STABILITY OF THE RAKE DURING THE 1992, LANDERS EARTHQUAKE - AN INDICATION FOR A SMALL STRESS RELEASE

We use the high quality data set of the Landers earthquake to study in detail the ability of strong motion inversions to resolve the rake during the rupture process. Initially, we constrain the rake to be constant on the fault. Several inversions with different rake values show that a pure strike slip gives the best fit to the data. In a second step, we have allowed the rake to vary spatially. Spatial rake variations are well resolved by the data and remain small: the data strongly constrain the earthquake to be a pure strike-slip event with almost no variation of the rake. We assume that the direction of the slip vector is always close to the direction of the applied stress, which itself is the vector sum of the pre existing stress and the dynamic stress changes. The stability of the rake on a fault with a dynamical rupture process which is clearly heterogeneous suggests that the pre-existing stress level is quite homogeneous and that it dominates over the dynamic stress changes.