Once a developer acknowledges the possible presence of faults on his land, he needs to consider such questions as their reality, location and extent, and likely rate and amount of movement. The final answers to these questions commonly must come from the subsurface, where they may be obtained by drilling a series of boreholes, 150 to 500 ft (45 to 150 m) deep. These are logged for electric spontaneous potential, and for resistivity using a single-point electrode to obtain maximum bed resolution and character. The borehole method has the advantages of economy, speed, and reliability; it can be done almost everywhere; and the electric logs provide a permanent, objective record of the strata penetrated. Airphotos, 1-ft (0.3 m) contour-interval topographic maps, and field ins ection are useful guides in determining locations for boreholes for maximum efficiency. The subsurface information enhances the accuracy of the surficial methods.

The client's reaction generally depends on whether he is siting an industrial building or developing residential lots. Industrial builders usually are very concerned about exact choice of location, and plan carefully taking the faulting into consideration. However, land developers have a very wide range of reactions, and even the most responsible lack flexibility for much replanning because of prior commitments to major thoroughfares, etc. Although the presence of faults frequently delays full development of a tract, it does not seem to prevent it. Where one operator drops his option because of faulting another (ignorantly?) will come along and build. Also, a few large (but low) buildings have been designed and constructed knowingly over fairly active faults, and certainly many more h ve been built unknowingly.

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