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An earthquake occurs when rock that has been deformed under stress rebounds elastically along a fault plane (Gilbert, 1884; Reid, 1911), radiating seismic waves through the surrounding earth. Rupture along the entire fault surface does not spontaneously occur at the same time, however. Rather the rupture starts in one tiny area, the rupture nucleation zone, and spreads sequentially along the fault. Like a row of dominoes, one bit of rebounding fault triggers the next. This triggering is understood to occur because of the large dynamic stresses at the tip of an active seismic rupture. The importance of these crack tip stresses is a central question in earthquake physics. The crack tip stresses are minimally important, for example, in the time predictable earthquake model (Shimazaki and Nakata, 1980), which holds that prior to rupture stresses are comparable to fault strength in many locations on the future rupture plane, with bits of variation. The stress/strength ratio is highest at some point, which is where the earthquake nucleates. This model does not require any special conditions or processes at the nucleation site; the whole fault is essentially ready for rupture at the same time. The fault tip stresses ensure that the rupture occurs as a single rapid earthquake, but the fact that fault tip stresses are high is not particularly relevant since the stress at most points does not need to be raised by much. Under this model it should technically be possible to forecast earthquakes based on the stress-renewaql concept, or estimates of when the fault as a whole will reach the critical stress level, a practice used in official hazard mapping (Field, 2008). This model also indicates that physical precursors may be present and detectable, since stresses are unusually high over a significant area before a large earthquake. The fact that fault tip stresses are high is critical in a second earthquake model, however, which argues that stress conditions over the the fault are in fact generally much lower than static fault strength. Failure occurs only because of the high fault tip stresses followed by the drop in fault strength that is known to occur at fast sliding speeds (Di Toro et al., 2011). In this case there would not be any special conditions over the fault before rupture, making the prediction of large earthquakes practically impossible. Likewise because the whole fault plane does not need to be at any particular stress before rupture, the time-predictable stress renewal model would not be effective. That is, with rupture time not closely tied to fault stress, a long quiescence since the last earthquake would not foretell an imminent quake. Because stress over most of the fault is initially far below failure strength special processes to decrease the stress/strength ratio at the nucleation point would likely be required to enable rupture initiation. As will be explained below, although this second model is more complicated, it is overwhelmingly supported by the observed data. The observed data shows that there is compelling evidence both that seismogenic faults are strong and that the average static stress on faults at the time of rupture is far below this strength. Borehole measurements adjacent to faults, for example, have shown a critically stressed crust with hydrostatic pore pressure and friction coefficients of 0.6–0.9 (Townend and Zoback, 2000), indicating that the total strength on well oriented faults should be on the order of 50–100 MPa. Multiple lines of evidence indicate, however, that deviatoric shear stress resolved onto the fault plane at the time of rupture is rarely more than 10 MPa. This evidence includes the lack of a heat flow anomaly around the San Andreas Fault (Lachenbruch and Sass, 1980; Fulton et al., 2004) which cannot be explained by fluid heat transport (Fulton et al., 2004), lack of heat flow anomalies around other major faults (Kano et al., 2006), slip striation rotations on mainshock fault planes that show that the initial stress was low (Spudich et al., 1998), regional rotations of focal mechanisms after large earthquakes that limit the amount of stress that could have been on the surrounding faults prior to the mainshock (Hardebeck and Hauksson, 2001; Hasegawa et al., 2011), and self healing pulses during rupture which require low stress/high strength conditions (Heaton, 1990; Noda et al., 2009). Direct borehole measurements of resolved shear stress along active faults have also found low values at the time of measurement (Zoback and Healy, 1992; Brudy et al., 1997). Furthermore, while there are some outliers (Allman and Shearer, 2009), earthquake stress drops generally fall in the range of 1–10 MPa (Abercrombie and Leary, 1993). This would tell us little if each earthquake released a small portion of the total stress on the fault, but observational seismic (Michael et al., 1990; Beroza and Zoback, 1993; Hasegawa et al., 2011) and borehole studies (Barton and Zoback, 1994) indicate that earthquake stress drops are

Pulverization provides a mechanism for the nucleation of earthquakes at low stress on strong faults