New insights into normal fault rupture propagation in sand

The mechanism of rupture propagation is fundamental to understanding the damage to structures within the fault zones. However, the traditional logarithmic spiral model could not describe some normal fault ruptures of physical tests. Therefore, this study conducted six centrifuge tests to give new insights into normal fault rupture propagation in the sand. The tests simulated normal faulting with dip angles of 60° in the free-filed condition, designed with different relative densities of sand and thicknesses of the overlying soil. The rupture propagation process and effects of relative density and soil thickness were discussed. The mechanism of normal rupture propagation in the sand was summarized, based on the test results. It is observed that two outcropping ruptures develop during normal faulting. The first one is a logarithmic spiral, with a surface angle of 45° + ψmax/2. However, its direction angle at the fault tip disagrees with the logarithmic spiral model. The second outcropping rupture experiences a combined trace due to the reduction in the dilation angle during faulting. The lower part follows a logarithmic spiral. The upper part propagates as a straight trace with an angle of 45° + φmax/2 when the dilation angle falls to the critical state. A slope forms on the ground surface, the angle of which is consistent with φres.