Figure 3

Effect of rock-fluid system compressibility on rupture speed. With the simulation setup of Fig. 1, we vary the specific storage coefficient, \(S_{\epsilon }\), which controls the extent of the undrained overpressure at the detachment front, \(\Delta P\). We plot the normalized rupture velocity, \({\bar{V}}_R/C_{S,0}\) (black line, left axis), and the undrained overpressure at the rupture front, \(\Delta P\) (blue line, right axis), against \(S_{\epsilon }\). For rather incompressible systems (small values of \(S_{\epsilon }\)), the overpressure is significant and the rupture is supershear. As the system storage capacity increases, thus reducing the coseismic undrained overpressure, propagation speed decreases. Beyond a threshold value of \(S_{\epsilon }\), the rupture front velocity falls within the sub-Rayleigh range. This abrupt parametric shift is compatible with the existence of a range of unstable rupture speeds: if the fault is large enough, ruptures would either become asymptotically sub-Rayleigh, with \({\bar{V}}_R\) \(\approx\) \(0.8C_S\), or accelerate to supershear with rupture speeds larger than the Eshelby speed, \(\sqrt{2}C_{S,0}\). The reported rupture speeds are calculated as the average during the rupture (see Supplementary Information, Section 3).