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Geologic Division ---- Coastal and Marine Geology ---- Woods Hole Field Center

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Ground Instability Due to Faulting

seismic profile of compaction faults In this seismic profile across the annular trough, we see a group of faults that cut all the sedimentary beds above the breccia, and extend to the bay floor. The current resolution of our seismic profiles allows us to trace the faults to within 10 m of the bay floor. These faults represent another result of the differential compaction and subsidence of the breccia. As the breccia continues to subside under the load of post-impact deposits, it subsides unevenly due to its variable content of sand and huge clasts. This eventually causes the overlying beds to bend and break, and to slide apart along the fault planes. These faults are zones of crustal weakness, and have the potential for continued slow movement, or of sudden larger offsets if reactivated by earthquakes.

Suppose you were building a new hotel complex in Norfolk. If you had good information about the fault distribution, you could make a better choice of the site location and perhaps modify the building construction.

faults in the area Or suppose that the faulting completely separated adjacent sides of the confining unit over the salt-water reservoir. This could allow the salty water to flow upward and contaminate the fresh water supply. So we need to know in detail the location, orientation, and amount of offset of these compaction faults.

Using the seismic profiles on hand, Wylie Poag has identified more than 100 faults or fault clusters around and over the crater, which reach to or near the bay floor .

The implication is that cities built over the crater or along its rim, like Norfolk, Portsmouth, Hampton, Newport News, Yorktown, and Virginia Beach, and the entire lower Eastern Shore, are potentially at higher risk from fault-related ground instability than cities distant from the crater.

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Last Modified: 12/30/97