Deep-seated downslope slip during strong seismic shaking

Abstract

[1] Strong seismic waves cause frictional failure within regolith on hillslopes. The material responds anelastically to the combined oscillating dynamic and static gravitational stresses. Gravity drives preferential downslope movement of the transiently weakened material. The net effect over many earthquake cycles is a tens of meter thick slow landslide, that is a sackung. The classical approach to hillside failure involves downslope dynamic accelerations or equivalently downslope dynamic shear tractions. Rate and state friction quantifies that the starting coefficient of friction is a few tenths higher that the apparent long-term coefficient of friction from the angle of repose, 0.96–1.25 in the calculations, retaining an extra digit. Furthermore, the dynamic Coulomb stress ratio for dynamic to lithostatic stress for S-waves refracted toward vertical paths is the dynamic acceleration in g's above the scale depth 1/k, the inverse of the wave number. In more detail, a dynamic acceleration of 0.13–0.35 g destabilizes slopes near the angle of repose. Seismologists expect such acceleration near active major faults, compatible with the widespread occurrence of sackungen on steep slopes. Accelerations of 0.96–1.25 = ∼1 g destabilize shallow slopes, leading to net downhill movement, analogous to the movement of an object down a vibrating ramp. However, sackungen are rare or even absent on shallow slopes in California indicating the rarity of sustained 1 g accelerations. A second mechanism may drive downhill movement on modest ∼19° slopes as observed in the San Gabriel Mountains of California near the San Andreas Fault. The near-field velocity pulse on strike-slip faults produces only a few tenths g acceleration but particle velocities of over 1 m s−1 persisting over >1 s. Dynamic stresses and strains scale with this velocity and extend all the way to the free surface. Coulomb failure criteria are exceeded at shallow depths. Failure greatly weakens the regolith allowing it to move downhill in response to gravity. This mechanism appears capable of producing prehistoric 1 m per event slip on modest slopes observed in the San Gabriel Mountains.