Seismotectonic Problem of en Echelon Fault System and Earthquake Source Mechanism

Abstract

The large earthquake occurs due to the emission of elastic waves during fracture propagation and abrupt displacement along a large fault system, which process is triggered by fracturing of a block of rock in the fault system or at fault edge. Earthquake source process is too much complicated for the precise modeling due to the complexity and variety of fault systems. However, the precise observation of fault development during fracture experiments suggests that a large fault is formed by connection of a set of smaller fractures or faults aligned en echelon. Many fault systems show multiple en echelon arrangement of smaller component fractures. Two mechanically different types of steps or bends are distinguished in the en echelon fault system or zigzag fault which is derived from en echelon fractures. One type is the step-up gap or bend which is compressed by the sliding of the fault. The other type is the step-down gap or bend which is extended by the slip of the fault.
The rough estimation of resistive strength of en echelon fault gaps against fault sliding shows that the step-up gap is much more resistant to fault displacement than the step-down fault gap is. The strength of gap can be roughly estimated from stepping ratio of en echelon steps. The energy of earthquake is proportional to the product of square of offset of en echelon step and length of fault.
The doughnut pattern of foreshock activity and migration of earthquake fracturing can be adequately described by the model that the earthquakes are generated by a process of fracturing of gaps or bends in a multiple en echelon fault system. This “multiple en echelon fault model of earthquake source mechanism” has been proposed in this paper in an attempt to make the “asperity model” more concrete with respect to fault geometry and fault development mechanism.
The gaps or bends of en echelon fault system make loci of stress concentration and are expected to be very sensitive to premonitory stress disturbance before earthquake. The gaps and bends of fault system make promising monitoring sites for earthquake prediction.