If they are not unleashed by geological faults, the tremors may come from asteroid impacts or volcanic activity.

In that situation, the oldest geological faults on the surface would have certain specific characteristics (dubbed compressional features).

Southern California, with its many geological faults, is prone to frequent quakes and many residents fear the next big one.

Environmentalists say that, given the region's torrential rains, nearby geological faults and the industry's track record on pollution, the walls would inevitably fail.