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From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference. Plain Language Summary: The largest earthquakes on Earth occur along faults that develop between two tectonic plates that come into contact. Termed megathrust earthquakes, these catastrophic events are responsible for generating both strong ground‐shaking and tsunamis. The Cascadia megathrust fault straddles the Pacific coastline of North America and from evidence in both the United States and Japan, we know this fault last slipped in 1700 A.D. We have combined models of strain buildup (geodetic coupling models) with state‐of‐the‐art 3‐D computer simulations to understand the potential hazard of a future earthquake in Cascadia and show what factors might lead to the fault slipping its entire length. We compare our simulations to geologic measurements of permanent ground movement from 1700 A.D. Our results demonstrate that no matter where the earthquake is allowed to start, coupling models showing strain accumulation to the top of the fault easily leads to big earthquakes. We also look into what 1700 A.D. event may have looked like and show several scenarios that fit the geologic data very closely. This work represents the first set of 3‐D simulations that use the laws of physics to see what may control the size of future earthquakes in Cascadia. Key Points: We design the first fully dynamic 3‐D earthquake simulations based on geodetic coupling models for the Cascadia megathrustSegmentation in the stress drop is needed to produce subsidence amplitudes consistent with observed megathrust earthquakesDynamic rupture simulations demonstrate how fault friction and stress levels may control margin‐wide rupture [ABSTRACT FROM AUTHOR]

There is a strong need to model potential rupture behaviors for the next Cascadia megathrust earthquake. However, there exists significant uncertainty regarding the extent of downdip rupture and rupture speed. To address this problem, we study how the transition region (i.e., the gap), which separates the locked from slow‐slip regions, influences coseismic rupture propagation using 2‐D dynamic rupture simulations governed by a slip‐weakening friction law. We show that rupture propagation through the gap is strongly controlled by the amount of accumulated tectonic initial shear stress and gap friction level. A large amplitude negative dynamic stress drop is needed to arrest downdip rupture. We also observe downdip supershear rupture when the gradient in effective normal stress from the locked to slow‐slip regions is dramatic. Our results justify kinematic rupture models that extend below the gap and suggests the possibility of high‐frequency energy radiation during the next Cascadia megathrust earthquake. Plain Language Summary: How large, deep, and damaging a future earthquake will be depends on factors such as energy release that must be constrained by precise observations of previous earthquakes in the same area. But such data are rarely available. Instead, computer models of earthquakes guided by the laws of physics can provide us with estimates of potential ground shaking for a future event. In our study, we design two‐dimensional earthquake simulations for the Cascadia fault below the northwestern United States coast and test different hypotheses for how stress may be accumulating at depth along this fault. Our models focus on a portion of the fault referred to as the "gap." The gap physically separates a shallow region that slips during large earthquakes from a deeper region that experiences intermittent slip between large earthquakes. A gap region similar to that in Cascadia is also found in Japan, Mexico, and around other active faults worldwide. We find that our simulated rupture is able to extend to deeper regions at faster speeds given the current understanding of stress levels and earthquake fault friction in the gap. While this work represents only a first step toward understanding how stresses and friction influence how the Cascadia fault might slip, it lays the foundation for modeling more complex physics that can help scientists better predict shaking from seismic waves. Key Points: We examine dynamic source effects on along‐dip rupture propagation for a Cascadia megathrust earthquakeSimulated earthquake rupture is able to penetrate through the transition zone and reach the deeper slow‐slip regionOur results underscore the potential for a deeper downdip rupture and faster rupture speed than previously assumed in kinematic models [ABSTRACT FROM AUTHOR]