The wave-like or undulatory motion is most common and least destructive.
It appears to be the normal one, and it is possible that the others may
be simply the result of various systems of waves intersecting one
another. The waves either advance in one direction, like waves of the
sea, or spread from a central point, like ripples produced by dropping a
pebble into still water.
The earthquakes of the Andes are chiefly linear, being propagated along
the mountains, with the undulations perpendicular to the direction of
the ranges. The destructive earthquake at Lisbon, was a central one, the
concentric waves gradually diminishing in intensity with increasing
distance from the place of origin.
The vertical motion acts from beneath like the explosion of a mine, and
when violent nothing can resist its force. The earthquake at Calcutta,
in September, 1828, owed its great destructiveness to the fact that the
main shock was vertical; and one in Murcia, Spain, in 1829, destroyed or
injured more than three thousand five hundred houses.
The rotary or whirling motion is the most dangerous, but happily the
rarest of all. In the great earthquake of Jamaica, in 1692, the surface
of the ground was so disturbed that fields changed places, or were found
twisted into each other.