Effects of fault interaction on moment, stress drop and strain energy release

Solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone in-teraction on determinations of moment, stress drop, and static energy release. Two models, the barrier model and the asperity model, are considered. In the asperity model, the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases: areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or 'asperities ' across which slip occurs only at the time of a seismic event. In the barrier model, slip zones separated by unbroken ligaments (barriers) are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation pro-posed by Das and Aki. The strain energy change due fo introducing collinear slip zones or due to break-ing the asperities between them is shown to be given by the usual formula for an isolated slip zone,with the stress drop replaced by the effective stress. Significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones. For the case of two collinear slip zones, fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones. For example, if the asperity length is 0.051, where l is the length of each adjacent slip zone, then fracture of the asperity causes a moment almost 1.8 times the moment caused by introducing a slip zone oflength l. For two collinear slip zones