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Tilt-beams and recording horizontal extensometers installed across the surface trace of three active faults in northwestern Houston, Harris County, Texas (the Long Point, the Piney Point, and the Eureka Heights faults), recorded 34 events during the 14 months from April 1971 through June 1972. The Long Point fault is an east-northeast-striking regional growth fault that is downthrown toward the coast. The Piney Point fault is antithetic to it. The Eureka Heights fault is associated with the long-abandoned Eureka Heights field.
The events recorded lasted from 1 to 96 hours and were separated by periods of apparent inactivity lasting from 4 to 60 days. The vertical separation recorded during an event ranged from 0.09 to 3.33 mm. The annual rate of vertical separation recorded during the period of observation ranged from 6.7 to 34 mm/year. Normal and reverse faulting resulted in 77 and 23% of the observed
events, respectively. The average vertical separation during episodes of normal and reverse faulting was 1.90 and 0.24 mm, respectively. The cumulative vertical separation recorded during episodes of reverse faulting was 20% of that recorded during episodes of normal faulting.
Events occurred at different times along the same fault. This suggests that faults that have not been demonstrated to be active still must be considered to be capable of moving at any time at possibly damaging rates.
The events appear to be caused primarily by differential compaction and expansion of the Chicot aquifer in response to changes of its piezometric surface. Lesser causes may be the release of tectonic extensional strain energy, the depressuring of hydrocarbon reservoirs, and the movement of salt domes.
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