Effect of real-world frictional strengthening layer near the Earth's free surface on rupture characteristics with different friction laws: Implication for scarcity of supershear earthquakes

More and more supershear rupture earthquakes, with rupture propagation velocity larger than the local shear wave speed, have been identified in recent years. However, the number of supershear events is scarce compared with large strike-slip earthquakes. This problem remains unresolved although several previous workers have offered some explanations. In this study, we utilize finite element method to simulate spontaneous dynamic rupture propagations in three dimensional half-space model with and without velocity strengthening friction layer (VSFL) near the Earth's free surface employing different frictional constitutive relations, including linear slip weakening law and rate-and state-dependent relations with aging form and slip form. The simulated results showed that the VSFL suppresses supershear rupture transition at the Earth's free surface occurring in the absence of a VSFL. In particular, we found that the minimum thickness of VSFL to prevent supershear rupture transition is-1.4-km. Meanwhile, we reviewed a great deal of previous work on thickness of VSFL along mature faults in nature, and its thickness generally exceeds-3-km, which is enough to suppress the transition to supershear rupture in the real-world. The presence of VSFL may be the primary reason why the supershear rupture earth-quakes in nature are so scarce. In addition, the modelling results indicated that spatial heterogeneity of VSFL on the surface along fault strike affects the supershear rupture transition to some extent. Therefore, this study contributes to understanding the earthquake source dynamics and seismic hazard mitigation.