Master joints converging downward and bounding Y-shaped blocks or wedges of rock of more or less acute angle and varying magnitude may be momentarily opened by tensile stress during the passage of earthquake vibrations. When this happens the unsupported or imperfectly supported V or keystone promptly drops, as into a vise, and is more or less extensively broken and crushed, at least in the upper part, by the powerful compressive stress of the succeeding phase of the earth vibration. This dual action, or alternation of tension and compression, simulating the operation of a jaw crusher, may be repeated many times during the passage of an earthquake, the depressed block, or keystone, tending to sink deeper with each successive pulsation; and the narrow graben, or trough fault, thus resulting is, in its simplest terms, the keystone fault. The advisability of the distinctive name will, it is believed, become apparent as . . .

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