Super-shear cascading ruptures envelopes

Faults are weak, geometrically complex, strongly interacting interfaces of crustal volumes dissipating tectonic stress. Earthquake ruptures propagate throughout them following uneven multiple paths, taking jumps and bends producing spatially heterogeneous slip amounts and stress drops. Nevertheless, routinely implemented dynamic and kinematic simulations model ruptures as uniform fronts travelling along smooth or slightly rough surfaces. How far can this assumption impact our view of coseismic dynamics? Super-shear earthquakes are seismic events whose apparent rupture speed exceeds the shear wave velocity. The possibility of ruptures propagating faster than shear waves was predicted theoretically and reproduced in the laboratory (Xia et al., 2004); while just a dozen of tectonic “supershear” earthquakes have been convincingly reported so far (Bao et al., 2022). Because of the small number of observations, their dynamics is still poorly understood. Moreover, super-shear speeds have been unambiguously detected only along faults with dominant strike-slip component. Here we propose a new interpretation of super-shear events: we speculate that apparent super-shear velocities should not be attributed to continuously propagating single fronts, but instead to envelopes of dynamically triggered multi-focal ruptures within rough fault zones allowing a faster motion of the two fault walls along strike. We also show that ruptures speed up in the presence of competent rock rheology and low local stress drops. On the other hand, the patches of sub-shear velocity can be interpreted as related to synthetic step overs. Our model, although more complex than the classical one, is more physics-based assuming intricated and wavy fault zones and geometry; moreover, it can reproduce the Mach cone. In addition, it allows to overcome some inconsistencies due to the anomalously intense high-frequency content in the power frequency spectrum expected whenever ruptures travel at super-shear speed (Bizzarri & Spudich, 2008), which is incompatible with observational evidence (e.g., Bouchon et al., 2010). Bao, H., Xu, L., Meng, L., Ampuero, J. P., Gao, L., & Zhang, H. (2022). Global frequency of oceanic and continental supershear earthquakes. Nature Geoscience, 1-8. Bizzarri, A., & Spudich, P. (2008). Effects of supershear rupture speed on the high‐frequency content of S waves investigated using spontaneous dynamic rupture models and isochrone theory. Journal of Geophysical Research: Solid Earth, 113(B5). Bouchon, M., Karabulut, H., Bouin, M. P., Schmittbuhl, J., Vallée, M., Archuleta, R., ... & Marsan, D. (2010). Faulting characteristics of supershear earthquakes. Tectonophysics, 493(3-4), 244-253. Xia, K., Rosakis, A. J., & Kanamori, H. (2004). Laboratory earthquakes: The sub-Rayleigh-to-supershear rupture transition. Science, 303(5665), 1859-1861.