Fault and Joint Measurements in Austin Chalk, SuperConducting Super Collider Site, Texas

Structure maps of 9.4 miles of nearly continuous tunnel excavations and more than 10 miles of other exposures and excavations in the Austin Chalk at the Superconducting Super Collider (SSC) site in Ellis County, Texas, record normal fault and joint populations in the subsurface within the northern segment of the Balcones Fault Zone that has unmatched resolution for such a long traverse. Small faults (less than 10 feet throw) occur in clusters or swarms that have as many as 24 faults. Fault swarms are as much as 2,000 feet wide, and spacing between swarms ranges from 800 to 2,000 feet, averaging about 1,000 feet. Predominantly northeast-trending joints are in swarms spaced 500 to more than 21,000 feet apart.