Why are there so few supershear rupture earthquakes in real-world nature?

The supershear rupture earthquake is more dangerous than the subshear one with the same magnitude, so it has been paid more attention. However, the number of supershear ruptures in nature is extremely rare; so far, only a dozen cases of supershear rupture earthquakes have been identified worldwide. However, the physical mechanism of this phenomenon has not been systematically studied. For this purpose, in this paper, using finite element method (FEM), we carry out quantitative analysis of the familiar trigger factors for the occurrence of supershear rupture. The simulation results show that the Earth's free surface is the most effective factor for triggering the supershear rupture transition, but when there is a velocity strengthening friction layer (VSFL) near the Earth's free surface resulted from sedimentary layer, unconsolidated fault gouge and the low level of normal stresses near the free surface, the subshear rupture cannot be transformed into supershear rupture, and supershear rupture will be stifled in this case. In addition, the modeling results also demonstrate that the fault stepover, barrier, and anti-asperity on the fault can promote the transition from subshear rupture to supershear one, but there is a temporal pause in the transition process, so the average rupture velocity of the whole fault is less than the speed of shear waves, that is, the local supershear rupture cannot be distinguished through the seismic signals received by the far-field seismic stations. We found that the conditions for producing a supershear rupture event that can be recognized at present are very tough, such as the length of the fault needs to be long enough, the geometry of the fault needs to be straight enough, and the initial stress level is high enough, the thickness of the VSFL is thin enough, and there are also enough seismic stations in the near field. Thus, all these factors above contribute to the dearth of identifiable supershear rupture earthquakes in nature. This study is helpful for us to deep understand the mechanism of the occurrence of supershear ruptures and the assessment of seismic hazards.

Automated summary

Supershear rupture earthquakes are more dangerous than subshear ones, but they are extremely rare in nature. In this study, the finite element method was used to analyze the triggering factors of supershear rupture, and it was found that the Earth's free surface is the most important factor. Additionally, the geometry of the fault, barriers, and anti-asperities can also promote the transition from subshear rupture to supershear rupture, but there is a temporal pause in the transition process.