Examinationof the map shows that nearly all of the faults are confined
to two well-defined,curvilinearbelts which bear a close relationshipto oil
fields in the area. The associationis not accidental.All major fields shown
on the map are known or thought to be producingfrom sedimentsabove or peripheral
to salt domes. It is believedthat the formationof the domes and the faulting
The faults are natural geologicfeaturesof antiquity.Seismic and drilling
data reveals that the faults persist to depths of thousandsof feet and show
clear indicationsof prehistoricmovementextendingover millions of years.
There is ample evidence for a prehistoricorigin of the faults, and for
prehistoricmotion along them. Unresolvedis the questionof why many faults
are so active today. It can be documentedthat natural movement on some faults
persisted into the very recent geologicpast, and is likely continuing.How-
ever, contemporyrates of movement along many faults, which range up to one-inch
per year, are in excess of what has occurred in the past.
If the contemporaryrates were characteristicof the recent past, there
should be many fault scarps over 30 feet. The largest fault on the map has a
scarp height of little more than three feet. The present anomalous rate of
movementmay be the result of human activity,specificallypetroleumand
ground water production.
The extractionof large quantitiesof water from shallow sedimentsbeneath
the city, and productionof petroleumfrom somewhatdeeper levels, has resulted
in large declines in fluid pressureswithin the subsurfacesediments.Inasmuch
as a link between fluid-pressuredeclinesand land-surfacesubsidenceis well
established,it may be that the withdrawalof water and petroleum products is
somehow triggeringor acceleratingmotion along preexistingfalts as well.