Echelon Cracks
The Hayward Fault in Oakland, California
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Just a few meters from the previous photo, these distinctive sets of offset cracks cross Revere Avenue. This geometry is called an echelon pattern, named from a formation of infantry troops. The right side of the line of cracks is moving toward the camera. The overall direction of the set of cracks, not the individual fissures, marks the trace of the fault. The echelon pattern reflects Riedel shearing, a phenomenon found from the microscopic scale to the map scale in very young faults.
Photo (c) 2005 Andrew Alden, licensed to About.com. (fair use policy)
