STABILITY OF A MINED GEOLOGIC REPOSITORY IF SUBJECTED TO SEISMIC EVENTS

Engineering calculations using rock properties at Yucca Mountain have led to the conclusion that seismic events would cause only minor damage to tunnels in the major rock units. These conclusions cannot be tested directly with analogues, but their reasonableness can be assessed. Caves (some millions of years old) have been investigated in areas where strong ground motion has occurred many times. Damage to speleo­thems has been reported widely, but roof collapse is rare. For example, two caves in Israel are about 40 km from the Dead Sea transform (a 1000 km long fault zone), and have been subjected to 13 to 18 seismic episodes with estimated magnitudes of 7.6 to 8.2. These caves exhibit speleothem damage and minor roof collapse. Perhaps one extreme example of stability is provided by a cave in Italy that had incurred 1 meter of displacement, but no roof collapse.

The record for anthropogenic underground openings spans a much shorter period of time than for natural open­ings, but it also attests to remarkable stability even in areas of strong seismicity. Thousands of tombs throughout Europe and the Middle East have been excavated in limestone and volcanic rock. Most of these are older than 2 ka. There are also several kilometers of tunnels in these same two rock types located within areas of strong seismicity, and these have not collapsed in more than 2,500 years. Several compilations of mines and tunnels have documented response to earthquakes during the last 200 years; reports of damage have been minimal. A few reports note heavy damage to surface structures from earthquakes that were not felt by miners working underground. Commonly, when damage has been reported, it was near the portal or in areas of shallow cover. Damage has been reported for tunnels displaced by faulting, but even in these cases, total collapse has not been reported.

The analogue data agree with engineering calculations and strongly suggest that a geologic repository should survive seismic events, especially if tunnels were not driven across capable faults and were located deep underground.