section, how much strain accumulates before a fault ruptures, and how much energy is then released. Four deformation models are used, reflecting different approaches to handling earthquake dynamics. The earthquake rate model (ERM) fits together all this data to estimate the long-term rate of rupturing. The probability model estimates how close (ready) each fault segment is to rupturing given how much stress has accumulated since its last rupture. The first three layers of modeling are used to determine the long-term, or Time Independent, estimates of the magnitude, location, and frequency of potentially damaging earthquakes in California. The Time Dependent model is based on the theory of elastic rebound, that after an earthquake releases tectonic stress there will be some time before sufficient stress accumulates to cause another earthquake. In theory, this should produce some regularity in the earthquakes on a given fault, and knowing the date of the last rupture is a clue to how soon the next one can be expected. In practice this is not so clear, in part because slip rates vary, and also because fault segments influence each other, so a rupture on one segment triggers rupturing on adjacent segments. One of the achievements of UCERF3 is to better handle such multifault ruptures. The various alternatives (see diagram), taken in different combinations, form a logic tree of 1440 branches for the Time Independent model, and, when the four probability models are factored in, 5760 branches for the Time Dependent model. Each branch was evaluated and weighted according to its relative probability and importance. The UCERF3 results are an average of all these weighted alternatives. "The Grand Inversion" In UCERF2 each fault was modeled separately, as if ruptures do not extend to other faults. This assumption of fault segmentation was suspected as the cause of UCERF2 predicting nearly twice as many earthquakes in the M 6.5 to 7.0 range then actually observed, and is contrary to the multifault rupturing seen in many earthquakes. UCERF3 subdivides each fault section (as modeled by the Fault Models) into subsections (2606 segments