WRINKLE-LIKE SLIP PULSE ON A FAULT BETWEEN DIFFERENT MATERIALS

Pulses of slip velocity can propagate on a planar interface governed b y a constant coefficient of friction, where the interface separates di fferent elastic materials. Such pulses have been found in two-dimensio nal plane strain finite difference calculations of slip on a fault bet ween elastic media with wave speeds differing by 20%. The self-sustain ing propagation of the slip pulse arises from interaction between norm al and tangential deformation that exists only with a material contras t. These calculations confirm the prediction of Weertman [1980] that a dislocation propagating steadily along a material interface has a ten sile change of normal traction with the same pulse shape as slip veloc ity. The self-sustaining pulse is associated with a rapid transition f rom a head wave traveling along the interface with the S wave speed of the faster material, to an opposite polarity body wave traveling with the slower S speed. Slip occurs during the reversal of normal particl e velocity. The pulse can propagate in a region with constant coeffici ent of friction and an initial stress state below the frictional crite rion. Propagation occurs in only one direction, the direction of slip in the more compliant medium, with rupture velocity near the slower S wave speed. Displacement is larger in the softer medium, which is disp laced away from the fault during the passage of the slip pulse. Motion is analogous to a propagating wrinkle in a carpet. The amplitude of s lip remains approximately constant during propagation, but the pulse w idth decreases and the amplitudes of slip velocity and stress change i ncrease. The tensile change of normal traction increases until absolut e normal traction reaches zero. The pulse can be generated as a second ary effect of a drop, of shear stress in an asperity. The pulse shape is unstable, and the initial slip pulse can change during propagation into a collection of sharper pulses. Such a pulse enables slip to occu r with little loss of energy to friction, while at the same time incre asing irregularity of stress and slip at the source.