. Bulletin of the United States Geological Survey--The San Francisco Earthquake and Fire of April 18, 1906 and their effects on structures and structural materials . obable that a subterranean slip-ping here occurred on a fault associated with the valley. In thatcase the geologic event causing the earthquake included coincidentor nearly coincident yielding on more than one fault plane of theCoast Range system, and various other peculiarities in the distribu-tion of intensity may be ascribed to local faulting. The region of high intensity, to which most of the destruction waslimited, is a belt
. Bulletin of the United States Geological Survey--The San Francisco Earthquake and Fire of April 18, 1906 and their effects on structures and structural materials . obable that a subterranean slip-ping here occurred on a fault associated with the valley. In thatcase the geologic event causing the earthquake included coincidentor nearly coincident yielding on more than one fault plane of theCoast Range system, and various other peculiarities in the distribu-tion of intensity may be ascribed to local faulting. The region of high intensity, to which most of the destruction waslimited, is a belt 20 to 40 miles wide, extending from the mouth ofEel River at the northwest to Priest Valley at the southeast (fig. 1).This belt includes the surface expression of the principal fault—afeature distinctively known as the fault trace—a large number ofcracks, and many local and superficial dislocations of soil and rockvariously characterized as landslides and slumps. THE FAULT TRACE. The plane of the earthquake-causing fault, ivhere it appears at thesurface, is approximately vertical. The movement which took placealong this plane was approximately Fig. 2.—Diagrams illustrating the nature of the earthquake-making fault. As the statement of these relations is sometimes found confusing,they are here illustrated diagrammatically. The upper diagram infig. 2 represents a rectangular block as if cut from the land, the thick-ness being 25 feet, the length east and west (right to left) 150 feet,and the width 100 feet. The dotted line NS indicates the surfaceoutcrop of the old fault plane, trending northwest and southeast,this plane cutting the face of the block in the vertical line SD. ABstands for any straight line on the surface of the land—such as wouldbe defined by a road, a fence, or a row of trees—crossing the faultoutcrop at right angles. The lower diagram represents the same THE EARTHQUAKE AS A NATURAL PHENOMENON. 5 rectangular block after the earthquake, its
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