COSEISMIC TEMPORAL CHANGES OF SLIP DIRECTION - THE EFFECT OF ABSOLUTESTRESS ON DYNAMIC RUPTURE

We investigate the dynamics of rupture at low-stress level. We show th at one main difference between the dynamics of high- and low-stress ev ents is the amount of coseismic temporal rake rotation occurring at gi ven points on the fault, Curved striations on exposed fault surfaces a nd earthquake dislocation models derived from ground-motion inversion indicate that the slip direction may change with time at a point on th e fault during dynamic rupture. We use a 3D boundary integral method t o model temporal rake variations during dynamic rupture propagation as suming a slip-weakening friction law and isotropic friction, The point s at which the slip rotates most are characterized by an initial shear stress direction substantially different from the average stress dire ction over the fault plane. We show that for a given value of stress d rop, the level of initial shear stress (i.e., the fractional stress dr op) determines the amount of rotation in slip direction. We infer that seismic events that show evidence of temporal rake rotations are char acterized by a low initial shear-stress level with spatially variable direction on the fault (possibly due to changes in fault surface geome try) and an almost complete stress drop. Our models motivate a new int erpretation of curved and cross-cutting striations and put new constra ints on their analysis, The initial rake is in general collinear with the initial stress at the hypocentral zone, supporting the assumptions made in stress-tensor inversion from first-motion analysis. At other points on the fault, especially away from the hypocenter, the initial slip rake may not be collinear with the initial shear stress, contradi cting a common assumption of structural geology. On the other hand, th e later part of slip in our models is systematically more aligned with the average stress direction than the early slip. Our modeling sugges ts that the length of the straight part of curved striations is usuall y an upper bound of the slip-weakening distance if this parameter is u niform over the fault plane, and the direction of the late part of sli p of curved striations should have more weight in the estimate of init ial stress direction.