Coupling between an evanescent P wave and a Rayleigh wave can occur when th
e. two modes share the game phase velocity at an interface. The result is a
new, distinctive monochromatic mode of large amplitude and finite duration
that may reverberate in a bounded layer, generating stationary patterns du
e to constructive and destructive, interference. Such modes may be responsi
ble for earthquake damage in the Valley of Mexico and perhaps in other sedi
mentary basins vulnerable to seismic disasters on soft ground.