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P. 9
(Below and right) As a land wave passes every 10 seconds, the ground ripples along, with waves in solid rock moving past with about ten metres between crests and a third of a metre between trough and crest. Because the land is a solid, the waves make the surface crack up. At a crest gaping fissures occur, while in a trough the rocks are squashed and forced upwards in a chaotic way. Then, as the wave passes, crests become troughs, and the fissures close. Troughs become crests, and new fissures open. This can happen for tens of seconds and sometimes for several minutes. The picture above right shows the ground surface after the 1906 San Francisco earthquake.
m
Cracks appear
Cracks close
10m
Surface material crushed
Cracks appear
(Below) During the 1964 Alaskan earthquake the up-and-down vibrating movement of the surface drove the wooden supporting piles up through the concrete deck of this bridge.
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