Ninety-one faults with an aggregate length of more than 170 km have been mapped recently within a test area of 500 sq km in southeastern metropolitan Houston. Of the four oil fields in this area, three are associated with known salt domes and the fourth is thought to overlie a more deeply seated dome. Eighty-seven of the 91 faults are confined to two complex but well-defined curvilinear grabens that are closely related to the oil fields and underlying domes. A north- to northeast-trending graben connects the South Houston and Mykawa fields. A second graben, which trends southeast, intersects the first graben over Mykawa field, turns east to Webster field, then continues northeast to Clear Lake field and extends northeast beyond the map area toward Goose Creek field. The f ur faults that appear to be unrelated to the grabens include one fault extending northward into the mapped area from the Hastings dome, two faults that are probably regional growth faults, and one fault whose origin remains unexplained.

The pattern of faulting requires a genetic link between the faults and salt domes. Thus, the faults are natural geologic features, probably of Tertiary age. However, most offset of the present land surface appears to have occurred within the last half century. Although scarps in excess of 1 m exist today, only a few that have a 1-ft contour interval can be recognized on topographic maps that were surveyed in 1915-16. Although some allowance must be made for equipment and film, the number of faults that can be recognized on aerial photographs has increased dramatically since 1930 when large-scale coverage was first obtained. Moreover, present rates of offset on the 48 faults known to be active exceed the estimated average prehistoric rates by several orders of magnitude. Evidence that ault movement has increased sharply during the last few decades supports the hypothesis that recent withdrawal of subsurface fluids has triggered or accelerated movement along these ancient and natural planes of weakness.

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