Faults in Earth’s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast, seismic-wave-producing rupture events perceived as earthquakes1,2,3. These types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment: one with stable, rate-strengthening friction and the other with rate-weakening friction that leads to stick-slip2,3,4,5. The 2011 Tohoku-Oki earthquake with moment magnitude Mw = 9.0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping6,7,8,9,10. Here we propose a model in which stable, rate-strengthening behaviour at low slip rates11,12 is combined with coseismic weakening due to rapid shear heating of pore fluids13,14,15,16, allowing unstable slip to occur in segments that can creep between events. The model parameters are based on laboratory measurements on samples from the fault of the Mw 7.6 1999 Chi-Chi earthquake17. The long-term slip behaviour of the model, which we examine using a unique numerical approach that includes all wave effects16,18, reproduces and explains a number of both long-term and coseismic observations—some of them seemingly contradictory—about the faults at which the Tohoku-Oki and Chi-Chi earthquakes occurred, including there being more high-frequency radiation from areas of lower slip8,19,20,21, the largest seismic slip in the Tohoku-Oki earthquake having occurred in a potentially creeping segment6,7, the overall pattern of previous events in the area8 and the complexity of the Tohoku-Oki rupture9. The implication that earthquake rupture may break through large portions of creeping segments, which are at present considered to be barriers, requires a re-evaluation of seismic hazard in many areas.
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This study was supported by the US National Science Foundation (NSF) (grant EAR 0548277), the Southern California Earthquake Center (SCEC) and the Gordon and Betty Moore Foundation. The SCEC is funded by NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. This is SCEC contribution no. 1675 and Caltech Tectonics Observatory contribution no. 213. Numerical simulations for this study were performed on the CITerra Dell cluster at the Division of Geological and Planetary Sciences of the California Institute of Technology.
In this period, whole patch B creeps before model-spanning events. Note that the timesteps between frames vary for orders of magnitude between interseismic and coseismic periods. For details, see Supplementary Information section 2.
In this period, model-spaning events take place while patch B is partially locked. For details, see Supplementary Information section 2.